WORKS    OF 

PROF.  IRA  O.  BAKER 

PUBLISHED   BY 

JOHN  WILEY  &  SONS,  Inc. 


A  Treatise  on  Masonry  Construction. 

Materials  and  Methods  of  Testing  Strength, 
etc.;  Combinations  of  Materials— Composi- 
tion, etc.;  Foundations — Bearing  Power  of 
Soils,  etc.;  Masonry  Structure — Dams,  Re- 
taining Walls,  Abutments,  Piers,  Culverts, 
Vousoir  Arches,  Elastic  Arches.  Tenth  Edi- 
tion, Re-written  and  Enlarged.  8vo,  745 
pages,  244  figures,  cloth,  $5.00. 

Engineers'  Surveying  Instruments. 

Their  Construction,  Adjustment  and  Use. 
Second  Edition,  Revised  and  Greatly  En- 
larged, 12mo,  ix+391  pages,  86  figures, 
cloth,  $3.00. 

A  Treatise  on  Roads  and  Pavements. 

8vo,  xi  +  667  pages,  235  figures,  80  tables, 
cloth,  $5.00. 


A  TREATISE 


ON 


ROADS  AND  PAVEMENTS 


BY 

IRA  OSBORN  BAKER,  C.E.,  D.  ENG'G 

Professor  of  Civil  Engineering,  University  of  Illinois;   Author  of  Masonry 
Construction,  Engineer's  Surveying  Instruments;     Member  of 
American  Society  of  Civil  Engineers,    Western 
Society  of  Engineers,  Society  for  Promo- 
tion of  Engineering  Education 


THIRD  EDITION 

RE-WRITTEN  AND  ENLARGED 

FIRST  IMPRESSION 


NEW  YORK 

JOHN   WILEY   &  SONS,   INC. 

LONDON;  CHAPMAN  &  HALL,  LIMITED 

1918 


Copyrighted  1903,   1913,   1918, 

BY 

IRA  OSBORN  BAKER 


PRESS    OP 

•  RAUNWORTH    It    CO. 

BOOK    MANUFACTURERS 

BROOKLYN.    N.    V. 


PREFACE 

FIRST.  EDITION 


THE  object  of  this  book  is  to  give  a  discussion  from  the  point  of 
view  of  an  engineer  of  the  principles  involved  in  the  construction  of 
country  roads  and  of  city  pavements.  The  attempt  has  been  made 
to  show  that  the  science  of  road  making  and  maintenance  is  based 
upon  well-established  elementary  principles,  and  that  the  art  depends 
upon  correct  reasoning  from  the  principles  rather  than  in  attempting 
to  follow  rules  or  methods  of  construction.  In  some  cases  prac- 
tical experience  has  not  yet  determined  the  best  method  of  pro- 
cedure, and  in  these  cases  the  conflicting  views  with  the  reasons  for 
each  are  fully  stated. 

Considerable  space  has  been  given  to  the  economics  and  location 
of  country  roads  and  to  the  construction  and  maintenance  of  earth 
roads,  since  such  roads  constitute  more  than  ninety-five  per  cent  of 
the  mileage  of  the  public  highways  and  are  greatly  in  need  of  careful 
consideration.  It  is  frequently  claimed  by  engineers  that  the  public 
would  be  benefited  by  placing  the  care  of  the  roads  in  the  hands  of 
engineers;  but  there  is  no  evidence  that  any  considerable  number  of 
engineers  comprehend  either  the  principles  of  road  making  necessary 
for  the  improvement  and  maintenance  of  our  country  roads,  or  the 
economic  limitations  and  political  d  fficulties  of  the  problem.  The 
first  five  chapters  of  this  book  are  offered  as  a  contribution  to  this 
phase  of  the  good-road  problem. 

The  remainder  of  the  book,  the  portion  that  considers  roads 
having  permanently  hard  surfaces,  which  may  not  unfittingly  be 
said  to  relate  to  urban  and  suburban  roads,  is  based  chiefly  upon 
American  experience,  because  the  principles  of  road  making  worked 
out  in  this  country  are  probably  best  suited  to  American  conditions, 
and  also  because  in  most  particulars  American  roads  and  pave- 
ments are  superior  to  any  other  in  the  world.  Some  countries 
have  more  hard  roads  than  this,  because  of  a  difference  in  condi- 
tions; but  in  no  country  does  the  quality  of  such  roads  average 

iii 

382053 


PREFACE — FIRST   EDITION 


better  than  in  this.  In  some  foreign  cities  the  pavements  seem  to 
be  better  cared  for  than  in  this  country,  owing  chiefly  to  different 
controlling  conditions;  but  the  principles  of  construction  employed 
here  are  equal  to  the  best.  Notwithstanding  the  general  excellence 
of  the  best  American  practice  in  constructing  hard  roads  and  pave- 
ments, there  is  still  room  for  improvement  in  adapting  the  particular 
form  of  construction  to  the  local  conditions  and  also  in  preserving 
the  surface  from  ruthless  destruction.  These  two  phases  of  the 
subject  have  been  emphasized  in  the  proper  places  in  this  volume. 
Throughout  the  attempt  has  been  to  state  fully  and  clearly  the 
fundamental  principles  of  the  construction  and  maintenance  of  roads 
and  pavements. 

In  the  preparation  of  the  book  the  endeavor  has  been  to  observe 
a  logical  order  and  a  due  proportion  between  the  different  parts; 
and  great  care  has  been  taken  in  classifying  and  arranging  the  matter. 
It  will  be  helpful  to  the  reader  to  notice  that  the  volume  is  divided 
successively  into  parts,  chapters,  articles,  sections  having  small- 
capital  black-face  side-heads,  sections  having  lower-case  black-face 
side-heads,  sections  having  lower-case  italic  side-heads,  and  sections 
having  simply  the  serial  number.  In  some  cases  the  major  subdi- 
visions of  the  sections  are  indicated  by  small  numerals.  The  con- 
stant aim  has  been  to  present  the  subject  clearly  and  concisely. 

Every  precaution  has  been  taken  to  present  the  work  in  a  form 
for  convenient  practical  use  and  ready  reference.  Numerous  cross 
references  are  given  by  section  number;  and  whenever  a  table  or  a 
figure  is  mentioned,  the  citation  is  accompanied  by  the  number  of 
the  page  on  which  it  may  be  found.  The  table  of  contents  shows 
the  general  scope  of  the  book;  the  running  title  assists  in  finding 
the  different  parts;  and  a  very  full  analytical  index  makes  every- 
thing in  the  book  easy  of  access. 

The  author  will  esteem  it  a  favor  if  any  errors  that  may  be  found 
are  at  once  brought  to  his  notice. 

I,  O.  B. 
CHAMPAIGN,  ILL., 

November  27,  1902. 


PREFACE 

THIRD  EDITION 


THE  numerous  changes  in  methods  of  road  and  pavement  con- 
struction have  made  necessary  a  radical  revision  of  this  volume. 
Five  years  ago,  before  many  of  the  changes  had  become  well  estab- 
lished, a  second  edition  was  issued  containing  a  supplementary 
chapter  which  briefly  treated  some  of  the  new  forms  of  construction. 
The  present  edition  has  been  thoroughly  revised  and  entirely  re- 
written. Five  chapters  of  minor  importance  have  been  dropped  to 
make  room  for  an  equal  number  of  new  ones.  The  number  of  illus- 
trations has  been  greatly  increased.  No  pains  have  been  spared  to 
bring  the  book  up  to  date  and  to  fully  present  the  best  modern 
practice. 

Attention  has  been  given  to  materials  and  forms  of  construction 
that  affect  the  quality  and  cost  of  the  road  and  pavement,  rather 
than  to  the  machines  employed  and  the  methods  of  doing  the  work. 
In  other  words,  the  book  is  intended  more  for  the  one  who  designs 
and  inspects  the  road  or  pavement  than  for  the  contractor  who 
constructs  it.  In  recent  years  there  have  been  developed  a  number 
of  machines  for  doing  road  work  and  for  handling  road-building  and 
paving  materials  that  have  greatly  reduced  the  cost  of  road  and 
pavement  construction;  but  to  have  included  an  adequate  discussion 
of  such  appliances  and  of  modern  methods  of  operation  and  organi- 
zation would  have  greatly  increased  the  size  of  the  book. 

Photographs  from  which  illustrations  have  been  made  were 
obtained  from  the  following: 

Austin  Manufacturing  Co.,  Chicago,  111. 

Baker  Manufacturing  Co.,  Springfield,  111. 

Barber  Asphalt  Paving  Co.,  Philadelphia,  Pa. 

Barrett  Manufacturing  Co.,  New  York  City,  N.  Y. 

Cressy  Contracting  Co.,  Boston,  Mass. 

Granite  Paving  Block  Manufacturing  Association  of  the  U.  S.,  Boston,  Mass. 

Illinois  Paving  Brick  Publicity  Bureau,  Chicago,  111. 

Illinois  State  Highway  Department,  Springfield,  111. 


VI  PREFACE — THIRD  EDITION 


Metropolitan  Paving  Brick  Co.,  Canton,  Ohio. 

National  Paving  Brick  Manufacturers  Association,  Cleveland,  O. 

Portland  Cement  Association,  Chicago,  111. 

Standard  Oil  Co.,  New  York  City,  N.  Y. 

U.  S.  Office  of  Public  Roads  and  Rural  Engineering,  Washington,  D.  C. 

Mr.  Walter  Buehler,  Chicago,  111. 

Mr.  John  S.  Crandell,  New  York  City,  N.  Y. 

Mr.  Harlan  H.  Edwards,  Danville,  111. 

Mr.  Richard  H.  Gillespie,  Chief  Engineer  of  Streets,  Bronx,  New  York  City. 

Dr.  Herman  von  Schrenk,  St.  Louis,  Mo. 

The  following  persons  have  generously  given  valuable  suggestions 
on  the  subject  stated. 

Mr.  Arthur  N.  Johnson,  Chicago,  Concrete  Roads. 
Mr.  Walter  Buehler,  Chicago,  Wood-block  Pavements. 
Mr.  Philip  P.  Sharpies,  New  York  City,  Bituminous  Materials. 
Mr.  John  W.  Stipes,  Champaign,  111.,  many  matters. 
Mr.  D.  T.  Pierce,  Philadelphia,  Sheet  Asphalt  Pavements. 
Mr.  W.  C.  Perkins,  Conneaut,  Ohio,  Brick  Pavements. 

Mr.  George  N.  Norton,  Buffalo,  N.  Y.,  Cost  of  Maintenance  of  Sheet  Asphalt 
Pavements. 

Mr.  Walter  L.  Weeden,  Boston,  Granite  Block  Pavements. 

Valuable  data  were  received  from  many,  which  are  specifically 
acknowledged  in  the  text. 

To  all  of  these  the  author  gratefully  acknowledges  his  obligations. 

I.  O.  B. 

URBANA,  ILLINOIS, 
January  8,  1918. 


TABLE  OF  CONTENTS 


PART  I.     COUNTRY  ROADS 

PAGE 

INTRODUCTION 1 

CHAPTER  I.     ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION 

ART.  1.  ROAD  ECONOMICS.  Advantages  of  Good  Roads.  Cost  of 
Wagon  Transportation.  Financial  Value  of  Road  Improvements.  Tractive 
Resistance.  Power  of  Horse.  Travel  Census.  Weight  and  Width  of 
Vehicles 3 

ART.  2.     ROAD  ADMINISTRATION.     Administrative    Unit.     State    Aid. 
National  Aid.     Classification  of  Roads.     Road  Taxes 31 

CHAPTER  II.     ROAD  LOCATION 

Elements  Involved.  Distance.  Grade.  Rise  and  Fall.  Curves. 
Width.  Cross  Section.  Placing  the  Line.  Establishing  Grade.  Example 
of  Re-location 41 

CHAPTER  III.    EARTH  ROADS 

ART.  1.  CONSTRUCTION.  Width.  Cross  Section.  Grades.  Drainage. 
Excavation  and  Embankment.  Improving  Old  Roads.  Road  Building 
Machinery.  Cost  of  Earthwork.  Bridges.  Waterways.  Culverts.  Retain- 
ing Walls.  Guard  Rails.  Guide  Posts.  Artistic  Treatment 70 

ART.  2.  MAINTENANCE.  Destructive  Agents.  Care  of  Surf  ace :  Road 
Drags  and  Rules  for  Using;  Scraping  Grader  and  Rules  for  Using;  V  Road- 
Leveler.  Care  of  Side  Ditches.  Care  of  Roadside.  Obstruction  by  Snow. 
Systems  of  Maintenance.  Expenditures  for  Maintenance 115 

ART.  3.  SURFACE  OILING.  Preventing  Dust.  Effect  of  Oiling  on 
Maintenance.  Preparing  Surface.  The  Oil.  Applying  the  Oil.  Cost 133 

CHAPTER  IV.     SAND  AND  SAND-CLAY  ROADS 

ART.  1.  SAND  ROADS.  Drainage.  Grading.  Shade.  Hardening  the 
Surface 139 

ART.  2.  SAND-CLAY  ROADS.  The  Design.  Natural  Mixture  of  Sand 
and  Clay.  Sand  on  Clay  Subgrade.  Clay  on  Sand  Suhgrade.  Cost. 

Maintenance 140 

vii 


Vlll  CONTENTS 


CHAPTER  V.     GRAVEL  ROADS 

PAGE 

ART.  1.  THE  GRAVEL.  Requisites  for  Road  Gravel.  Distribution  of 
Gravel.  Characteristics  of  Different  Gravels 150 

ART.  2.  CONSTRUCTION.  Drainage.  Width.  Maximum  Grade.  Crown. 
Forms  of  Construction:  Surface  Construction;  Trench  Construction. 
Bottom  Course.  Screening  the  Gravel.  Hauling  the  Gravel.  Measuring 
the  Gravel.  Cost.  Economic  Value  of  Gravel  Surface.  Specifications. . .  .  165 

ART.  3.  MAINTENANCE.  Destructive  Agents.  Method  of  Maintenance. 
Cost 178 

ART.  4.  DUST  PALLIATIVES.  Dust  Preventives:  Fresh  Water,  Sea 
Water,  Deliquescent  Salts,  Proprietary  Compounds,  Sprinkling  with  Oil. .  .  .  181 

CHAPTER  VI.     WATER-BOUND   MACADAM   ROADS 

HISTORY 185 

ART.  1.  THE  STONE.  Requisites  for  Road  Stone.  Methods  of  Testing 
Stone 186 

ART.  2.  CONSTRUCTION.  Forms  of  Construction.  Width.  Crown. 
Thickness.  Cross  Section.  Permissible  Grades.  Preparing  Subgrade. 
Crushing  the  Stone.  Spreading  the  Stone.  Road  Rollers.  Rolling  the 
Stone.  Binding  the  Stone.  Cost  of  Construction.  Specifications 189 

ART.  3.  MAINTENANCE.  Leveling.  Ruts.  Patching.  Rolling.  Sprink- 
.  ling.  Cost 223 

CHAPTER  VII.     PORTLAND-CEMENT  CONCRETE  ROADS 

DEFINITIONS.     HISTORY .• 227 

ART.  1.  THE  MATERIALS.  Cement.  Fine  Aggregate.  Coarse  Aggre- 
gate. Theory  of  Proportions.  Methods  of  Proportioning.  Data  for 
Estimates 227 

ART.  '2.  THE  CONSTRUCTION.  Drainage.  Preparing  the  Subgrade. 
One-  vs.  Two-course  Pavements.  Cross  Section.  Maximum  Grade. 
Width.  Thickness.  Crown.  Side-forms.  The  Concrete:  proportions, 
mixing,  consistency,  placing,  striking,  finishing,  curing,  protecting.  Con- 
traction Joints.  Reinforcement.  Shoulders.  Curbs.  Cost  of  Concrete 
Roads.  Characteristics  of  Concrete  Roads.  Concrete  Street  Pavements.  .  238 

ART.  3.  MAINTENANCE.  Character  of  Work  Required.  Cost  of  Main- 
tenance    264 

CHAPTER  VIII.     BITUMINOUS  ROAD   MATERIALS 

DEFINITIONS 267 

ART.  1.  ASPHALT.  Definitions.  Characteristics  of  Asphalt.  Sources 
of  Asphalt.  Specifications  for  Asphalt:  for  Bituminous  Surfaces,  Binder  for 
Bituminous  Macadam,  Binder  for  Bituminous  Concrete,  Sheet  Asphalt, 

Filler  for  Block  Asphalt.     Cost .267 

ART.  2.  PETROLEUM.  Classification.  Methods  of  Refining.  Ship- 
ping. Asphaltic  Content  of  Road  Oils.  Specifications  for  Oil:  for  Park 
Drives,  Earth  Roads,  Gravel  Roads,  Macadam  Roads.  Cost 283 


CONTENTS  IX 


PAGE 

ART.  3.  TAR.  Definitions.  Characteristics  of  Tar.  Specifications: 
for  Bituminous  Surface,  Bituminous  Macadam,  Bituminous  Concrete,  Joint 
Filler  for  Block  Pavements.  Cost. . 289 

CHAPTER  IX.  BITUMINOUS  SURFACES  FOR  ROADS 

KINDS  OF  BITUMINOUS  SURFACES 296 

ART.  1.  PROTECTIVE  COATING.  Bituminous  Material 297 

ART.  2.  BITUMINOUS  CARPETS.  Bituminous  Material.  Cleaning  Road 

Surface.     Applying  Bituminous  Material.     Value  of  Bituminous  Carpets. 

Maintenance.     Cost 298 

CHAPTER  X.     BITUMINOUS   MACADAM   AND   BITUMINOUS  CON- 
CRETE ROADS 

ART.  1.  BITUMINOUS  MACADAM  ROADS.  Foundation.  Maximum  Grade. 
Crown.  Wearing  Course.  Bituminous  Binder.  Tar-sand  Macadam.  Char- 
acteristics of  Bituminous  Macadam.  Costs.  Maintenance 306 

ART.  2.  BITUMINOUS  CONCRETE  ROADS.  The  Aggregate.  The  Binder. 
Mixing.  Laying.  Seal  Coat.  Cost.  Comparison  of  Bituminous  Mac- 
adam and  Bituminous  Concrete , .  . . 312 


PART  II.    STREET  PAVEMENTS 

CHAPTER  XI.     PAVEMENT  ECONOMICS  AND  PAVEMENT 
ADMINISTRATION 

ART.  1.  PAVEMENT  ECONOMICS.  Benefits  of  Pavements.  Investment 
in  Pavements 318 

ART.  2.  PAYEMENT  ADMINISTRATION.  Importance  of  Problem.  Ap- 
portionment of  Cost.  Special  Assessments.  Guaranteeing  Pavements. 
Tearing  Up  Pavements 321 

CHAPTER  XII.     STREET  DESIGN 

Street  Plan:  checkerboard,  diagonal,  concentric.  Size  of  Lots  and 
Blocks.  Width  of  Streets.  Area  of  Streets.  Width  of  Pavement.  Street 
Grades.  Crown  of  Pavement.  Cross  Sections  of  Side-hill  Streets.  Plans 
and  Specifications.  Street  Trees 336 

CHAPTER  XIII.     STREET  DRAINAGE 

Underdrainage.  Catch  Basins.  Gutters.  Drainage  at  Street  Inter- 
section. Surface  Drainage.  Crown,  Rules  for 361 

CHAPTER  XIV.  CURBS  AND  GUTTERS 

Curb:  Stone,  Concrete,  Combined  Concrete  Curb  and  Gutter.  Radius 
of  Curb  at  Street  Corner.  Combined  Curb  and  Walk  .  .  378 


X  CONTENTS 


CHAPTER  XV.     PAVEMENT  FOUNDATIONS 

PAGE 

ART.  1.     PREPARATION  OF  SUBGRADE.     Drainage.     Rolling  the  Sub- 
grade.     Filling  Trenches 392 

ART.  2.  THE  FOUNDATION.  Portland-Cement  Concrete:  Thickness, 
Proportions,  Mixing  and  Placing,  Finishing,  Curing.  Cost.  Old  Macadam. 
Bituminous  Concrete  Foundation 399 

ART.  3.  FOUNDATIONS  FOR  STREET-RAILWAY  TRACKS.  Foundation. 
Ties.  Rails.  Paving 407 

CHAPTER  XVI.  ASPHALT  PAVEMENTS 

ART.  1.  SHEET  ASPHALT  PAVEMENTS.  Classification.  History.  Foun- 
dation: portland-cement  concrete,  bituminous  concrete,  other  forms.  Binder 
Course:  open,  closed;  cement;  bitumen  in  binder;  mixing;  laying;  thick- 
ness. Wearing  Coat:  sand;  filler;  cement;  bitumen  in  wearing  coat;  mixing; 
laying;  rolling;  thickness.  Asphalt  Adjacent  to  Tracks.  Causes  of  Fail- 
ure. Methods  of  Repairing.  Cost  of  Construction.  Cost  of  Mainte- 
nance. Maximum  Grade.  Crown.  Merits  and  Defects.  Specifica- 
tions   411 

ART.  2.  ASPHALT  CONCRETE  PAVEMENTS.  Definitions:  Bitulithic, 
Warrenite,  Amiesite,  Topeka  mixture,  stone-filled  sheet-asphalt,  asphalt- 
concrete  pavement.  Mixing  and  Laying.  Cost.  Merits  and  Defects. 
Specifications 461 

ART.  3.     ROCK  ASPHALT  PAVEMENTS.     Construction 469 

ART.  4.  ASPHALT  BLOCK  PAVEMENTS.  The  Blocks.  Cost.  Merits  and 
Defects...  .  470 


CHAPTER  XVII.    BRICK  PAVEMENTS 

ART.  1.  THE  BRICK.  The  Clay.  Manufacture  of  the  Brick.  Kinds 
of  Brick.  Size.  Testing  the  Brick.  Service  Tests 474 

ART.  2.  CONSTRUCTION.  Foundation.  Bedding  Course*  sand 
cement-sand,  mortar.  Laying  the  Brick.  Joint  Filler:  sand,  hydraulic 
grout,  bituminous  cement,  tar-sand.  Expansion  Joints.  Comparison  of 
Types.  Brick  Adjacent  to  Track.  Maximum  Grade.  Brick  Streets. 
Brick  Roads.  Cost.  Merits  and  Defects.  Specifications 503 

ART.  3.  MAINTENANCE.  Repairs:  soft  brick,  shrinkage  of  sand  cush- 
ion, settlement  of  foundation,  settlement  of  trench,  defective  grouting, 
bulges,  re-laying  pavement,  cracks.  Re-surfacing:  asphalt  top,  tar  top, 
turning  the  brick,  monolithic  brick  top.  Cost  of  maintenance 552 


CHAPTER  XVIII.     STONE-BLOCK  PAVEMENTS 

Nomenclature:  Roman  roads,  cobble-stone,  Belgian-block,  oblong-block, 
durax  pavements 566 

ART.  1.  THE  STONE:  granite,  Medina  sandstone,  Potsdam  sandstone, 
Sioux  Falls  quartzite,  Kettle  River  sandstone,  limestone 569 


CONTENTS 


PACE 

ART.  2.  CONSTRUCTION.  Foundation.  Bedding  Course:  sand,  mortar. 
The  Blocks:  dressing,  re-cutting,  size,  measuring.  Setting.  Ramming. 
Filling  Joints.  Expansion  Joints.  Paving  Adjacent  to  Track.  Maximum 
Grade.  Merits  and  Defects.  Durax  Pavement.  Cost:  the  blocks,  granite- 
block,  Medina-sandstone  block,  and  durax  pavements ,  .  572 

ART.  3.  MAINTENANCE.  Re-laying.  Re-filling  Joints.  Spalling  Joints. 
Raising  Low  Blocks.  Settlement  of  Foundation.  Settlement  of  Trench. . . .  597 

CHAPTER  XIX.     WOOD-BLOCK  PAVEMENTS 

KINDS  OF  WOOD-BLOCK  PAVEMENTS.     HISTORY 601 

ART.  1.  MATERIALS  AND  TREATMENT.  The  Timber.  Causes  of  Decay. 
Preservative.  Treatment 603 

ART.  2.  CONSTRUCTION.  Bedding  Course:  sand,  cement-sand,  mor- 
tar, bituminous  cement.  Laying  the  Blocks.  Joint  Filler:  sand,  tar  pitch. 
Open-joint  Construction.  Expansion  Joints.  Cost.  Merits  and  Defects. 
Specifications 612 

ART.  3.  MAINTENANCE.  Removing  Poor  Blocks.  Raising  Low  Spots. 
Re-laying  over  Trenches.  Lowering  Bulges.  Bleeding.  Cost  of  Main- 
tenance    628 


CHAPTER  XX.     SELECTING   THE   BEST  PAVEMENT 

KINDS  OF  PAVEMENTS 633 

ART.  1.  THE  DATA  FOR  THE  PROBLEM.  Durability.  Requirements  of 
the  Ideal  Pavement:  cost  of  construction,  cost  of  maintenance,  tractive 
resistance,  slipperiness,  ease  of  cleaning,  noiselessness,  healthfulness,  freedom 

from  dust  and  mud,  temperature 635 

ART.  2.  THE  SOLUTION  OF  THE  PROBLEM.  Economic  Solution.  Non- 
economic  Solution , 651 


ROADS  AND  PAVEMENTS 


INTRODUCTION 

THE  problems  involved  in  the  construction  and  maintenance 
of  rural  highways  differ  materially  from  those  which  are  encountered 
in  the  improvement  and  care  of  city  streets,  and  therefore  this 
discussion  of  the  subject  of  Roads  and  Pavements  will  be  divided 
into  Part  I,  Country  Roads,  and  Part  II,  City  Pavements.  In 
each  division  of  the  subject  certain  general  principles  will  first  be 
considered,  and  the  further  discussion  will  be  divided  according 
to  the  several  materials  in  use  for  road  surfaces.  It  will  not  always 
be  possible  to  keep  the  several  portions  entirely  distinct,  but  a 
knowledge  of  the  intention  in  this  respect  will  make  it  easier  to 
understand  the  method  of  presentation  or  to  turn  readily  to  the 
discussion  of  any  particular  phase  of  the  subject. 

The  classification  of  road  surfaces  into  country  Roads  and 
City  Pavements  is  partly  according  to  the  most  general  use  of 
each,  and  partly  according  to  the  elaborateness  of  the  construction. 
According  to  the  somewhat  loose  classification  here  adopted  road 
surfaces  for  country  roads  consist  of  two  parts,  subgrade  and 
wearing  coat;  while  city  pavements  consist  of  four  parts,  sub- 
grade,  foundation,  a  binder  or  cushion  course,  and  a  wearing  coat. 

1 


PART  I 

COUNTRY   ROADS 

PART  I  will  include  matters  relating  to  earth,  sand  and  sand- 
clay,  gravel,  water-bound  macadam,  bituminous  macadam,  and 
concrete  roads  in  rural  districts,  although  some  of  the  discussion 
is  also  applicable  to  these  road  surfaces  when  employed  in  city 
streets. 


CHAPTER  I 
ROAD   ECONOMICS  AND   ROAD  ADMINISTRATION 

ART.  1.    ROAD  ECONOMICS 

1.  ADVANTAGES  OF  GOOD  ROADS.  Good  roads  are  so  im- 
portant to  the  financial,  social  and  educational  well-being  of  a  rural 
community  that  no  enumeration  of  their  advantages  is  likely  to 
include  all  the  benefits;  but  a  brief  consideration  of  some  of  the 
chief  advantages  of  good  roads  will  be  of  value  in  determining  the 
amount  of  money  that  may  justifiably  be  expended  to  secure  road 
improvement  and  in  deciding  who  should  in  equity  bear  this  ex- 
pense. The  principal  advantages  of  good  roads,  i.  e.,  of  permanently 
hard  ones,  are  as  follows: 

1.  Good  roads   decrease  the  cost  of  transportation, — at  some 
seasons  of  the  year  considerably,  but  at  others  very  little.     This 
item  will  be  considered  more  fully  later  (see  §  4-9). 

2.  Good  roads  make  the  marketing  of  crops  easier.     This  ad- 
vantage results  in  extending  the  area  devoted  to  the  cultivation 
of  fruits  and  vegetables,  and  is  most    effective   in  the  vicinity  of 
a  large  city. 

3.  Good  roads  give  a  wider  choice  of  time  for  the  marketing  of 
crops.     In  some  instances  good  roads  permit  the  marketing  of  crops 
when  the  labor  of  production  is  not  pressing;    but  this  advantage 

3 


4  ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION          JCHAP.  I 

accrues  only  to  the  producers  of  imperishable  crops,  and  is  not 
of  great  importance  since  the  labor  required  to  market  the  product 
is  small  in  comparison  with  that  of  production. 

4.  Good  roads  permit  the  marketing  to  be  done  when  prices 
are  most  favorable.     This  advantage  is  more  important  with  per- 
ishable than  with  imperishable  products.     As  far  as  perishable  prod- 
ucts are  concerned,  this  advantage  is  virtually  included    in  par- 
agraph 2  above.     As  far  as  imperishable  products  are  concerned, 
this  advantage  is  important  only  near  a  large  city,  i.  e.,  where  the 
producer  hauls  to  the  market.     Prices  of  staple  farm  products  (not 
garden  products)  are  not  much  affected  by  roads,  since  the  con- 
dition of  the  roads  is  local  while  prices  are  governed  by  world-wide 
conditions.     Writers  on  good-road  economics  usually  greatly  over- 
estimate this  advantage  as  far  as  the  ordinary  producer  of  imper- 
ishable products  is  concerned.     If  this  advantage  were  anything 
like  as  great  as  is  frequently  claimed,  producers  would  store  such 
products  at  the  local  shipping  point,  or  in  the  great  city,  or  at  the 
port  of  export,  awaiting  a  favorable  price.     Such  storage  would 
also  permit   the  delivery  at  a  time  when   other  work  was    least 
pressing.     The  expense  of  storage  at  the  local  shipping  point  is  a 
small  per  cent  of  the  value  of  the  product.     It  is  frequently,  but 
erroneously,  claimed  that  hard  roads  would  save  the  Illinois  farmer 
3  to  5  cents  per  bushel — an  amount  10  to  15  times  the  cost  of 
storage.     Since  producers  do  not  so  store  their  products,  it  is  safe 
to  assume  that  this  advantage  of  good  roads  as  a  rule,  is  not  very 
great.     The  present  method  of  doing  business  makes  this  advantage 
comparatively  unimportant. 

5.  Good  roads  give  a  wider  choice  of  the  market  place.     This 
advantage  affects  perishable  products  chiefly,  and  for  geographical 
reasons  is,  as  a  rule,  not  very  great. 

6.  Good  roads  tend  to  equalize  the  produce  market  between 
different  weather  conditions.     In  the  absence  of  railroad  transpor- 
tation and  cold  storage,  this  advantage  might  be  of  considerable 
local  importance;  but  under  ordinary  conditions  it  is  comparatively 
unimportant. 

7.  Good   roads  tend  to   equalize   railroad   traffic   between   the 
different  seasons  of  the  year.     Impassable  wagon  roads  over  any 
considerable  area  materially  decrease  the  amount  of  agricultural 
products  to  be  transported  by  railroads,  and  a  return  of  good  roads 
will  for  a  time  congest  the  railroad  transportation  facilities.     The 
effect  of  good  roads  in  equalizing  railroad  transportation  is  partially 


ART.  1]  ROAD  ECONOMICS 


neutralized  by  the  fact  that  agricultural  products  are  only  one  of 
many  classes  of  commodities  transported  by  the  railroads;  and 
also  by  the  fact  that  most  railroads  transport  agricultural  products 
originating  over  a  considerable  area,  and  bad  wagon  roads  are  not 
likely  to  occur  over  all  the  contributory  area  at  the  same  time; 
and  further  by  the  fact  that  the  storage  capacity  of  warehouses 
helps  to  equalize  the  traffic. 

8.  Good  roads  tend  to   equalize  mercantile   business  between 
different  seasons   of  the  year.     Merchants   having  a  considerable 
rural   custom   could   do  business  more  economically  if  the  trade 
were   distributed   uniformly   throughout  the   yea*.     However,   the 
succession  of  good  and  bad  wagon  roads  is  only  one  cause  of  the 
unequal  distribution  of  rural  patronage. 

9.  Good  roads  permit  more  easy  intercourse  between  the  mem- 
bers of  rural  communities,  and  also  between  rural  and  urban  pop- 
ulations.    This  is  an  important  benefit,  particularly  under  a  demo- 
cratic form  of  government. 

10.  Good  roads  facilitate  the  consolidation  of  rural  schools,  and 
thereby  increase  their  economy  and  efficiency.     This  is  an  impor- 
tant matter  to  coming  generations. 

11.  Good  roads  facilitate  rural  mail  delivery,  and  thereby  tend  to 
improve  the  social  and  intellectual  condition  of  the  rural  population. 

12.  Good  roads  sometimes  change  rural  into  suburban  property. 

13.  Good   roads   are   a   material   factor   in   stimulating  tourist 
travel  and  making  rural  communities  attractive  to  vacation  residents. 

2.  It  is  customary  to  include  the  increase  in  the  price  of  farming 
land  as  one  of  the  benefits  of  good  roads;   but  the  increase  in  price 
of  land  is  simply  the  measure  of  the  value  of  all  the  above  advan- 
tages, and  hence  should  not  be  included. 

3.  Notice  that  the  first  eight  advantages  mentioned  above  relate 
to  the  financial  benefits  of  good  roads,  and  the  last  four  to  the 
social  benefits.     In  the  past  writers  upon  good-road  economics  have 
given  much  attention  to  the  supposed  financial  benefit  of  hard  roads 
and  little  or  none  to  the  social  advantage.     Any  considerable  ex- 
penditure for  the  improvement  of  rural  highways  can  not  be  justi- 
fied on  financial  grounds  alone  (§  12).     Good  roads  are  desirable 
for  the  same  reason  that  a  man  buys  an  automobile  or  builds  a 
fine  house,  i.  e.,  because  they  are  a  comfort  and  a  pleasure.     Good 
roads  should  be  urged  principally  for  the  same  reason  that  good 
schools  are  maintained,  namely,  because  they  increase  the  intel- 
ligence and  value  of  the  citizen  to  society. 


6  ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION          [CHAP.  I 

4.  COST  OF  WAGON  TRANSPORTATION.    The  chief  financial 

advantage  of  hard  roads  is  the  decreased  cost  of  transportation. 
It  is  proposed  to  inquire  briefly  concerning  the  cost  of  wagon  trans- 
portation with  a  view  of  determining  the  proportion  of  this  cost 
that  may  be  saved  by  road  improvement. 

In  this  connection,  a  distinction  must  be  made  between  the 
cost  to  those  whose  chief  business  is  to  sell  transportation,  and 
the  cost  to  those  to  whom  transportation  is  a  mere  incident  of  a 
business  organized  for  some  other  purpose.  The  first  class  is  rep- 
resented by  a  freighter,  and  the  second  by  a  farmer.  The  former 
maintains  his  teams  and  wagons  only  to  transport  freight,  while 
the  latter  ordinarily  keeps  his  teams  and  wagons  primarily  for 
general  farm  work  of  which  transportation  on  the  roads  is  only 
a  small  part.  In  some  cases  the  traffic  to  be  considered  is  prin- 
cipally that  by  freighters,  but  usually  the  chief  traffic  over  country 
roads  is  that  connected  with  agricultural  operations. 

Again,  consideration  should  be  given  only  to  hauling  in  which 
the  load  is  equal  to  the  full  capacity  of  the  team  for  the  particular 
condition  of  the  roads.  A  farmer  may  employ  a  two-horse  team 
to  take  a  bushel  of  potatoes  to  town,  or  a  grocery  wagon  may  make 
a  trip  to  deliver  a  pound  of  cheese;  but  the  partial  load  is  entirely 
independent  of  the  condition  of  the  roads. 

Further,  it  is  necessary  to  notice  that  only  the  rate  for  full  loads 
should  be  considered.  If  a  number  of  packages  are  carried  in  the 
same  load  for  different  parties,  part  of  the  charge  is  to  cover  the 
cost  of  collection,  distribution,  possible  partial  loads,  etc.;  and 
therefore  only  part  of  the  charge  is  for  transportation  proper. 

5.  Cost  to    Freighters.    The    cost  will  vary  greatly  with  the 
conditions  of  the  service,  i.  e.,  with  the  character  of  the  road  surface, 
the  average  grade  of  the  road,  the  maximum  grade,  return  load,  etc. 

Except  in  rare  cases,  the  cost  per  ton-mile  for  loads  one  way 
upon  earth  roads  will  not  be  more  than  25  cents,  and  ordinarily  it 
will  not  be  more  than  15  to  20  cents;*  while  with  easy  grades  and 
favorable  road  surface  it  may  be  as  low  as  10  to  15  cents,  and  with 
long  hauls,  return  loads,  and  favorable  road  surface,  it  may  be  8 
to  10  cents.  When  the  last  price  is  obtained  there  is  little  need  or 
opportunity  for  road  improvement. 

6.  Cost  to  Farmers.     In  this  division  of  the  subject,  a  distinc- 
tion should  be  made  between  producers  of  perishable  products  and 

*  See  "Cost  of  Wagon  Transportation,"  by  the  author,  in  Proceedings  of  Illinois  Society 
of  Engineers,  Vol.  16  (1901),  p.  3&-44;  full  abstract  of  the  same  in  Engineering  News,  Vol. 
45  (1901),  p.  86. 


ART.  1]  ROAD  ECONOMICS 


producers  of  non-perishable  products.  The  first  class  is  represented 
by  gardeners,  dairymen,  fruit-growers,  etc.;  and  the  second,  by 
producers  of  hay,  grain,  cotton,  etc. 

The  cost  of  transportation  is  much  greater  for  perishable  than 
for  non-perishable  products.  In  the  first  place,  the  marketing  of 
perishable  products  is  an  important  factor  in  comparison  with 
the  cost  of  production,  and  frequently  necessitates  an  independent 
transportation  department;  while  the  labor  of  marketing  non- 
perishable  products  is  comparatively  small — particularly  as  in  most 
localities  where  there  is  much  of  this  class  of  produce,  the  distance 
from  the  farm  to  the  railroad  station  is  short.  Further,  perishable 
products  must  go  to  market  whatever  the  condition  of  the  roads, 
while  non-perishable  ones  can  wait  for  comparatively  favorable 
conditions;  and  finally,  the  former  frequently  go  to  market  in 
partial  loads,  and  the  second  usually  in  full  loads.  Except  in  com- 
paratively limited  districts,  non-perishable  products  make  up  the 
bulk  of  the  traffic  on  the  country  roads.  According  to  the  U.  S. 
Census  of  1890,  the  gardeners,  fruit-growers,  dairymen,  vine-growers, 
florists,  and  nurserymen  constitute  only  1.8  per  cent  of  the  so-called 
farming  class. 

7.  The  cost  of  transporting  perishable  products  is  probably 
greater  than  that  for  any  other  class  of  traffic  over  the  country 
roads;  but  as  it  is  next  to  impossible  to  secure  any  reliable  data 
no  attempt  will  be  made  to  present  any  general  conclusions.  For 
several  reasons,  this  traffic  will  usually  justify  larger  expense  for 
road  improvement  than  any  other. 

The  cost  of  transportation  to  farmers  proper,  i.  e.,  producers  of 
non-perishable  products,  depends  chiefly  upon  the  condition  of  the 
road  surface  and  upon  the  demands  of  general  farm  work.  Loam 
or  clay  roads  are  reasonably  good  when  dry,  and  are  therefore  at 
least  passable  most  of  the  year;  while  sand  roads  are  at  their  worst 
when  dry,  and  are  therefore  in  their  worst  condition  during  the 
greater  part  of  the  year.  Fortunately  sand  roads  are  less  common, 
the  country  over,  than  clay  or  loam  roads.  In  the  crop  season, 
with  a  little  choice  as  to  the  time  of  doing  the  work  the  cost  on 
fairly  level  loam  or  clay  roads  is  probably  not  more  than  10  to 
12  cents  per  ton-mile;  and  when  farm  work  is  not  pressing,  the  cost 
is  not  more  than  8  to  10  cents  per  ton-mile.* 

*  See  "Cost  of  Wagon  Transportation,"  by  the  author,  in  Proceedings  of  Illinois  Society 
of  Engineers,  Vol.  16  (1901),  p.  36-44;  full  abstract  of  the  same  in  Engineering  News,  Vol. 
45  (1901),  p.  86. 


8 


ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION          [CHAP.  I 


8.  A  Conflicting  View.  In  current  literature  on  road  economics, 
the  claim  is  frequently  made  that  the  cost  of  wagon  transportation 
to  the  farmer  is  considerably  more  than  stated  in  §  7.  Apparently 
most  of  these  claims  are  based,  either  directly  or  indirectly,  upon 
data  published  in  Circular  19  of  the  Road  Inquiry  Office  of  the 
United  States  Department  of  Agriculture  under  date  of  April  4, 
1896.  Table  1  is  a  brief  summary  from  that  circular. 

The  conclusions  of  this  circular  have  been  so  widely  quoted  and 
so  generally  accepted  as  to  justify  a  brief  consideration.  In  former 
editions  of  this  book  these  conclusions  were  somewhat  carefully 
considered,  and  the  following  is  a  brief  summary  of  that  investigation. 


TABLE  1 
AII/EGED  COST  OP  WAGON  TRANSPORTATION 


Ref. 

No. 

Locality. 

Average 
Distance 
Hauled, 
Miles. 

Average 
Load 
Hauled, 
Tons. 

Average 
Cost  per 
Ton-mile, 
Cents. 

Total  Cost 
from  Farm 
to  Market, 
per  Ton. 

I 

Eastern  States. 

5  9 

1.108 

32 

$1    89 

9 

Northern  States        

6  9 

27 

1  86 

3 

Middle-Southern  States  

8.8 

31 

2  72 

4 

Cotton  States  

12.6 

0.688 

25 

3  05 

5 

Prairie  States  

8.8 

1.204 

22 

1  94 

6 

Pacific  Coast  and  Mountain.  .  .  . 

23.3 

1.098 

22 

5.12 

7 

Whole  United  States  

12  1 

1  001 

25 

3  02 

Circular  19  concludes  that  313,349,227  tons  were  hauled  over 
the  highways  of  the  United  States  in  1895  at  a  cost  of  $3.02  per 
ton-mile,  or  a  total  cost  of  $946,314,665.54;  and  that  the  annual 
cost  of  transporting  the  crops  of  the  United  States  to  market  was 
26.6  per  cent  of  the  price  of  the  crops  at  the  market.  These  con- 
clusions are  greatly  in  error  chiefly  for  the  following  reasons: 

1.  The  investigation  was  not  very  elaborate,  since  replies  were 
received  from  only  one  county  in  thirty. 

2.  The  average   distance  hauled  seems  to  be  about  twice  too 
great. 

3.  The    value   for   the   average   load  hauled    is   approximately 
twice  too  great.     The  mean  between  the  maximum  and  the  mini- 
mum load  may  be  one  ton;   but  the  great  bulk  of  teaming  is  done 
when  the  roads  are  at  least  in  fair  condition,  when  the  load  is  con- 


ART.  1]  ROAD  ECONOMICS  9 

siderably  more  than  one  ton.  The  author  has  examined  the  records 
of  several  grain  buyers  in  central  Illinois,  where  at  times  the  roads 
are  as  bad  as  anywhere,  and  finds  that  the  average  load  is  nearly 
a  ton  and  a  half.  Statistics  for  marketing  over  300,000  bushels 
of  corn  in  Illinois  gives  the  average  load  as  almost  exactly  2 
tons. 

4.  The  cost  per  ton-mile  indicates  that  this  value  was  obtained 
by  assuming  that  the  wages  of  wagon,  team,  and  driver  are  35  cents 
per  hour;    that  the  team  travels  3  miles  per  hour;    and  that  the 
team  hauls  a  load  only  one  way.     The  price  per  hour  for  a  team 
is  too  great,  since  the  cost  per  day  as  reported  by  316  farmers  in 
76  counties  in  Illinois  varied  during  crop  time  from  $1.40  to  $2.74, 
the  average  being  only  $2.13,  or  say  21  cents  per  hour. 

5.  No  account  was  taken  of  the  relative  amounts  of  traffic  in 
the  several  states. 

9.  Under  date  of  Feb.  28,  1907,  the  Bureau  of  Statistics  of  the 
U.  S.  Department  of  Agriculture  published  Bulletin  49 — Cost  of 
Hauling  Crops  from  Farm  to  Shipping  Point.  In  the  latter  investi*. 
gation  the  cost  of  hauling  the  twelve  leading  farm  products  from 
the  farm  to  the  shipping  point  during  the  crop  year  of  1905-06 
is  said  to  be  $84,684,000;  whereas  in  the  first  investigation  (§8), 
the  cost  in  1895  of  hauling  all  crops  from  the  farm  to  the  market 
was  said  to  be  $652,000,000.  Notice  that  the  cost  according  to 
the  later  and  more  elaborate  investigation  is  only  about  one  eighth 
of  that  by  the  former  investigation,  notwithstanding  the  fact  that 
the  weight  of  the  seven  leading  crops  was  almost  exactly  50  per 
cent  greater  in  1905  than  in  1895;  in  other  words,  on  the  face  of 
the  returns,  the  result  in  the  first  bulletin  is  about  sixteen  times 
too  great. 

Further,  the  result  of  the  second  investigation  is  subject  to 
errors  2,  3,  and  4  of  §  8.  Besides,  the  second  bulletin  is  greatly 
in  error  for  the  following  reasons:  It  finds  the  cost  of  hauling  crops 
from  the  farm  to  the  market  to  be  $72,984,000;  and  then  adds 
$11,700,000  as  the  cost  of  hauling  wheat  to  local  mills  to  be  ground. 
This  allowance  is  altogether  too  great,  since  it  assumes  that  more 
than  one  third  of  the  wheat  not  used  for  seed  is  ground  at  the  local 
mill;  while  only  an  inappreciable  quantity  is  so  used. 

Correcting  the  above  errors  would  reduce  the  total  of  the  second 
bulletin  to  one  half  or  one  third,  and  make  the  result  in  the  first 
bulletin  thirty  to  fifty  times  too  great.  Unfortunately  the  results 
of  the  first  investigation  are  frequently  used  in  discussions  on  road 


10  ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION          [CHAP.  I 

economics,  and  the  object  of  this  note  is  to  show  their  utter  unre- 
liability.* 

10.  Possible  Annual  Saving.    The  Office  of  Road  Inquiry,  in 
Circular  No.  19,  to  which  reference  has  been  made,  estimates  the 
possible    annual    saving   by   road    improvement    as    $628,000,000. 
This  estimate  is  based  upon  a  comparison  of  the  data  in  Circular 
No.  19  with  that  on  the  "Cost  of  Hauling  Farm  Products  to  Market 
or  Shipping  Point  in  European  Countries,  Collected  by  U.  S.  Con- 
sular Agents,"  published  in  Circular  No.  27  (Feb.  5,  1897)  of  the 
Office  of  Road  Inquiry  of  the  U.  S.  Department  of  Agriculture. 
The  average  cost  as  given  in  the  latter  circular  is  10  cents  per  ton- 
mile,  and  the  difference  between  this    and  the    average  stated  in 
Table  1  is  15  cents  per  ton-mile,  which  is  two  thirds  of  the  average 
value  in  Table  1. 

Concerning  the  data  for  America,  notice  that  they  are  taken 
from  Circular  No.  19,  and  are  greatly  in  error  as  has  already  been 
shown.  Concerning  the  data  for  Europe,  notice  in  the  first  place 
that  they  are  open  to  most  of  the  criticisms  made  against  the  data 
in  Table  1.  In  the  second  place,  the  twelve  results  given  in  Cir- 
cular 27  vary  from  4  to  30  cents  per  ton-mile,  which  is  too  wide  a 
range  and  too  few  results  for  an  accurate  determination  of  the 
average  cost  of  wagon  transportation  in  Europe.  In  the  third 
place,  some  of  the  results  are  professedly  the  cost  to  transportation 
companies,  and  some  the  cost  to  farmers  to  whom  the  hauling  of 
the  crops  to  market  is  merely  an  incident  of  farm  work.  And, 
finally,  the  data  for  the  cost  of  hauling  not  done  by  transportation 
companies  are  for  hauling  garden  products,  etc.,  to  large  cities, 
and  are  therefore  not  representative  of  the  cost  of  transporting 
general  farm  products  to  market. 

11.  It  is  very  unfortunate  that  the  conclusions  from  the  two 
Circulars  referred  to  above,  have  been  so  generally  accepted  by 
speakers  and  writers  upon  good-road  economics.     Country  roads 
are  used  chiefly  by  farmers,  and  if  improvements  are  made  they 
must  be  paid  for  largely,  if  not  entirely,  by  farmers;   and  therefore 
the  cooperation  of  farmers  must  be  secured  before  any  improve- 
ment of  the  country  roads  is  possible.     Farmers  know  that  con- 
clusions such  as  are  deduced  above  from  Table  1  are  ridiculous; 
and  not  unnaturally  distrust  the  motives  prompting  the  argument, 

*  For  a  further  discussion  of  the  Circular  see  the  following:  In  defense  of  the  Circular, 
Engineering  News,  Vol.  34,  p.  410-11;  do.,  Vol.  45,  p.  50-51.  Controverting  the  Circular: 
Engineering  News,  Vol.  34,  p.  377-78;  do.,  Vol.  34,  p.  410-11;  do.,  Vol.  44,  p.  337-44;  do., 
Vol  5,  p.  48-49;  do.,  Vol.  57,  p.  428. 


ART.  1]  ROAD  ECONOMICS  11 

and  are  hostile  to  all  propositions  for  road  improvement  supported 
by  such  arguments. 

Further,  it  is  not  possible  to  determine  either  the  cost  of  wagon 
transportation  or  the  financial  value  of  road  improvement  in  the 
wholesale  manner  proposed  in  the  above  Circulars.  The  cost  of 
haul  and  the  value  of  unproved  roads  vary  greatly  with  local  con- 
ditions; and  consequently  a  special  investigation  should  be  made 
for  each  particular  case. 

However,  it  should  be  borne  in  mind  in  discussing  road  eco- 
nomics that  financial  profit  is  only  one  of  the  advantages  of  good 
roads  (see  §  1-3). 

12.  FINANCIAL  VALUE  OF  ROAD  IMPROVEMENT.    It  is  not 

possible  to  present  any  valuable  general  conclusions  as  to  the  saving 
in  cost  of  transportation  attainable  by  any  proposed  road  improve- 
ment. 

For  any  particular  road  where  the  traffic  is  principally  by 
"freighters"  as  defined  in  §5,  it  is  possible  to  arrive  at  a  rough 
approximation  by  (1)  taking  a  census  of  the  traffic,  (2)  making 
an  estimate  of  the  present  cost  per  ton-mile,  and  (3)  making  an 
estimate  of  the  cost  after  the  improvement.  The  amount  of  traffic 
varies  with  the  condition  of  the  road  surface,  and  the  chief  difficulty 
is  to  determine  the  advantage  of  being  able  to  move  freight  at  any 
time.  This  advantage  will  depend  upon  the  proportion  of  the  time 
that  the  roads  are  "good,"  which  depends  entirely  upon  the  local- 
ity and  the  nature  of  the  road  surface,  and  varies  greatly  from 
year  to  year.  Ordinarily  the  road  is  used  by  a  variety  of  team- 
sters, and  the  cost  varies  with  the  particular  circumstances  of  each. 
There  will  rarely  be  conditions  to  which  this  method  of  investiga- 
tion can  be  applied  with  any  degree  of  certainty.  At  best  the 
results  of  such  an  investigation  must  be  regarded  as  mere  approx- 
imations, since  no  factor  of  the  problem  can  be  determined  accu- 
rately, and  since  any  slight  error  in  the  estimated  saving  per 
ton-mile  is  greatly  magnified  when  multiplied  by  the  number  of  ton- 
miles.  Nevertheless  such  an  investigation  is  desirable  to  aid  the 
judgment,  but  its  approximate  nature  must  not  be  forgotten. 

For  roads  where  the  travel  is  by  farmers  the  difficulties  are  still 
greater.  The  number  of  users  is  greater,  the  cost  of  transportation 
to  the  different  users  varies  very  greatly,  and  the  value  of  being 
able  to  use  the  road  at  any  time  is  very  different  with  different 
users,  and  for  the  same  class  of  users  varies  with  the  locality  and 
the  nature  of  the  road. 


12  ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION  [CHAP.  I 

The  amount  of  money  that  may  justifiably  be  expended  for  any 
proposed  road  improvement  will  depend  upon  the  present  condition 
of  the  road,  the  amount  and  the  nature  of  the  travel,  and  the  cost 
of  constructing  and  maintaining  the  improved  road.  The  ques- 
tion is  a  local  one,  and  can  be  answered  approximately  correctly 
only  after  careful  study  of  the  conditions.  Ordinarily  the  saving 
in  transportation,  except  near  large  cities,  will  not  justify  any 
radical  road  improvement;  but  with  a  miscellaneous  travel,  the 
social  advantages  of  road  improvement  should  be  taken  into  con- 
sideration, even  though  they  can  not  be  computed  in  dollars  and 
cents. 

13.  TRACTIVE  RESISTANCE.    The  solution  of  some  problems  con- 
nected with  road  improvement  requires  a  knowledge  of  the  tractive 
resistance.     Until  recently  all  vehicles  were  horse-drawn;    but  now 
many  are  propelled  by  motors.     The  passenger  automobile  with 
its  wide  range  of  speed  and  power  is  able  to  surmount  almost  any 
grade  likely  to  be  encountered  upon  a  road  used  also  by  horse- 
drawn  vehicles;    and  automobile  trucks  are  not  common,  at  least 
yet,  upon  rural  roads,  and  besides  the  factors  governing  the  tractive 
power  of  such  vehicles  are  not  yet  well  established.     Hence  the 
road  problems  involving  tractive  resistance    relate    almost  exclu- 
sively to  horse-drawn  vehicles;    and  consequently  only  this  class 
will  be  considered  under  this  head. 

The  resistance  to  traction  of  a  vehicle  on  a  road  consists  of  three 
independent  elements:  axle  friction,  rolling  resistance,  and  grade 
resistance. 

14.  Axle  Friction.    The  resistance  of  the  hub  to  turning  on 
the  axle  is  the  same  as  that  of  a  journal  revolving  in  its  bearing, 
and  has  nothing  to  do  with  the  condition  of  the  road  surface.     The 
coefficient  of  journal  friction  varies  with  the  material  of  the  journal 
and  its  bearing,  and  with  the  lubricant.     It  is  nearly  independent 
of  the  velocity,  and  according  to  observations  made  by  the  author 
seems  to  vary  about  inversely  as  the  square  root   of  the  pressure. 
For  light  carriages  when  loaded,  the  coefficient  of  friction  is  about 
0.020  of  the  weight  of  the  axle;   for  heavier  carriages  when  loaded, 
it  is  about  0.015;    and  for  the  ordinary  thimble-skein  American 
wagon  when  loaded,  it  is  about  0.012.     The  above  results  are  for 
good  lubrication;    if  the  lubrication  is  deficient,  the  axle  friction 
is  two  to  six  times  as  much  as  above.     The  above  figures  agree 
reasonably  well  with  results  obtained  for  journal  friction  of  ma- 
chines.    Apparently  the  value  of  this  coefficient  in  Morin's  experi- 


ART.  1] 


ROAD  ECONOMICS 


13 


ments  (§  20)  was  0.065.*  The  greater  axle  friction  is  probably 
due  to  the  inferior  mechanical  construction  of  French  carriages 
and  wagons  of  that  date. 

The  tractive  power  required  to  overcome  the  above  axle  friction 
for  American  carriages  of  the  usual  proportions  is  about  3  to  3J  Ib. 
per  ton  of  the  weight  on  the  axle;  and  for  truck  wagons,  which 
have  medium-sized  wheels  and  axles,  is  about  3J  to  4J  Ib.  per  ton. 

15.  Rolling  Resistance.     The  resistance  of  a  wheel  to  rolling 
along  on  a  road  is  due  to  the  yielding  or  indentation  of  the  road, 
which  causes  the  wheel  to  be  continually  climbing  an  inclination. 
The  resistance  is  measured  by  the  horizontal  force  necessary  at  the 
axle  to  lift  the  wheel  over  the  obstacle  or  to  roll  it  up  the  inclined 
surface;    and  varies  with  the  diameter  of  the  wheel,  the  width  of 
the  tire,  the  speed,  the  presence  or  absence  of  springs  on  the  vehicle, 
and  the  nature  of  the  road  surface. 

16.  Diameter  of  Wheel.     The  rolling  resistance  varies  inversely 
#s  some  function  of  the  diameter  of  the  wheel,  since  the  larger  the 
wheel  the  less  the  force  required  to  lift  it  over  the  obstruction  or 
to  roll  it  up  the  inclination  due  to  the  indentation  of  the  surface. 
Table  2  shows  the  results  obtained  by  Mr.  T.  I.  Mairs  at  the  Mis- 
souri Agricultural  Experiment  Station,*  with  three  different-sized 


TABLE  2 

EFFECT  OF  SIZE  OF  WHEELS  ON  TRACTIVE  RESISTANCE 
Resistances  in  Pounds  per  Ton 


Ref. 

No. 

Description  of  Road  Surface. 

MEAN  DIAMETER  OF   FRONT 
AND  REAR  WHEELS. 

50" 

38" 

26" 

1 

2 
3 

4 
5 
6 

7 
8 
9 

10 

Macadam:  slightly  worn,  clean,  fair  condition.  .  .  . 
Gravel  road:  dry,  sand  1"  deep,  some  loose  stones  . 
Gravel  road:  up  grade  2.2%,  \"  wet  sand,  frozen 
below  

57 

84 

123 

69 
101 
132 
173 

178 
252 

61 
90 

132 
75 
119 
145 
203 
201 
303 

70 
110 

173 

79 
139 
179 
281 
265 
374 

Earth  road  :  dry  and  hard  

"         "      £"  sticky  mud,  frozen  below,  rough.  . 
Timothy  and  blue-grass  sod'  dry  grass  cut 

"wet  and  spongy  
Corn-field:  flat  culture,  across  rows,  dry  on  top.  .  . 
Plowed  ground  :  not  harrowed,  dry  and  cloddy  .  .  . 

Average  value  of  the  tractive  power  

130 

148 

186 

*  Lowe's  Strassebaukunde,  page  75.     Wiesbaden,  1895. 

t  Missouri  Agricultural  Experiment  Station,  Bulletin  No.  52,  Columbia,  1902. 


14  ROAD  ECONOMICS  AND  ROAD  ADMINISTRATION          [CHAP.  I 

wheels.  The  50-inch  represents  44-inch  front  and  56-inch  hind 
wheels;  the  38-inch  represents  36-inch  front  and  40-inch  hind  wheels; 
and  the  26-inch  represents  24-inch  front  and  28-inch  hind  wheels. 
The  tires  were  6  inches  wide.  The  load  was  practically  If  tons 
in  each  case. 

Morin  concluded  that  the  resistance  varies  inversely  as  the  first 
power  of  the  diameter  of  the  wheel;  Dupuit  that  it  varies  as  the 
square  root;  and  Clarke  claims  that  it  varies  as  the  cube  root.* 
According  to  some  experiments  made  in  England  in  1874,f  the 
tractive  resistance  varied  more  rapidly  than  the  first  power  of  the 
diameter  of  the  wheels.  The  mean  results  in  Table  3  vary  nearly 
inversely  as  the  square  root  of  the  mean  diameter — certainly  more 
nearly  than  as  either  the  first  power  or  the  cube  root.  For  obvious 
reasons,  the  experiments  can  not  be  very  exact;  and  apparently 
the  tractive  resistance  varies  differently  for  different  surfaces.  The 
exact  determination  of  the  law  of  variation  is  of  no  great  importance. 

17.  Width  of  Tire.  If  the  wheel  cuts  into  the  road  surface,  the 
tractive  resistance  is  thereby  increased;  but  with  surfaces  for  which 
there  is  little  or  no  indentation,  the  traction  is  practically  inde- 
pendent of  the  width  of  tire. 

Table  3,  page  15,  shows  the  results  of  an  elaborate  series  of 
experiments  by  the  Missouri  Agricultural  Experiment  Station,  t 
The  load  in  each  case  was  1  ton.  These  results  show  that  on  poor 
macadam,  poor  gravel,  and  compressible  earth  roads,  and  also  on 
agricultural  land,  the  broad  tire  gives  less  resistance  except  as 
follows:  (1)  when  the  earth  road  is  sloppy,  muddy,  or  sticky  on  top 
and  firm  underneath;  (2)  when  the  surface  is  covered  with  a  very 
loose  deep  dust  and  is  hard  underneath;  (3)  when  the  mud  is  very 
deep  and  so  sticky  that  it  adheres  to  the  wheel;  or  (4)  when  the 
road  has  been  rutted  with  the  narrow  tire.  The  last  conclusion 
was  established  by  a  large  number  of  experiments  not  included  in 
Table  3. 

Table  4,  page  16,  gives  data  on  the  effect  of  width  of  tire  upon 
the  tractive  power,  obtained  by  the  Studebaker  Bros.  Manufactur- 
ing Co.,  South  Bend,  Ind.,  in  1892,  with  an  ordinary  3J-inch  thimble- 
skein  wagon.  Notice  that  on  a  hard  and  incompressible  road  sur- 
face, e.  g.,  wood  block  pavement  and  gravel,  the  narrower  tire  draws 

*  Clarke's  Construction  of  Roads  and  Streets,  p.  294.     London,  1890. 

t  Clarke's  Manual  of  Rules,  Tables  and  Data  for  Mechanical  Engineers,  p.  962.      London, 
1877. 

t  Missouri  Agricultural  Experiment  Station,  Bulletin  No.  39,  Columbia,  Mo.,  July,  1897. 


ART.   1] 


ROAD  ECONOMICS 


15 


TABLE  3 

TRACTIVE  RESISTANCE  OF~  BROADBAND  NARROW  TIRES  * 
Resistances  in  Pounds  per  Ton 


Ref 

WIDTH  ( 

)F  TIRE. 

Nr»    of 

No. 

Description  of  the  Road  Surface. 

li". 

6'V 

Trials. 

1 

Broken  Stone  Road: 
Hard,  smooth,  no  dust,  no  loose  stones,  nearly 
level                                          

121 

98 

2 

2 

Gravel  Road: 
Hard  and  smooth,  few  loose  stones  size  of  black 
walnuts.                   .  *  

182- 

134 

2 

3 

4 

Hard,  no  ruts,  large  quantity  of  sand  which 
prevented  packing  
New,  gravel  not  compact,  dry  

239 
330 

157 

260 

1  ' 
1 

5 

Wet  loose  sand  1"  to  1\"  deep 

246 

254  * 

2 

6 
7 
8 
9 
10 
11 

Earth  Roads: 
Loam,  —  dry,  loose  dust  2"  to  3"  deep  
.  '  '         "      hard,  no  dust,  no  ruts,  nearly  level. 
stiff  mud,  drying  on  top,  spongy  below  . 
1  '        mud  1\"  deep,  very  sticky,  firm  below. 
Clay,  —  sloppy  mu,d  3"  to  4"  deep,  hard  below  . 
dry  on  top  but  spongy  below,  narrow 
tires  cut  in  6"  to  8" 

90 
149 

497 
251 

286 

472 

106" 
109 
307 
325? 
406*^ 

422 

2 
3 

1 
1 
1 

2 

12 

13 

dry  on  top  but  spongy  below     
stiff  deep  mud 

618 
825 

464 
551 

5 
1 

14 

Mowing  Land: 
Timothy  sod,  —  dry,  firm,  smooth,  narrow  tire 
cuts  in  1" 

317 

229 

\ 

15 
16 

moist,  narrow  tire  cuts  in  3^".  . 
soft    and    spongy,    grass    and 
stubble  3"  high,  narrow  tire 
cuts  in  6"  

421- 
569 

305 
327 

1 
1 

17 
18 
19 

20 
21 

Pasture  Land: 
Blue-grass  sod,  —  dry,  firm,  smooth  
soft,  narrow  tire  cuts  in  3".  .  . 
narrow  tire  cuts  in  4"  

Stubble  Land: 
Corn  stubble,  —  no  weeds,  nearly  dry  enough 
to  plow  
some   weeds   and   stalks,    dry 
enough  to  plow  

218 
420 

578 

631 
423 

156 
273 
436 

418 
362 

2 
2 
1 

2 
1 

?,?, 

in  autumn,  dry  and  firm  

404 

256 

2 

23 
24 

Plowed  Land: 
Freshly  plowed,  not  harrowed,  surface  rough  .  . 
harrowed,  smooth,  compact.  .  . 

510 

466 

i*  /•> 

283 
323 
M 

1 
1 

*  Missouri  Agricultural  Experiment  Station,  Bulletin  No.  39,  Columbia,  Mo.,  July,  1897. 


16 


ROAD   ECONOMICS   AND   ROAD   ADMINISTRATION         [CHAP.  1 


TABLE  4 

EFFECT  OF  WIDTH  OF  TIRE  UPON  TRACTIVE  POWER  * 
Resistances  in  Pounds  per  Ton 


Ref. 

No. 

Description  of  the 
Road  Surface. 

DIAMETERS  OF  THE  FRONT  AND  REAR  WHEELS  RESPECTIVELY. 

3'  6"  and 
3'  10". 

3'  6"  and 
3'  10". 

3'  8"  and 
4'  6". 

3'  6"  and 
3'  10". 

3'  8"  and 
4'  6". 

WIDTH  OF  THE  TIRE. 

H" 

4" 

H" 

4" 

U" 

4" 

H" 

3" 

If" 

3" 

228 
114 
228 

1 
2 
3 
4 
5 
•  6 
7 

Sod 

283 
152 

239 
152 

189 
114 
265 

Earth  Road,  hard.  .  .  . 
"       "       muddy.. 
Sand  Road,  hard  
"       "      deep..  .. 
Gravel  Road  good 

199 
371 

108 
243 
162 
351 

268 

171 

304 
164 

236 
141 

254 
168 

98 
61 

117 
70 

83 
35 

80 
46 

.  ;  .  . 

'  54 

66 

28 

76 
38 

Wood  Block,  round  .  . 

51 

49 

the  easier;  while  upon  the  soft  or  spongy  surface  the  wider  tire 
draws  the  easier. 

Morin  experimented  (see  §  20)  with  tires  2|,  4J,  and  6J  inches 
wide;  and  concluded  that  on  a  solid  road  or  pavement  the  resist- 
ance was  independent  of  the  width  of  the  tire,  but  on  a  compressible 
surface  the  resistance  decreased  as  the  width  of  the  tire  increased, 
the  rate  depending  upon  the  nature  of  the  surface. 

For  a  further  discussion  of  the  relative  merits  of  broad  and  narrow 
tires,  see  §  200-202. 

18.  Effect  of  Speed.  The  rolling  resistance  increases  with  the 
velocity,  owing  to  the  effect  of  the  shocks  or  concussions  produced 
by  the  irregularities  of  the  road  surface.  This  increase  is  less  for 
vehicles  having  springs  than  for  those  not  having  them,  and  is  also 
less  for  smooth  road  surfaces  than  for  rough  ones. 

Table  5,  page  17,  is  a  summary  of  Morin's  results  (see  §  20) 
showing  the  effect  of  a  variation  of  speed  for  vehicles  provided 
with  springs.  In  a  rough  way  the  three  speeds  are  2J,  5,  and  7J 
feet  per  second,  or  about  2,  4,  and  6  miles  per  hour  respectively. 
According  to  these  results  the  resistance  on  broken-stone  roads 
increases  roughly  as  the  fourth  root  of  the  speed,  and  on  stone-block 
pavement  about  as  the  square  root.  For  springless  vehicles  the 
increase  would  be  greater.  The  above  is  for  metal  tires;  for  pneu- 


*  Pamphlet  by  Studebaker  Bros,  Manufacturing  Co.,  South  Bend,  Ind.,  1892, 


ART.    1] 


ROAD    ECONOMICS 


17 


TABLE  5 

EFFECT  OF  SPEED  ON  TRACTIVE  POWER  * 
The  figures  give  the  resistance  in  pounds  per  ton 


Ref. 

No. 

Description  of  the  Road  Surface. 

STAGE  COACH. 

CARRIAGE. 

Walk. 

Trot. 

Fast 
Trot. 

Walk. 

Trot. 

Fast 
Trot. 

1 

2 
3 

4 
5 

6 

7 
8 

9 
10 
11 

Broken  Stone  Road: 
Good  condition,  dry  and  compact  . 
Very  firm  large  stones  visible.    .  . 

42 
59 
49 
77 
95 
112 
146 
-164— 

49 
75 
75 
92 
108 
127 
161 
100 

50 
81 
88 

100 
117 
134 
169 

41 
58 
48 
76 
93 
110 
145 
Ifi2 

48 
73 
74 
91 
108 
126 
160 
302 

49 
81 

88 
99 
116 
132 
168 

Little  moist,  or  little  dirty.  ....... 

Firm,  little  soft  mud  

'  '     ruts  and  much  mud  

Portions  worn  out,  thick  mud  
Much  worn,  ruts  3"  deep,  thick  mud 
Vrrv  hnd   rutr  1"  dccn   vcr«  fnmrh 

Stone  Block  Pavement: 
Very  smooth  narrow  joints  

32 
35 
35 

48 
52 
49 

55 
61 
56 

31 
34 

44 

47 
51 
60 

54 
67 
67 

Fair  condition,  dry            

Moist  covered  with  dirt  

matic  tires  there  is  very  little  increase  of  resistance  with  increase 
of  speed,  f 

The 'preceding  data  refer  to  the  effect  of  speed  upon  the  tractive 
power  after  the  load  is  in  motion.  It  requires  from  two  to  six  or 
eight  times  as  much  force  to  start  a  load  as  to  keep  it  in  motion  at 
2  or  3  miles  per  hour.  The  extra  force  required  to  start  a  load  is 
due  in  part  to  the  fact  that  during  the  stop  the  wheel  may  settle 
into  the  road  surface,  in  part  to  the  fact  that  the  axle  friction  at 
starting  is  greater  than  after  motion  has  begun,  and  further  in  part 
to  the  fact  that  energy  is  consumed  in  accelerating  the  load. 

19.  Effect  of  Springs.     Springs  decrease  the  tractive  resistance 
by  decreasing  the  concussions  due  to  irregularities  of  the  road  sur- 
face, and  are  therefore  more  effective  at  high  speeds   than   at  low 
ones,  and  on  rough  roads  than  on  smooth  ones.     Apparently  no 
experiments  have  been  made  upon  the  effect  of  springs;  but  a  few 
data  on  this  subject  may  be  obtained  by  comparing  the  last  and  the 
sixth  columns  of  Table  6,  page  18. 

20.  Results  of  Early  French  Experiments.     Immediately  before 
and  shortly  after  the  introduction  of  railroads,  European  engineers 
made  many  experiments  on  the  force  necessary  to  draw  different 
vehicles  over  various  surfaces.     The  experiments  by  Morin,  *  made  in 

*  Experiences  sur  le  tirage  des  voitures  et  sur  les  effets  destructeurs  qu'elles  exercent  sur 
les  routes,  executees  en  1837  et  1838  par  ordre  du  Ministre  de  la  Guerre,  et  en  1839  et  1841 
par  ordre  du  Ministre  des  Travaux  Publics,  A.  Morin.  Paris,  1842. 

t  Proc.  of  lust,  of  Mecb.  Engrs.  (London),  for  1890,  Part  No.  2,  p.  195. 


J. 


18 


ROAD   ECONOMICS   AND   ROAD   ADMINISTRATION          [CHAP.    I 


TABLE  6 
TRACTIVE  RESISTANCE  OF  DIFFERENT  VEHICLES  ON  VARIOUS  LEVEL  ROADS,  AT  THREE  MILES  AN  HOUR,  IN  POUNDS  PER  TON  | 

|  VEHICLES  WITH  SPRINGS. 

sjlb5  £,$0 

i-l           TH  T-H  <M  (M                                                          i-H  —I 

^        ^  ^ 

ricoojcoio 

1  II  II  II  II 

CO     •        GOCOOO        TH  GO  00  CO  CO  O  »C  00        i-HTfHTtH        00 

£tj^  ii  ii  ii 

'.'.'.'.'. 

£*-•*)*        Oi  CO  «C  CO        TH  HH  1C  !>•  Cl  i—  i  rfi  CO        CO  CO  ^f        rP 



|  VEHICLES  WITHOUT  SPRINGS. 

Freight  Wagon. 
t  =  4.5" 
a  =  2.5" 

^00 

1     ....        cOC^        COOOrH        CO^  ^  COt^O^C^CO        CNC^CO        °< 



ebb 

COCO 

II  II 

^_^^                                       .       .                      .       ^.^^                                ^^.^ 

N.  ^^        O^  Ol  "^  *C        ^^  ^  *C  t>»  O5  CO  ^  ^C        CO  CO  T^        Tt^ 

Itiifcli 

3  «  II  II  II  II 

C^  O        OO        CD  1C        C75  00  CD  b-        I>  CO  (N  CO  T—  t  CO  CD  O        O5  rH  O        CO 
GOO        COCO        CO(M        CO  O5  rH  T^        CO  "*  1C  CO  00  O5  (N  T^        (M  CO  Tt<        rf 
rHrHC^                 i—  ti—  IrHC^C^                                                      r—  1  rH 

Freight  Cart. 
t  =  4.5" 
a  =  2.5" 

b 

00 

II 

rH  i    1  .    1  rH 

s 

II 

Q 

1C  *C        CO  00  O  O        O  1C  CO  »C  GO  O  *C  00        CO  >C  CO        »C 
ICO        TJHCDOOO5        CO  CO  ^  1C  CO  00  O  I-H        (N  (M  CO        CO 

O  E  II 

II  II 

b-OOO5T^      .        ICO        Tt^b-0005        CO  CO  -tf  »C  b-  00  O  CN        (N  <N  CO        CO 

i—  1       •                  _           _  _  _  _                                                      rHrH 

1 

05 

1 

» 

1 

*o 

fl 
Q, 

i 
5 

. 

••••«.         .     .         .     .     .^         ... 

f3 

•         •                  •         •            rC                        ^                  73         . 

•    •     "§         °       '.    gii      •    •    • 

>j     •          ®^     s.     C§         "0      •      •      •      •      -^    § 

•g  £»;   g"  "  -g   s^  •    "d^^i    • 

1   -il   1         -  II  -:  :-  -:1    ^   ^     :     : 

^s  i  1  L  =            ! 
Hft?»i^tjiwH?«S 

gK^S^SK"    ^J>O    e  rH*CQ  THrH"oOK*r-^PH          Pn^K*    o*->f^'^    CH 

^5                       B^         ^              '     ^fe                                    GQ              S^ 

SI 

rH<NCOTt^iC       COI>       OOOSOi-H       (N  CO  T^  1C  CO  b-  00  O5       OI-HIM       CO 

ART.    1] 


ROAD   ECONOMICS 


19 


1837-41  for  the  French  Government,  were  much  the  most  elaborate 
and  appear  to  have  been  made  with  great  care.  Table  6,  page  18, 
is  a  summary  of  Morin's  results  showing  the  tractive  resistance  for 
different  vehicles  on  various  road  surfaces.  The  table  represents 
about  700  experiments. 

21.  Results  of  Modern  American  Experiments.  Table  7,  page 
20,  shows  data  obtained  by  the  author.  The  tractive  power  was 
determined  with  a  Baldwin  dynograph,  Fig.  1 .  The  instrument  con- 
sists of  two  long  flat  springs  fastened  together  at  their  ends  and 


TOP  VIEW 


BOTTOM  VIEW 
FIG.  1. — BALDWIN  DYNOGRAPH. 


20 


ROAD   ECONOMICS  AND   ROAD   ADMINISTRATION         [CHAP. 


having  their  centers  slightly  farther  apart  than  their  ends.  One 
end  of  the  apparatus  is  attached  to  the  wagon,  and  the  team  is 
hitched  to  the  other.  The  pull  of  the  team  causes  the  centers  of  the 
flat  springs  to  approach  each  other.  One  spring  supports  a  gradu- 
ated disk,  and  the  other  is  connected  to  an  index  arm  which  is  pivoted 
at  the  center  of  the  disk.  From  one  end  of  this  index  arm,  the  pull 
can  be  read  directly  from  the  graduated  disk.  There  are  two  extra 
index  arms — one  to  indicate  the  maximum  power  developed  and  one 
to  indicate  a  rough  average.  The  former  (the  upper  one  in  Fig.  1)  is 
simply  pushed  around  by  the  main  index  arm  and  is  left  at  the  highest 
point.  The  latter  (the  middle  arm  in  Fig.  1)  has  a  transverse  slot 
in  which  plays  a  stud  on  the  main  index  arm.  When  making  an 

TABLE  7 
TRACTIVE  RESISTANCE  ON  LEVEL  PAVEMENTS 


Expt. 
No. 


Location  and  Description  of  the  Pavement. 


Pounds 
Ton. 


1 
2 
3 

4 
5 

6 

7 
8 
9 

10 

11 
12 
13 
14 
15 

16 

17 


IS 
19 
20 
21 
22 
23 
24 

2f> 
26 


Asphalt:     Chicago — Calumet  Ave.,  bet.  43d  and  44th  Sts.;  smooth,  clean, 

no  cracks,  52°  F 

Chicago — Calumet  Ave.,  bet.  43d  and  44th  Sts.;  smooth,  clean, 

no  cracks,  84°  F 

Chicago — Washington   Boul.,    bet.    Halsted    and    Green    Sts.; 

smooth,  clean,  no  cracks,  42°  F 

Brick:     Champaign — University  Ave.,  west  of  New  St.;    3  X  9-in.  brick 

on  concrete,  corners  rounded,  sand  filler,  not  worn,  clean 

Champaign — Second  South  St.;   same  as  No.  4,  except  newer  and 

covered  with  2-in.  of  dust 

Champaign — First  South  St.;   same  as  No.  4,  except  cement  filler, 

just  completed 

Chicago — Peoria  St.,  between  Washington  and  Randolph;   2£  X  8- 

in.  brick  on  concrete,  pitch  filler,  new , 

Chicago — Laurel  St.  Stock  Yards;    3  X  8-in.  brick  on  gravel  and 

cinders,  sand  filler,  corners  not  rounded 

Chicago — Exchange  Ave.,  Stock  Yards;   2j  X  8-in.  brick  on  sand 

and  old  macadam,  tar  filler,  new 

Granite  block:    Chicago-^Exchange  Ave.,  Stock  Yards;  smoothly  dressed 
3  X  9-in.  blocks  on  concrete,  joints  J  in.,  tar  filler,  not 

worn „ 

Chicago — Randolph  St.,  between  Desplaines  and  Halsted; 
smoothly  dressed  blocks  on  concrete,  pitch  filler,  new.  . 
Chicago — Halsted  St.,  between  Randolph  and  Washing- 
ton; ordinary  granite,  9  years  old 

Macadam:    Chicago— Michigan  Ave.,  between  42d  and  43d  Sts.:  granite 

top,  no  dust,  no  mud .  .  

Plank  road:  Chicago— Packer's  Ave.,  Stock  Yards;   oak  plank,  3  X  12-in., 

nearly  new 

Exactly  same  as  above  after  worn  down  \  in.  in  many  places, 

clean 

Substantially  same  as  above;   covered  with  i-in  fine  loose  dirt 

bteel    wheelway:    Chicago— Transit    Ave.,   Stock    Yards;     8-in.    1H  lb. 

channel  on  2  X  8-in.  pine,  that  on   macadam,  covered 

with  7f-in.  powdered  stone 

Same  when  scraped  with  a  shovel 

Same  when  covered  with  J-in.  fine  dust 

Wood  block:    Rectangular  blocks  3  X  12-in.  considerably  worn 

Cylindiical  cedar  block  covered  with  5-in.  silica  pea  gravel 
Exactly  same  as  above  covered  with  J-in.  crushed  gravel .  . 
Cylindrical  cedar  block;  clean,  blocks  slightly  convex  on  top 
Cylindrical  cedar  block  on  2-in.  plank  and  2  in.  of  sand,  clean, 

not  worn 

Same  as  above;  clean,  slightly  worn 

Same  as  above;  clean,  considerably  worn i 


37 
70 
34 
17 
31 
22 
24 
37 
25 

29 
30 
36 
18 
32 

38 
40 


40 
10 
28 
30 
90 
50 
53 

37 
51 

54 


ART.    1] 


ROAD   ECONOMICS 


21 


experiment  the  main  index  arm  is  continually  in  motion,  and  the 
position  of  the  auxiliary  arm  roughly  indicates  the  average  power 
exerted.  The  end  of  the  index  arm  opposite  the  graduated  arc 
records  the  amount  of  tractive  resistance  upon  a  strip  of  paper  which 
is  wound  from  one  cylinder  to  another  by  clock-work  located  behind 
the  lower  right-hand  corner  of  the  top  view  of  Fig.  1.  The  auto- 
graphic record  is  more  accurate  than  the  indicated  reading. 

The  wagon  employed  was  the  usual  thimble-skein  four-wheel 
farm  wagon  with  a  2-inch  tire.  Experiments  3,  4,  and  5  were  made 
with  wheels  averaging  42 J  inches  in  diameter,  and  the  remainder 
with  wheels  averaging  47  inches. 

22.  From  a  study  of  the  preceding  experiments  and  also  others 
not  here  described,  it  is  concluded  that  the  average  tractive  resist- 
ance on  different  road  surfaces  is  about  as  in  Table  8  which  is  given 
for  use  in  comparing  different  roads  and  pavements. 

TABLE  8 
STANDARD  TRACTIVE  RESISTANCE  OF  DIFFERENT  ROADS  AND  PAVEMENTS 


Ref. 

TRACTIVE  ] 

IESISTANCE. 

No. 

Kind  of  Road  Surface. 

Pounds  per  Ton. 

In  Terms  of  Load. 

1 
2 

Asphalt  —  artificial  sheet  
Brick.             

30-  70 
15-  40 

A-A 

T^T—  JG    , 

3 

Cobble  stones  . 

50-100 

13l3     5l° 

4 
5 
6 

.7 
8 

Portland  cement  concrete,  unsurfaced 
Earth  roads  —  ordinary  conditions.  .  . 
Gravel  roads  
Water-bound  macadam  
Plank  road  

27-  30 
50-200 
50-100 
20-100 
30-  50 

li 

-Ar-A 
TTTo~2~o 
TT1--46 

9 

Sand  —  ordinary  condition  

100-200 

2TT-T& 

10 

Stone  block 

30-  80 

11 

11 

Steel  wheelway 

15r-  40 

iJPS 

1? 

Wood  block  —  rectangular  . 

30-  50 

X-t£ 

13 

cylindrical  

40-  80 

S-S 

23.  GRADE  RESISTANCE.     This  is  the  force  required  on  a  grade 
to  keep  the  load  from  rolling  down  the 
slope.     It  is  independent  of  the  nature 
of  the  road  surface,  and  depends  only 
upon  its  angle  of  inclination. 

In  Fig.  2,  P  is  the  grade  resistance, 
and  W  is  the  weight  of  the  wheel  and 
its  load.  From  the  diagram  it  is  easily  seen  that  P  —.  W  X  B  C  -r- 
A  C.  For  all  ordinary  cases,  A  C  may  be  considered  as  equal  to 
A  B,  and  then  P  =  WXBC+AB. 


FIG.  2. 


22  ROAD  ECONOMICS  AND   ROAD   ADMINISTRATION 

The  preceding  analysis  is  approximate  for  three  reasons:  (1) 
assuming  A  C  =  A  B,  i.  e.,  assuming  the  sine  of  inclination  to  be 
equal  to  the  tangent;  (2)  assuming  the  normal  pressure  on  the 
inclined  road  surface  to  be  equal  to  the  weight,  i.  e.,  assuming  the 
cosine  of  the  inclination  to  be  unity;  and  (3)  neglecting  the  fact  that 
the  hind  wheels  carry  a  greater  proportion  of  the  load  on  an  inclina- 
tion than  on  the  level.  The  resulting  error,  however,  is  wholly 
inappreciable. 

Grades  are  ordinarily  expressed  in  terms  of  the  rise  or  fall  in  feet 
per  hundred  feet,  or  as  a  per  cent  of  the  horizontal  distance.  If 
A  B  be  100  feet,  then  the  number  of  feet  in  B  C  is  the  per  cent  of  the 
grade;  and  therefore  the  grade  resistance  is  equal  to  the  load  mul- 
tiplied by  the  per  cent  of  the  grade.  Or  the  grade  resistance  is 
equal  to  20  Ib.  per  ton  multiplied  by  the  per  cent  of  the  grade. 

24.  POWER  OF  A  HORSE.  The  horizontal  pull  which  a  horse  can 
exert  depends  upon  its  weight,  its  build,  the  method  of  hitching,  the 
foothold  afforded  by  the  surface,  the  speed,  the  length  of  duration  of 
the  effort,  the  rest-time  between  efforts,  and  the  tax  upon  the  future 
efficiency  of  the  horse.  The  chief  of  these  are  the  weight,  the  speed, 
and  the  length  of  the  effort. 

Horses  vary  in  weight  from  800  to  1,800  Ib.  The  larger  horses 
do  not  usually  travel  more  than  2|  or  3  miles  per  hour.  With 
reasonably  good  footing  a  horse  can  exert  a  pull  equal  to  one  tenth 
of  its  weight  at  a  speed  of  2j  miles  per  hour  (3|  feet  per  second) 
for  10  hours  per  day  for  6  days  per  week  and  keep  in  condition. 
This  is  a  common  rate  of  exertion  by  farm  horses  in  pulling  plows, 
mowers,  and  other  agricultural  implements.  On  this  basis  a  1500- 
Ib.  horse  would  develop  550  foot-pounds  per  second  (the  conven- 
tional horse-power),  and  16,500,000  foot-pounds  per  day.  This 
may  be  considered  about  the  limit  of  endurance.  A  lighter  horse  will 
exert  a  proportionally  less  force.  If  the  time  of  the  effort  is  decreased, 
the  draft  may  be  proportionally  increased;  or  if  the  speed  is  increased, 
the  draft  must  be  decreased  in  a  like  proportion.  In  other  words,  the 
foot-pounds  of  energy  that  can  be  developed  per  day  by  any  particular 
horse,is  practically  constant. 

The  maximum  draft  for  a  horse  is  about  half  of  its  weight, 
although  horses  have  been  known  to  exert  a  pull  of  two  thirds  of 
their  weight.  Most  horses  can  exert  a  tractive  power  equal  to  half 
their  weight,  at  a  slow  walk  for  about  100  feet.  On  the  road  in 
emergencies,  as  in  starting  the  load  or  in  overcoming  obstacles,  a 
horse  may  be  expected  to  exert  a  pull  equal  to  half  its  weight,  but 


ART.    1]  ROAD   ECONOMICS  23 

at  this  rate  it  would  develop  a  day's  energy  in  about  2  hours;  and 
consequently  if  it  is  expected  to  work  all  day,  it  should  not  be 
called  upon  to  exert  its  maximum  power  except  for  a  short  time. 
Similarly,  a  horse  can  exert  a  draft  equal  to  one  quarter  of  its  weight 
for  a  longer  time.  The.  working  tractive  power  of  a  horse  may  be 
taken  as  one  tenth  of  its  weight,  with  an  ordinary  maximum  of  one 
quarter,  and  in  great  emergencies  a  maximum  of  one  half  its  weight.* 

25.  Increasing  the  number  of  horses  does  not  increase  the  power 
proportionally — for  somewhat  obvious  reasons.     It  is  stated  that 
for  a  two-horse  team  the  efficiency  of  each  horse  is  about  95  per 
cent;   for  a  three-horse  team,  about  85  per  cent.     Of  course  such 
data  are  not  much  more  than  guesses. 

26.  Effect  of  Grade.    The  effective  tractive  power  of  a  horse 
upon  an  inclined  road  surface  is  decreased  by  the  fact  that  the 
horse  must  lift  his  own  weight  up  the  grade.     If  T=  the  tractive 
power,  W  =  the  weight  of  the  horse,  t  =  the  tractive  power  on  the 
level  in  terms  of  the  weight  of  the  horse,  and  g  =  the  rise  of  the 
grade  per  unit  of  horizontal  distance,  then,  with  sufficient  accuracy, 

T  =  tW  -gW  =  (t-  g)W (1) 

If  it  be  assumed  that  the  average  working  tractive  power  of  the 
horse  is  one  tenth  of  its  weight,  then  t  =  10  per  cent;  and  equation 
(1)  shows  that  on  a  1  per  cent  grade  the  horse  can  exert  an  effective 
tractive  power  of  9  per  cent  of  its  weight,  and  also  that  it  will  be 
able  to  carry  its  own  weight  up  a  10  per  cent  grade.  If  it  be  as- 
sumed that  the  horse  exerts  a  tractive  power  equal  to  20  per  cent  of 
its  weight,  then  equation  (1)  shows  that  on  a  10  per  cent  grade  it 
can  take  its  own  weight  up  and  in  addition  exert  a  tractive  power  of 
10  per  cent  of  its  weight  upon  the  load.  By  assigning  values  to  t 
and  g,  equation  (1)  readily  shows  the  effective  draft  of  a  horse  upon 
any  grade. 

Equation  (1)  is  not  mathematically ,  correct,  since  it  assumes 
that  the  weight  of  the  horse  is  always  normal  to  the  road  surface. 
However,  the  formula  is  sufficiently  accurate  for  use  in  comparing 
the  relative  tractive  power  of  a  horse  on  different  grades  (§  27). 
At  best  such  a  formula  can  be  only  approximate,  since  the  tractive 
power  varies  greatly  with  the  foothold. 

*  For  the  results  of  experiments  made  at  the  Kansas  State  Agricultural  College,  showing  that, 
a  horse  in  pulling  from  500  to  1500  feet  probably  exerted  from  26  to  42  per  cent  of  its  weight, 
see  Engineering  and  Contracting,  Vol.  38  (1912),  p.  515. 


ROAD   ECONOMICS  AND   ROAD   ADMINISTRATION          [cHAP.   I 


PA 
W 


27.  Maximum  Load  on  a  Grade.  On  a  grade  the  effective  trac- 
tive power  as  given  by  equation  (1)  is  used  up  in  moving  the  load 
over  the  road  surface  and  in  lifting  the  load  vertically.  If  L  =  the 
load,  and  M  the  coefficient  of  road  resistance,  then 


(t  -  gW  = 


and 


*  + 


(2) 
(3) 


Equation  (3)  gives  the  load  that  a  horse  can  draw  up  any  grade. 
Table  9  is  computed  from  equation  (3)  for  a  value  of  t  equal 
to  one  tenth  of  the  weight  ,of  the  horse^  The  top  line  of  the 
table  shows  the  loads  that  a  horse  can  draw  on  the  level  on  the 
various  road  surfaces;  and  any  column  of  the  table  shows  the  load 

at  a  horse  can  pull  on  any  grade  for  that  particular  road  surface. 

As  showing  the  different  effects  of  grades  upon  different  roads, 
notice  that  on  a  muddy  earth  road  a  1  per  cent  grade  reduces  the 
load  less  than  one  tenth,  while  on  asphalt  a  1  per  cent  grade  reduces 

,e  load  more  than  one  half;  or,  again,  notice  that  with  a  5  per 
cent  grade,  on  iron  rails  the  load  is  less  than  one  twentieth  of  the 
load  on  the  level,  while  on  the  best  earth  road  the  load  is  one  fifth 
of  that  on  the  level. 

TABLE  9 

EFFECT  OF  GRADE  UPON  THE  LOAD  A  HORSE  CAN  DRAW  ON  DIFFERENT  ROADS 
The  Load  is  in  terms  of  the  Weight  of  the  Horse 


± 

j, 

EARTH  ROAD. 

Rate  of 

Iron 

Sheet 

Broken 

Stone 

NT     ' 

Grade. 

Rails. 

Asphalt. 

Stone. 

Block. 

T 

No. 

Per  Cent. 

M  =  5*0- 

M  =  iJu- 

Best. 

Spongy. 

Muddy. 

M=  A- 

/*  =  ]&• 

M  =  its1' 

1 

0 

20.00 

10.00 

6.00 

5.00 

3.00 

2.00 

1.00 

2 

1 

6.00 

4.50 

3.33 

3.00 

2.09 

1.50 

0.91 

3 

2 

3.20 

2.67 

2.16 

2.00 

1.51 

1.14 

0.67 

4 

3 

2.00 

1.75 

1.49 

1.40 

1.11 

0.87 

0.54 

5 

4 

1.33 

1.20 

1.05 

1.00 

0.82 

0.66 

0.43 

6 

5 

0.91 

0.83 

0.75 

0.71 

0.60 

0.50 

0.33 

7 

6 

0.62 

0.57 

0.52 

0.50 

0.43 

0.36 

0.25 

8 
9 

7 
8 

0.40 
0.2? 

0.38 
0.22 

0.34 
0.21 

0.33 
0.20 

0.29 
0.18 

0.25 
0.15 

0.18 
0.11 

10 

9 

0.15 

0.10 

0.09 

0.09 

0.08 

0.07 

0.05 

11 

10 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

Table  9  shows  the  load  a  horse  can  draw  upon  different  grades 
and  different  road  surfaces  when  exerting  a  uniform  pull  equal  to 


AKT.    1] 


ROAD    ECONOMICS 


25 


one  tenth  of  its  weight.  If  we  desire  to  know  the  maximum  load 
which  a  horse  can  draw  up  any  grade,  we  must  insert  in  equation 
(3)  the  maximum  value  of  t  and  compute  the  corresponding  value 
of  L.  The  value  of  t  to  be  used  in  this  computation  will  depend 
upon  the  length  of  the  grade  and  upon  the  frequency  with  which  it 
occurs.  If  the  grade  is  only  a  few  hundred  feet  long,  it  will  probably 
be  safe  to  assume  a  pull  equal  to  one  fourth  of  the  weight  of  the 
horse;  but  this  value  should  be  assumed  only  for  the  maximum  grade, 
since  such  pulling  is  too  exhausting  for  continuous  work. 

Table  10  presents  the  same  data  as  Table  9,  but  in  a  slightly 
different  form. 

TABLE  10 

LOAD  WHICH  A  HORSE  CAN  DRAW  ON  A  GRADE  IN  TERMS  OF  THE  LOAD  ON  THE 
LEVEL  WHEN  EXERTING  A  UNIFORM  FORCE  EQUAL  TO  ONE  TENTH  OF  ITS 
WEIGHT 


EARTH  ROAD. 

Rate  of 

Iron 

Sheet 

Broken 

Stone 

Ref. 

Grade, 

Rails. 

Asphalt. 

Stone. 

Blocks. 

No. 

Per  Cent. 

M  =  ui0. 

M  =  yJo- 

M=&. 

«  =  A. 

Best. 

Spongy. 

Muddy. 

M=  A- 

M=   A- 

P-A. 

1 

0 

1.00 

1.00 

1.00 

1.00 

1.00 

1.00 

1.00 

2 

1 

0.30 

0.45 

0.56 

0.60 

0.62 

0.75 

0.91 

3 

2 

0.16 

0.27 

0.36 

0.40 

0.50 

0.57 

0.67 

4 

3 

0.10 

0.18 

0.25 

0.28 

0.37 

0.44 

0.54 

5 

4 

0.07 

0.12 

0.17 

0.20 

0.27 

0.33 

0.43 

6 

5 

0.04 

0.08 

0.12 

0.14 

0.20 

0.25 

0.33 

7 

6 

0.03 

0.06 

0.08 

0.10 

0.14 

0.18 

0.25 

8 

7 

0.02 

.0.04 

0.06 

0.06 

0.10 

0.12 

0.18 

9 

8 

0.01 

0.02 

0.04 

0.04 

0.06 

0.08 

0.11 

10 

9 

0.01 

0.01 

0.02 

0.02 

0.03 

0.04 

0.05 

11 

10 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

28.  The  maximum  load  which  a  horse  can  draw  upon  any  road, 
particularly  upon  the  steepest  grade,  is  not,  however,  necessarily 
proportional  to  the  rate  of  grade  and  to  the  resistance,  since  the 
pull  that  a  horse  can  exert  depends  upon  the  foothold.     Owing 
to  the  danger  of  slipping  on  steep  grades,  particularly  when  the 
road  is  wet  or  icy,  it  is  not  customary  to  lay  sheet  asphalt  on  grades 
of  more  than  4  per  cent,  or  ordinary  stone  blocks  on  grades  of  more 
than  10  per  cent.     On  steeper  grades,  special  forms  of  stone  blocks 
are  sometimes  employed  to  increase  the  tractive  power  by  affording 
better  foothold  for  the  horses. 

29.  TRAVEL  CENSUS.    A  knowledge  of  the  use  made  of  a  road  or 
pavement  has  an  important  bearing  on  questions  of  construction, 


26  ROAD    ECONOMICS   AND   ROAD   ADMINISTRATION          [CHAP.    I 

maintenance,  and  cleaning.  The  travel  determines  the  amount  of 
money  that  may  economically  be  spent  in  reconstruction,  and  also 
fixes  the  width  and  character  of  the  improved  portion.  The  use 
made  of  the  road  or  street  is  necessary  to  determine  the  amount  of 
service  obtained  from  any  particular  road  surface.  Further,  the 
cost  of  cleaning  a  pavement  depends  upon  the  character  of  the  sur- 
face and  the  travel;  and  unless  the  latter  is  known,  it  is  impossible 
to  make  any  instructive  comparison  between  the  cost  of  cleaning 
different  surfaces. 

30.  History.     It  is  surprising  that  but  few  travel  censuses  have 
ever  been  taken.     Except  in  France,  not  much  attention  has  been 
given  to  this  subject  in  Europe.     The  French  engineers  have  made  a 
very  careful  study  of  the  amount  and  effect  of  travel  on  the  rural 
roads.     Previous  to  1844  travel  data  had  been  collected  in  certain 
localities;    but  from  1844  to  1903  ten  censuses  of  national  scope 
were  taken,  and  it  is  planned  to  take  one  every  10  years. 

The  results  of  the  French  observations  are  not  of  much  value  in 
America  because  of  the  difference  in  conditions;  and  results  before 
about  1906  are  not  of  much  value  because  of  the  recent  introduction 
of  the  automobile. 

31.  American  Roads.    The  Massachusetts  Highway  Commission 
in  1909  had  a  travel  census  taken  upon  state-aid  highways  for  14 
hours  per  day  for  seven  days  at  238  stations,  and  in  1912  a  similar 
count  was  made  at  156  stations,  and  in  1915  at  192  stations.    At 
the  same  time  a  travel  census  was  taken  at  a  number  of  points  on 
the  roadways  of  the  Metropolitan  and  the  Boston  Park  Systems. 
The  methods  and  results  are  presented  in  the  respective  annual 
reports. 

The  observer  separated  motor-driven  vehicles  into  three  classes, 
and  horse-drawn  into  four.  These  classes  and  the  assumed  weight 
of  each  (taken  according  to  the  prevailing  practice  in  Great  Britain) 
are  as  follows : 

MOTOR-DRIVEN  VEHICLES:  WEIGHT 

Runabouts 1 . 35  tons 

Touring  cars 2 . 23     ' *' 

Trucks 6.25     " 

HORSE-DRAWN  VEHICLES: 

1-horse,  light 0.36  " 

1-     "      heavy 1.12  " 

2  or  more  horses,  light 0 . 54  ' ' 

2         "         "        heavy 2.46  " 


ART.    1] 


ROAD   ECONOMICS 


27 


In  the  first  three  years  motor  travel  on  the  rural  roads  increased 
130  per  cent,  and  in  the  second  three  years  150  per  cent;  while  the 
horse-drawn  vehicles  decreased  almost  exactly  20  per  cent  in  each 
period.  In  1912  the  motor-driven  vehicles  were  63  per  cent  of  the 
horse-drawn,  but  in  1915  they  were  497  per  cent.  The  total  travel 
increased  about  in  proportion  to  the  number  of  motor  cars  registered. 
As  the  number  of  motor  cars  in  this  country  is  rapidly  increasing 
(for  example,  the  number  made  in  1916  was  80  per  cent  greater  than 
in  1915),  it  is  likely  that  travel  on  public  highways  will  continue  to 
increase. 

A  summary  of  this  census  for  a  few  roads  is  shown  in  Table  11; 
and  incidentally  this  table  also  shows  the  use  of  travel  census 
data  in  determining  the  unit  cost  of  maintenance.  The  wide 
variation  in  the  cost  of  maintenance  of  these  roads  is  probably 
due  to  differences  in  the  age  or  character  of  the  surface  and  to  dif- 
ferences in  the  character  of  the  traffic. 

TABLE  11 
TRAVEL  AND  COST  OF  REPAIRS  ON  MASSACHUSETTS  STATE-AID  ROADS 


Road. 

MOTOR-DRAWN 
VEHICLES. 

HORSE-DRAWN 
VEHICLES. 

TOTAL 
TRAFFIC. 

COST  OF 
MAINTE- 
NANCE. 

Runabout. 

O 
bf 

a 

I 

a 

Single 
Horse. 

Two  or 
More. 

Tons  per  Day. 

g^ 
X£ 

1° 

Jr 

1 

flj 

iij 

£ 

Cents  per  Ton- 
Mile  per 
Year 

>> 

>> 

1 

1 

t* 

a 
<u 

w 

Ashley 

14 
60 
86 
194 
44 
15 
15 
76 
7 
115 

65 
278 
334 
1365 
121 
50 
58 
407 
63 
533 

4 
8 
31 
13 
49 
0 
78 
17 
1 
30 

70 
66 
75 
28 
47 
30 
25 
64 
15 
167 

16 
46 
39 
19 
198 
77 
190 
60 
14 
98 

5 
4 
2 
1 
2 
2 
3 
4 
1 
5 

14 

12 
27 
14 
193 
88 
65 
36 
3 
59 

271 
1  618 
1  199 
3468 
1332 
1  140 
1  022 
1305 
186 
1918 

81  150 
485  220 
359  730 
1  040  430 
399  570 
342  210 
306  660 
391  550 
55770 
575  280 

$  266 
1  104 
200 
1  081 
1031 
592 
1334 
510 
143 
1  040 

0.38 
0.23 
0.06 
0.10 
0.26 
0.17 
0.44 
0.13 
0.25 
0.18 

Beverly  (No.  1)  
Hamilton 

Lynn  .  .  
Medford-Somerville. 
Milton  

Sangus  
Shrewsbury  
Truro  

Weston  . 

The  use  of  a  travel  census  in  determining  the  character  of  a  road 
surface  fitted  to  particular  traffic  conditions  is  incidentally  illustrated 
in  Table  26,  page  177. 

32.  In  1906  the  Illinois  Highway  Commission  took  a  census  of 
travel  at  71  stations  at  various  times  during  one  year.*  Observa- 


*  Annual  Reports  for  1906  and  1907. 


28  ROAD   ECONOMICS   AND   ROAD   ADMINISTRATION          [CHAP.    I 

tions  were  made  of  only  the  number  of  vehicles  without  distinction 
as  to  their  character  or  weight.  Twelve  of  the  roads  had  about  75 
vehicles  per  day,  twenty-seven  about  145,  and  ten  about  250.  The 
results  seem  to  show  that  there  is  no  relation  between  the  travel  on  a 
road  and  the  population  of  the  near-by  town;  or,  in  other  words, 
that  there  are  roads  in  the  vicinity  of  even  very  small  towns  that 
have  as  much  travel  as  roads  near  large  cities. 

33.  In  the  summer  of  1917  the  Iowa  Highway  Commission  took  a 
census  of  travel  on  a  few  of  the  main  roads.    Observations  were  made 
at  each  station  for  ten  days;  and  all  vehicles  were  actually  weighed. 
A  record  is  to  be  made  of  farm  traffic,  of  town  and  city  traffic,  and  of 
tourist  travel,  with  the  hope  of  securing  data  for  an  equitable  appor- 
tionment of  road  expenditures.     A  preliminary  report  on  observa- 
tions made  during  the  tourist  season  at  eight  stations  on  an  earth- 
surface   tourist  highway   leading    into    important   market   centers, 
showed  3  per  cent  tourist  travel,  87  per  cent  interurban  traffic,  and 
10  per  cent  farm  traffic. 

34.  American  Streets.     The  first  travel  census  in  the  United 
States  was  carried  out  by  the  Barber  Asphalt  Co.  in  New  York  City 
and   less   elaborately  in   ten    other    cities   in    1885.*    The   record 
shows  the  total  number  of  vehicles  and  the  number  of  tons  per 
foot  of  width  of  pavement.     The  same  company  took  a  similar 
census  in  New  York  City  in  1904.f     In  both  cases  the  count  was 
limited  mainly  to  asphalt  and  granite-block  pavements.     Since  1913 
the  city  of  St.  Louis,  Mo.,  has  taken  an  annual  travel  census — at 
first  on  business  streets,  but  later  also  on  residence  streets.  |     In 
New  York  and  St.  Louis  there  was  a  very  great  annual  increase  in 
the  amount  of  travel.     In  St.  Louis  from  1914  to  1915  the  increase 
was  20  per  cent,  the  increase  in  motor-driven  traffic  being  53  per  cent 
and  the  decrease  in  horse-drawn  15  per  cent.     A  few  other  records 
of  various  kinds  have  been  made  in  several  cities. 

35.  Classification  of  Traffic.     The  data  collected  in  a  travel  census 
should  be  such  that,  in  addition  to  being  used  for  local  comparisons, 
they  should  be  such  as  to  permit  comparisons  with  data  taken  in 
other  localities.     There  is  no  standard  method  of  classifying  the 
vehicles,  or  of  the  assumed  weight  of   the  different  vehicles,  or  of 
fixing  the  width  of  the  traveled  way.     Further,  the  density  of  travel 
is  sometimes  stated  by  giving  simply  the  number  of  vehicles  per  day 

*  Trans.  Amer.  Soc.  of  C.E.,  Vol.  15  (1886),  p.  123. 

t  Ibid.,  Vol.  57  (1906),  p.  181-90. 

t  Engineering  News,  Vol.  76  (1916),  p.  832-34. 


ART.    1]  ROAD    ECONOMICS  29 

or  per  year;  but  usually  by  giving  the  number  of  tons  per  year  per 
foot  of  width.  The  unit  for  comparing  the  cost  of  maintenance 
is  either  the  tons  per  year  per  foot  of  width  or  the  ton-miles  per 
year. 

The  classification  and  weights  of  the  vehicles  in  the  Massachusetts 
census  are  shown  in  §  31.  Apparently  the  width  of  the  traveled 
way  was  taken  as  the  full  width  of  the  improved  portion, — as  it  prob- 
ably is  in  a  narrow  rural  road.  For  a  somewhat  similar  classifica- 
tion and  schedule  of  weights  employed  by  several  road  and  pave- 
ment constructing  companies,  see  page  149  of  the  1912  Proceedings 
of  the  American  Society  of  Municipal  Improvements.  s 

36.  Weight  of  Vehicles.     The  following  classification  and  sched- 
ule of  weights  has  been  recommended.*     The  weight  of  the  horse  is 
to  be  considered  as  a  part  of  that  of  the  vehicle ;  and  the  ton  is  2,000 
pounds. 

HORSE-DRAWN  VEHICLES  MOTOR-DRIVEN  VEHICLES 

Number  of  Horses.  ^ons*'  Style  of  Automobile.  ™tona*' 

Single. horse  without  vehicle.  ...  0.50        Motorcycle  or  bicycle 0.15 

1-horse  vehicle,  light 1 .20        2-passenger  automobile 1 .30 

1-horse  vehicle,  heavy 2 . 00  Over  2-passenger  automobile. .  .  2 . 20 

2-horse  vehicle,  light 2.00        Freight  motor-truck,  light 6.30 

2-horse  vehicle,  heavy 4.00  "          "            medium. .  6.00 

3-horse  vehicle 5.00  "           "            heavy 8.50 

4-horse  vehicle 6 . 00 

37.  Width  of  Traveled  Way.     The  effective  traveled  width  of  a 
street  is  sometimes  taken  as  3  feet  less  than  the  width  between  curbs. 
In  many  cases,  particularly  adjacent  to  car  tracks  and  where  auto- 
mobiles are  parked  along  the  curb,  most  of  the  travel  is  concentrated 
upon    a    comparatively    narrow    portion    of   the    pavement.t    The 
results  should  be  stated  in  tons  per  foot  of  total  width,  and  also  in 
tons  per  foot  of  maximum  traveled  width. 

If  the  several  classes  of  traffic  are  segregated  into  different  lines 
of  travel,  the  details  should  be  stated. 

38.  Diversion  of  Travel.     In  any  investigation  of  traffic  conditions 
preliminary  to  any  improvement  in  either  the  location  or  the  surface 
of  a  road,  careful  attention  should  be  given  to  the  probable  effect 
of  the  improvement  in  diverting  travel  to  the  road  or  street.     Some- 


*  W.  H.  Council,  Chief  of  Bureau  of  Highways,  Philadelphia,  Engineering  and  Contracting, 
Vol.  47  (1917),  p.  227. 

t  For  diagrams  showing  this  concentration,  see  Engineering  News,  Vol.  78  (1917),  p.  201-2. 


30  ROAD   ECONOMICS   AND   ROAD   ADMINISTRATION         [CHAP.   I 

times  a  small  change  in  the  condition  of  the  road  makes  a  radical 
change  in  the  amount  and  character  of  the  travel. 

39.  WEIGHT  AND  WIDTH  OF  VEHICLES.  Formerly  the  only 
excessive  loads  hauled  over  rural  roads  or  city  streets  were  heavy 
pieces  of  building  or  bridge  material,  machinery,  etc.,  hauled  on 
horse-drawn  vehicles,  and  steam  traction-engines;  but  as  these 
vehicles  were  not  very  numerous  and  as  the  speed  was  low,  no  serious 
harm  was  done,  particularly  where  traction  engines  were  required  to 
remove  or  cover  the  mud  lugs.  In  the  last  ten  years  the  advent  of 
heavy  high-speed  motor  trucks  has  greatly  increased  the  loads  and 
speeds  of  vehicles  using  the  highways.  Certain  types  of  motor 
vehicles  now  in  use  are  too  heavy  for  the  present  roads  or  pavements, 
and  much  damage  is  being  done;  and  furthermore  the  number  and 
weight  of  such  vehicles  is  rapidly  increasing.  It  is  inequitable  and 
impracticable  to  reconstruct  all  or  even  many  of  the  roads  and  pave- 
ments so  as  to  enable  them  to  carry  safely  such  motor-driven  vehicles. 
Therefore  special  taxes  are  being  levied  upon  heavy  motor-driven 
vehicles,  partly  to  make  them  partially  pay  for  the  damage  done  to 
the  highways,  but  chiefly  to  prevent  further  increase  in  their  number 
and  weight.  Some  of  the  states  have  laws  regulating  the  load  per 
width  of  tire  and  also  the  speed,  and  some  regulate  also  the  diameter 
of  the  wheel.*  The  following  is  from  an  ordinance  recently  passed 
in  New  York  City.f 

LICENSE  SCHEDULE  FOR  MOTOR  TRUCKS  IN  NEW  YORK  CITY 

"  (a)  Vehicles  carrying  or  intending  to  carry  a  total  gross  load  of  6,000  Ib. 
or  less  upon  any  wheel  shall  be  charged  the  following  annual  license  fee: 

Load  in  Pounds  License  Fee 

per  Inch  Width  for 

of  Tire.  Each   Vehicle. 

700  or  less $1 

751  to  800 3 

801  to  850 6 

851  to  900 12 

901  to  950 25 

951  to  1  000 50 

"  (6)  In  addition  to  the  fees  provided  in  subdivision  (a),  further  fees  shall  be 
charged  for  loads  greater  than  6,000  Ib.  upon  any  wheel,  but  not  exceeding  10,000 
Ib.,  as  follows: 

*  For  the  laws  regulating  motor  trucks  in  a  number  of  states  and  cities,  see  Engineering  News, 
Vol.  76  (1916),  p.  938-39. 

t  Engineering  Record,  Vol.  73  (1917),  p.  790, 


ART.    2]  ROAD   ADMINISTRATION  31 

Weight  in  License'Fee 

Pounds  per  for 

Wheel.  Each   Vehicle. 

6  000  to   6  500 $75 

6  501  to    7000 110 

7  001  to    7  500 150 

7  501  to    8  000 200 

8  001  to    8  500 : . . .     300 

8  501  to    9  000 500 

9  001  to    9  500 • 750 

9  501  to  10  000 1  000 

"  For  loads  greater  than  10,000  Ib.  per  wheel,  license  fees  shall  be  charged 
for  each  vehicle  at  the  additional  rate  of  $500  for  each  1,000  Ib.  per  wheel  increase 
in  weight,  provided  no  load  greater  than  1,000  Ib.  per  inch  width,  of  wheel  shall 
in  any  case  be  permitted,  except  as  specified  in  subdivision  (d). 

"  In  lieu  of  the  fees  hereinabove  provided  for  in  subdivisions  (a)  and  (6)  for 
loads  of  6,000  Ib.  or  more  on  any  wheel,  special  permits  may  be  issued  for  single 
trips  and  fees  charged  therefor  at  the  rate  of  10  per  cent  of  the  fees  therein  pro- 
vided, except  that  no  single  fee  shall  be  less  than  $25. 

"  (c)  Vehicles  6  feet  6  inches  or  more  in  width  over  all  shall  be  charged,  in 
addition  to  the  fees  specified  in  subdivision  (a)  and  (6),  the  following  annual 
fee; 

WIDTH  OF  VEHICLE  LICENSE  FEE 

for  each  inch  in 

width  in  excess 

of  6  feet  6  inches. 

6  feet  6  inches  to  7  feet  0  inches $5 

7  "    0        "        7   "    6    "    10     :, 

7  "    6        "        8   "    8    "    15 

8  "    0        "        8   "    6    "    20 

8   "    6        "        9   "    6    "    25 

"  (d)  In  lieu  of  the  fees  provided  in  subdivision  (c)  for  excess  width  of  vehicle, 
special  permits  for  single  trips  may  be  granted  upon  payment  of  single  fee  of  not 
less  than  $10." 

ART.  2.     ROAD  ADMINISTRATION 

41.  ADMINISTRATIVE  UNIT.  In  this  country  until  recently,  the 
management  of  roads  rested  upon  local  authorities,  either  those  of 
townships  or  counties.  In  those  cases  in  which  the  administration  of 
road  affairs  was  nominally  in  the  hands  of  the  county  authorities, 
nothing  was  usually  done  except  to  divide  the  county  into  road  dis- 
tricts and  virtually  transfer  all  authority  to  local  officials  appointed 
for  that  purpose.  Apparently  it  is  impossible  to  secure  either  good 
roads  or  an  efficient  road  administration  by  the  action  of  officials 
who  have  only  local  authority,  and  who  are  necessarily  swayed  by 
purely  local,  if  not  individual,  interests.  This  is  not  peculiar  to 


32  ROAD   ECONOMICS   AND    ROA±>    ADMINISTRATION 

America,  since  great  difficulties  have  always  been  encountered  in 
maintaining  a  system  of  public  highways  by  any  locally  governed 
community. 

The  fundamental  difficulty  is  that  the  small  administrative  unit 
makes  it  impossible  to  secure  efficient  supervision,  since  the  time 
necessarily  required  in  road  administration  is  but  an  incident  among 
private  or  official  duties.  Another  difficulty  is  that  the  official  is 
usually  elected  for  political  reasons,  rather  than  for  his  ability  in 
matters  relating  to  the  roads.  A  further  difficulty  is  that  the  tenure 
of  office  is  short,  and  successive  officials  have  conflicting  views  as  to 
road  administration  and  road  improvement. 

Another  objection  to  the  small  administrative  unit  is  the  improb- 
ability of  the  district's  having  suitable  machinery  in  sufficient 
quantity  to  effectively  and  economically  care  for  the  roads. 

42.  State  Aid. — In  1891  the  state  of  New  Jersey  inaugurated  a  new 
departure  in  road  administration  in  the  United  States — that  of  state 
aid  in  road  construction.     In  1917  all  of  the  states  except  Mississippi 
and  South  Carolina  had  adopted  some  form  of  state  aid.     The  fun- 
damental principle  of  state  aid  is  that  some  roads  are  built  at  the 
joint  expense  of  the  state  and  local  authorities.     The  states  differ 
greatly  as  to  (1)  the  proportion  paid  by  the  state,  (2)  the  amounts 
paid  by  the  county,  township,  and  abutting  property,  (3)  the  amount 
and  the  method  of  the  supervision  over  the  construction,  and  (4) 
the  authority  that  maintains  state-aid  roads. 

The  adoption  of  state  aid  led  to  the  establishment  of  state  high- 
way departments  in  many  of  the  states;  but  to  participate  in  federal 
aid  (see  §  45)  it  was  necessary  for  a  state  to  have  a  state  highway 
department,  and  hence  all  of  the  states  now  have  such  departments. 

One  of  the  most  important  factors  in  bringing  about  the  rapid 
adoption  of  state  aid  and  state  supervision,  has  been  the  introduction 
of  the  automobile  and  the  consequent  development  of  greater  interest 
in  good  roads. 

43.  Table  12  gives  data  concerning  road  improvement  in  the 
several  states. 

44.  The  principle  of  state  aid  is  defended  on  the  ground  that 
(1)  it  secures  centralized,  and  therefore  more  efficient,  control;  (2) 
makes  the  wealth  of  the  city  bear  part  of  the  expense  of  maintaining 
the  country  roads;   and  (3)  compels  the  railroads  and  other  state- 
wide corporations  to  bear  part  of  the  expense  of  local  improvements. 
The  chief  advantage  of  state  aid  is  that  it  secures  uniform  and  more 
intelligent  supervision  than  is  possible — on  state-aid  roads  at  least — 


ART.    2] 


ROAD    ADMINISTRATION 


33 


TABLE  12 
ROAD  MILEAGE  AND  ROAD  EXPENSES  IN  THE  SEVERAL  STATES 


MILES  OF 

RURAL  PUB 

Lie  ROADS. 

State. 

Total 
Surfaced 
Roads  in 
State. 

Total 
Public 
Rural 
Roads  in 
State. 

Percent- 
age of 
Surfaced 
Roads  in 
1915. 

Year 
Original 
State- 
Aid  Law 
Passed. 

State 
Aid  in 
in  1915. 

Total 
Cash 
Expendi- 
tures in 
1915. 

Alabama  
Arizona           .    . 

5915 
350 

55446 
12075 

10.7 
2.9 

1911 
1909 

$126  134 
476  178 

$4  283  207 
1  076  178 

Arkansas  

1  200 

50743 

2.3 

1913 

25000 

2  803  000 

California 

13  000 

61  038 

21.3 

1895 

8  301  149 

20  753  281 

Colorado  
Connecticut 

1  750 
3200 

39691 
14  061 

4.4 
22.7 

1909 
1895 

203  000 
2  084  944 

2  193  000 
3  484  944 

Delaware  
Florida 

300 
3  500 

3674 
17  995 

8.0 
19  4 

1903 
1915 

31000 
1  135 

397500 
5  501  135 

Georgia  

13000 

84770 

15.3 

1908 

3  700  000 

Idaho 

950 

23  109 

4  1 

1905 

200000 

1  974  636 

Illinois  

11  000 

94  141 

11.7 

1905 

818  638 

9  263  995 

Indiana 

27  000 

63  370 

42  6 

1917 

13  000  000 

Iowa.    . 

1  000 

106  847 

1  0 

1904 

80935 

13  606  299 

Kansas  
Kentucky 

1250 
13  000 

111  536 
57  916 

1.1 

22   1 

1911 
1912 

10000 
573  715 

5  510  000 
3  122  430 

Louisiana.  . 

2  250 

24  562 

9  1 

1910 

144  821 

3  569  709 

Maine 

3  000 

25  528 

11  7 

1901 

1  009  345 

3  293  902 

Maryland  . 

2950 

16  458 

17.9 

1898 

3  330  000 

5  630  000 

Massachusetts.  .  .  . 
Michigan 

8800 
8  600 

18681 
74  089 

46.6 
11  6 

1892 
1905 

2  634  567 
975  000 

6  557  279 
10  174  738 

Minnesota  

Mississippi  
Missouri  .  . 

5500 

2500 
8  000 

93500 

45778 
96  124 

5.9 

5.5 
8  3 

1905 
i907 

1580000 
369  i89 

8  292  000 

2  900  000 
8  369  189 

Montana  

Nebraska.  .  . 
Nevada  

775 

500 
75 

39204 

80338 
15  000 

2.0 

.6 
5 

1913 

1911 
1911 

18346 
120000 

3  676  318 

3  520  000 
250  000 

New  Hampshire.  .  . 

New  Jersey  
New  Mexico  
New  York 

1800 

4600 
450 
17  500 

14020 

14817 
11  873 
80  112 

12.8 

31.0 
3.8 
21  8 

1903 

1891 
1909 
1898 

666  339 

1  163  308 
152  122 
13  983  769 

2  363  414 

7  163  584 
584  919 
24  255  648 

North  Carolina.  .  .  . 
North  Dakota  .... 
Ohio  

6500 
1  100 
30  920 

50758 
68000 
86  453 

12.8 
1.6 
35  8 

1901 
1909 
1904 

10000 
3  442  604 

5  510  000 
2  500  700 
12  975  688 

Oklahoma  
Oregon  
Pennsylvania  

Rhode  Island  
South  Carolina.  .  .  . 

300 

7780 
9883 

1246 
3  500 

107  916 
36819 
91556 

2  121 
42220 

.3 

21.1 
10.8 

58.8 
8  3 

1911 
1913 
1903 

1902 

10000 
230  000 
6  541  257 

204  119 

3410000 
6  182  000 
12  541  257 

594  119 
1  000000 

South  Dakota  

850 

96306 

.9 

1911 

1  450  000 

Tennessee  
Texas 

8625 
12  000 

46050 
128  960 

18.7 
9  3 

1915 
1917 

3-500 

3  503  500 
9  500  000 

Utah  

1  053 

15  000 

7  0     • 

1909 

121  000 

1  213  100 

Vermont  

3  478 

15082 

23  i 

1898 

485  145 

1  475  145 

Virginia 

4  760 

53  388 

8  9 

1906 

526  645 

4  018  399 

Washington  

West  Virginia  
Wisconsin 

5460 

1  200 
14  050 

42428 

32024 
75  702 

12.8 

3.7 
18  5 

1905 

1909 
1911 

1  435  020 

9212 
1  389  51  5 

6  670  702 

2  759  212 
9  960  980 

Wyoming.  .  

500 

14381 

3.5 

1911 

5000 

441  291 

Total  

276  920 

2  451  660 

11.3 

53  491  651 

266  976  399 

34  ROAD   ECO    JMICS   AND   ROAD   ADMINISTRATION          [CHAP.    I 

with  a  smaller  administrative  unit;  and  besides  the  standards  set  on 
state-aid  roads  tend  to  become  the  ideals  for  the  other  roads. 

45.  National  Aid.     In  1916  Congress  passed  a  law  granting  to 
the  several  states  federal  aid  in  the  construction  of   roads,  which 
was  another  new  departure  in  road  administration  in  this  country. 
The  law  appropriated  $5,000,000  for  federal  aid  in  1917,  and  pro- 
vided to  increase  the  amount  $5,000,000  each  year  until  in  1921,  when 
the  appropriation  will   be   $25,000,000.     The  U.  S.  Department  of 
Agriculture  may  deduct  3  per  cent  for  administrative  expenses,  and 
the  remainder  is  divided  among  the  several  states  as  follows:   One 
third  in  proportion  to  the  areas  of  the  states,  one  third  in  proportion 
to  the  mileage  of  star  and  rural  mail  routes  in  the  states,  and  one 
third  in  proportion  to  the  population  according  to  the  preceding 
federal  census.     The  federal  money  can  be  used  to  pay  not  to  exceed 
one  half  of  the  total  cost  of  the  construction  of  any  road  or  system 
of  roads,  the  plans  for  which  have  been  previously  approved  by 
the  proper  federal  authority.     Table   13,  pages  36-37,  shows  the 
official  figures  employed  in  making  the  distribution,  and  the  amounts 
for  each  state  in  1917. 

In  Europe  nearly  all  countries  give  national  aid  in  some  form  for 
building  roads. 

46.  CLASSIFICATION  OF  ROADS.     It  has  long  been  known  by 
close  students  that  the  problems  of  road  administration  would  be 
greatly  improved  if  the  roads  were  classified  according  to  their  im- 
portance, into  state,  county  and  township  roads,  or  into  county, 
township  and  neighborhood  roads,  the  roads  of  each  class  to  be  under 
a  corresponding  administrative  authority.     One  of  the  incidental, 
but  not  unimportant,  results  of  the  adoption  of  the  state  aid  has  been 
the  classification  of  the  wagon  roads.     It  has  been  found  that  10 
to  15  per  cent  of  the  roads  carry  from  80  to  90  per  cent  of  the  travel. 
These  principal  roads  are  called  state  or  county  roads,  and  are  the 
ones  upon  which  the  state  aid  is  expended,  either  directly  or  indirectly 
under  the  supervision  of  state  authorities. 

The  modification  of  the  state  road  laws  incident  to  the  introduc- 
tion of  the  principle  of  state  aid  has  usually  resulted  in  giving  to  some 
county  authority  supervision  over  township  road  officials. 

47.  ROAD   TAXES.     How  shall  the  expense  of  constructing  and 
maintaining  roads  be  distributed?     This  question  has  been  answered 
in  various  ways  in  different  parts  of  this  country  and  in  different 
countries  of  Europe.     There  are  three  forms  of  road  taxes  which 
have  long  been  in  use;  viz.:   (1)  a  tax  upon  the  traveler,  (2)  a  cap- 


ART.    2]  ROAD    ADMINISTRATION  35 

itation  tax,  and  (3)  a  property  tax.     The  first  leads  to  toll  roads; 
and  the  second  is  usually  called  a  poll  tax. 

In  1901  the  State  of  New  York  introduced  a  method  of  raising 
revenue  for  road  purposes,  viz. :  a  license  for  operating  automobiles. 
For  present  purposes  this  will  be  referred  to  as  the  automobile  road 
tax. 

48.  Toll  Roads.     These  are  conducted  on  the  theory  that  the 
travelers  over  a  road  are  the  recipients  of  its  benefits  and  should 
pay  for  its  support.     Toll  roads  are  justifiable  only  in  a  new  country 
where  the  amount  of  taxable  property  is  small,  and  where  for  long 
stretches  of  territory  there  are  few  inhabitants,  since  such  roads 
induce  the  investment  of  capital  that  possibly  the  pioneer  or  the  new 
community  could  not  afford;   and  even  under  these  conditions  they 
are  practicable  only  where  there  is  considerable  traffic.     In  early 
times  the  government  collected  the  toll  and  used  it  for  the  main- 
taining and  extension  of  the  road;  but  later  toll  roads  were  usually 
in  the  hands  of  private  capitalists. 

Toll  roads  are  objectionable  owing  to  the  proportionally  great 
expense  of  collecting  the  revenue,  and  owing  to  the  fact  that  they 
are  managed  solely  with  reference  to  securing  returns  upon  the 
capital  invested  and  without  regard  to  the  interests  of  the  public. 
The  only  remedy  for  the  evils  of  the  system  is  for  the  public  to  sup- 
port the  roads.  Roads  are  an  indispensable  public  convenience 
—a  benefit  to  every  citizen,  whether  a  direct  user  of  the  road  or  not,— 
and  consequently  should  be  maintained  by  a  universal  tax.  At 
present  the  toll  system  is  regarded  as  unwise  for  both  economic  and 
political  reasons;  and  toll  roads  have  almost  entirely  been  abolished 
both  in  this  country  and  in  Europe. 

49.  Poll  Tax.     Notwithstanding  the  fact  that  most  writers  on 
political  economy  consider  a  capitation  tax  an  undesirable  form 
of  taxation,  nearly  all  of  the  states  levy  a  poll  tax  for  road  purposes. 
Apparently  it  is  the  only  capitation  tax  in  this  country.     It  is  not 
wise  to  occupy  space  here  to  inquire  into  either  the  wisdom  or  reason 
for  this  form  of  road  tax. 

Almost  universally  the  law  permits  the  payment  of  the  poll 
road-tax  in  money  or  labor,  and  it  is  usually  paid  in  labor.  In  the 
poorer  and  less  populous  states,  this  tax  is  nearly  the  sole  support 
of  the  road  system.  In  many  states  there  are  numerous  exemptions, 
and  in  all  states  the  tax  is  difficult  to  collect,  and  consequently 
the  poll  tax  is  an  unimportant  element  in  road  construction  and 
maintenance. 


36 


ROAD    ECONOMICS   AND   ROAD   ADMINISTRATION          [CHAP.    1 


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ART.    2] 


ROAD   ADMINISTRATION 


37 


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38  ROAD   ECONOMICS   AND   ROAD   ADMINISTRATION         [CHAP.    I 

50.  Property  Tax.     There  are  three  forms  of  the  property  road 
tax:  the  special  assessment,  the  direct  tax,  and  the  general  tax. 

In  many  states  when  any  considerable  road  improvement  is 
contemplated,  part  or  all  of  the  cost  of  the  same  is  laid  as  a  special 
tax  or  assessment  on  all  real  estate  within  some  certain  distance  of 
the  improvement.  In  Indiana  at  one  time,  this  distance  was  two 
miles;  and  in  Wisconsin,  three.  Ordinarily  this  tax  is  not  uniform 
over  the  included  area,  but  is  graded  according  to  the  supposed 
benefits.  This  tax  is  usually  payable  in  money. 

In  most  of  the  states,  the  territory  is  divided  into  small  units, 
called  road  districts,  and  a  uniform  road  tax  is  laid  upon  all  prop- 
erty within  the  district.  Usually  this  tax  may  be  paid  in  either 
money  or  labor;  and  when  so  permitted,  is  usually  paid  in  labor. 

In  most  states  there  is  also  a  general  property  tax  for  road  and 
bridge  purposes,  which  must  be  paid  in  money. 

In  poorer  communities  the  roads  are  cared  for  principally  by 
the  district  road  tax,  which  is  usually  paid  in  labor;  but  in  wealthier 
communities  the  general  property  road  and  bridge  tax  (cash  tax) 
is  greater  than  the  district  road  tax  (labor  tax). 

51.  Labor  vs.  Money  Tax.    In  most  of  the  states  the  labor  tax 
is  still  regularly  employed,  although  it  is  gradually  disappearing. 
The  labor-tax  system  was  inherited  from  England,  and  is  a  survival 
of  the  feudal  method  of  requiring  all  able-bodied  men  to  render  public 
service.     England  and  France  have  a  labor  road-tax,  but  upon  a 
much  less  extensive  scale  than  has  this  country. 

The  roads  and  streets  of  the  cities,  towns,  and  villages  are  usually 
under  the  control  of  the  municipalities,  in  which  as  a  rule  the  labor 
tax  does  not  exist;  and  therefore  the  labor-tax  system  applies  chiefly-, 
if  not  wholly,  to  rural  communities.  Further,  since  a  very  large  pro- 
portion of  the  roads  are  of  Dearth,  the  labor-tax  system  is  usually 
applied  to  the  construction  and  care  of  only  earth  roads. 

It  is  common  to  assume  that  the  labor-tax  system  is  all  wrong, 
and  that  its  evils  would  be  escaped  by  paying  road  taxes  in  money. 
The  labor  tax  has  inherent  disadvantages,  but  many  of  the  defects 
charged  to  it  belong  rather  to  defective  administration  and  to  the 
system  that  leaves  the  control  of  the  public  highways  to  a  small 
locally-governed  community. 

The  objections  to  the  labor-tax  system  are:  1.  The  labor  is 
indifferent  and  inefficient.  2.  It  is  impossible  to  get  the  work  done 
at  the  most  suitable  time.  3.  The  system  allows  no  selection  of  the 
laborer.  All  of  these  are  important  considerations. 


ART.    2]  ROAD    ADMINISTRATION  39 

The  reply  to  the  above  objections  is  usually  about  as  follows: 
1.  The  farmer  is  willing  to  pay  more  in  labor  than  in  money,  which 
compensates  in  part,  at  least,  for  the  objections  to  the  labor-tax 
system.  This  preference  is  not  peculiar  to  the  American  farmer. 
In  France,  if  the  road  tax  is  paid  in  money,  a  reduction  of  40  to  50 
per  cent  is  made;  but  still  60  per  cent  of  the  people  prefer  to  pay  in 
labor.  Farmers  not  infrequently  give  more  both  in  labor  and 
money  than  is  exacted  as  road  taxes,  because  they  are  interested 
in  better  roads.  2.  In  many  rural  communities  it  is  impossible  to 
secure  any  one  to  do  road  work  at  reasonable  wages  at  the  most 
suitable  season.  3.  If  the  tax  were  paid  in  money,  there  is  no 
certainty  that  the  labor  would  be  any  more  efficient.  Streets  are 
maintained  under  the  cash-tax  system,  but  the  labor  is  not  ideally 
efficient.  The  authority  that  virtually  wastes  the  labor  tax  will 
probably  also  waste  the  cash  tax. 

52.  The  labor  tax  is  not  necessarily  the  cause  of  inferior  roads, 
nor  the  cash-tax  system  in  itself  the  cause  of  improved  roads.  Town- 
ships under  the  labor-tax  system  often  have  better  roads  than 
adjoining  townships  under  the  cash-tax  system.  The  one  thing 
absolutely  necessary  for  successful  road  management  is  effective 
supervision  of  the  work.  Without  it  neither  system  .will  accom- 
plish much,  and  with  it  either  system  will  do  reasonably  well. 

Many  townships  have  changed  from  the  labor-tax  system  to  the 
cash-tax  system  with  a  marked  improvement  in  the  condition  of  the 
roads — due  chiefly,  if  not  wholly,  to  better  administration.  In  these 
cases  the  public  sentiment  that  demanded  road  improvement  secured 
the  change  from  the  labor  tax  to  the  cash  tax;  and,  consciously  or 
unconsciously,  also  secured  a  more  efficient  road  administration.  In 
many  of  these  cases  the  so-called  cash-tax  system  is  practically 
only  a  change  in  the  method  of  administering  the  labor-tax  system, 
since  farmers  desiring  to  do  so  are  given  an  opportunity  to  work  out 
their  road  taxes  under  the  cash  system.  Under  the  labor-tax  system 
those  working  upon  the  roads  receive  credit  on  their  road  taxes, 
while  in  the  so-called  cash  system  the  laborer  usually  receives  an 
order  which  is  accepted  as  cash  in  paying  taxes. 

The  labor-tax  system  is  more  objectionable  with  roads  having  a 
hard  surface  than  with  earth  ones,  since  the  construction  of  the 
former  is  more  difficult  and  their  maintenance  requires  intimate 
knowledge  and  constant  attendance,  and  also  since  the  former  are 
built  only  where  there  is  much  travel  and  where  the  labor  of  main- 
tenance is  greater,  This  subject  will  be  considered  incidentally 


40  ROAD    ECONOMICS   AND   ROAD    ADMINISTRATION          [CHAP.    I 

under  Maintenance  in  the  chapters  on  earth,  gravel,  and  broken- 
stone  roads. 

53.  Automobile  Tax.     Since  1901  the  several  states  have  adopted 
the  system  of  licensing  automobiles  as  a  means  of  securing  revenue 
for  road  purposes.     In   1916  the  gross  revenue  from  this  source 
amounted  to  $25,865,370,  of  which  92  per  cent  was  applicable  for  road 
work;   and  the  net  amount  so  applied  was  nearly  9  per  cent  of  the 
total  expenditures  for  rural  roads  and  bridges  in  the  United  States. 
About  70  per  cent  of  the  total  automobile  revenue  is  expended  under 
the  supervision  of  the  State  Highway  Departments. 

In  1916  there  was  an  average  of  1.4  motor  cars  for  each  mile  of 
rural  public  roads  in  the  United  States,  and  the  number  of  motor  cars 
is  increasing,  the  increase  in  1915  being  40  per  cent  and  in  1916  43 
per  cent.  The  average  annual  registration  or  license  fee  per  motor 
varies  between  the  several  states  from  50  cents  to  $19.67.  The 
tendency  in  all  of  the  states  is  to  increase  the  fee. 

54.  Comparison  of  Road  Expenditures.     From  1904  to  1915  the 
annual  expenditures  on  the  rural  roads  and  bridges  in  the  United 
States  increased  from  about  $80,000,000  to  about  $282,000,000,  an 
increase  of  more  than  2j  times.     During  the  same  period  the  annual 
expenditures  for  state-aid  road  and  bridge  construction  and  main- 
tenance increased  from    $2,550,000  to    $53,492,000,  an  increase  of 
20  times.     In   1904  the  local  bond  issues  for   roads   and    bridges 
amounted  to  $3,530,000,  but  in  1915  amounted  to  about  $40,000,000. 
In  1904  the  expenditure  for  roads  under  state  supervision  was  6  per 
cent  of  the  total  road  expenses;  but  in  1915  it  was  30  per  cent,  or 
more  than  the  total  expenditure  for  roads  in  1904. 

In  1904  about  one  fourth  of  the  total  expenditures  for  roads  and 
bridges  was  paid  in  labor.  From  1904  to  1915,  while  the  total 
expenditures  for  roads  and  bridges  have  increased  3J  times,  the  por- 
tion from  local  bond  issues  about  1 1  times,  and  that  from  state  aid 
20  times,  the  portion  from  the  labor  road-tax  decreased  about  half. 

In  1904  the  actual  cash  road  and  bridge  expenditure  in  the 
United  States  averaged  slightly  less  than  $28  per  mile  of  rural  roads; 
but  in  1915  it  had  increased  to  an  average  of  $109  per  mile  of  road. 

55.  The  magnitude  of  the  above  sums  shows  the  importance  of 
the  present  road  expenditures,  and  also  the  probability  that  such 
expenditures  will  increase  greatly  in  the  future.     Road  construction 
and  maintenance  is  already  a  matter  of  great  importance  to  the  public 
and  to  the  engineering  profession,  and  is  likely  to  increase  rapidly. 


CHAPTER  II 
ROAD  LOCATION 

58.  ELEMENTS  INVOLVED.  In  general  the  determination  of  the 
best  location  for  a  road  requires  a  study  of  the  topographical  fea- 
tures of  the  region  through  which  the  road  is  to  pass,  and  also  an 
investigation  of  the  nature  and  amount  of  the  traffic  to  be  provided 
for.  Viewed  as  a  question  of  economics,  the  best  location  is  that 
for  which  the  sum  of  the  interest  on  the  cost  of  construction  and  of 
the  annual  cost  of  maintaining  the  road  and  of  conducting  trans- 
portation over  it,  is  a  minimum.  The  location  of  a  wagon  road  is 
not,  however,  entirely  a  question  of  economics,  since  the  location 
should  be  made  with  reference  to  the  convenience  and  comfort, 
and  perhaps  also  to  the  pleasure,  of  those  who  use  it;  and  is  frequently 
more  of  a  social  question  than  one  of  economics.  Only  the  economic 
features  of  location  will  be  considered  here,  and  they  but  briefly. 

However,  in  locating  a  new  road,  it  is  well  to  remember  that  the 
location  will  probably  serve  for  many  generations,  and  perhaps  for  all 
time,  as  the  growing  importance  of  the  surrounding  country  and  the 
location  of  buildings  and  of  division  lines  of  the  land  with  reference 
to  the  road  make  it  increasingly  more  difficult  and  expensive  to  change 
the  location.  Thus  the  location  of  a  road  is  a  field  where  costly 
errors  and  permanent  blunders  may  creep  in  and  forever  fasten 
themselves  upon  the  road  and  its  users;  and,  worst  of  all,  these  errors 
become  more  costly  as  the  use  of  the  road  increases. 

Over  most  of  the  United  States,  the  roads  are  in  the  main  already 
located,  and  the  necessity  for  the  location  of  new  ones  does  not  often 
arise;  and  hence  as  a  rule,  the  only  application  of  the  principles  of 
economic  location  will  be  in  the  re-location  of  comparatively  short 
stretches  of  road.  The  original  location  may  have  been  fit  and  proper 
when  the  region  was  new  and  undeveloped,  but  the  increase  in  the 
amount  and  the  change  in  the  character  of  the  traffic  may  justify  a 
very  considerable  change.  There  are  many  rural  roads  that  could 
be  materially  improved  by  a  careful  re-location. 

41 


42  ROAD   LOCATION  [CHAP.    II 

59.  Rural  roads  are  used  by  both  horse-drawn  and  motor-driven 
vehicles;  and  strictly  each  class  of  vehicles  should  be  considered  in 
solving  problems  of  road  location.     However,  the  passenger  auto- 
mobile need  not  be  considered,  since  with  the  variable  speed  and  high 
power  of  its  engine,  it  can  overcome  any  grade  that  can  be  econom- 
ically used  by  horse-drawn  vehicles,  and  since  the  cost  of  transpor- 
tation by  a  motor-driven  vehicle  is  small  it  may  be   neglected   in 
computing  the  effect  of  slight  differences  in  distance;  and  therefore 
the  passenger  automobile  may  be  disregarded  in  problems  of  road 
location.     Automobile  trucks  need  not  be  considered  for  the  same 
reasons  as  above,  and  also  as  they  are  much  less  common  on  typical 
rural  roads  than  horse-drawn  vehicles. 

60.  The  principles  to  be  observed  and  the  methods  to  be  em- 
ployed in  making  the  location  of  a  wagon  road  are  substantially  the 
same  as  those  used  in  the  location  of  a  railroad.     The  method  of 
examining  the  country  and  of  making  surveys  will  not  be  considered 
here,  as  such  subjects  are  elaborately  presented  in  treatises  on  rail- 
road location. 

The  fundamental  principles  applicable  in  locating  a  new  rural  road 
or  in  improving  an  old  one  will  be  briefly  considered;  but  no  hard 
and  fast  rules  can  be  laid  down,  for  each  road  must  be  designed  for 
the  place  it  is  to  occupy  and  the  service  it  is  to  render.  In  the  loca- 
tion of  any  road  there  will  always  be  an  opportunity  to  exercise 
keen  insight  and  good  judgment. 

The  subject  will  be  considered  under  the  five  heads:  distance, 
grades,  curves,  width,  and  placing  the  line. 

61.  DISTANCE.     Other  things  being  equal,  the  shorter  the  road 
the  better,  since  any  unnecessary  length  causes  a  constant  threefold 
waste:   (1)  the  interest  on  the  cost  of  constructing  the  extra  length; 
(2)  the  ever-recurring  cost  of  repairing  it,  and  (3)  the  time  and  labor 
employed  in  traveling  over  it.     However,  the  advantage  of  straight- 
ness,  i.  e.,  of  shortness,  is  usually  greatly  over-estimated.     The  dif- 
ference in  length  between  an  absolutely  straight  line  and  one  deflect- 
ing a  little  to  one  side  is  not  very  great.     For  example,  in  Fig.  3,  if 

A  B  =  B  C  =  1,000  feet,  and  B  D  '=  10 
feet,  the  line  A  B  C  is  only  one  tenth  of  a 
foot  longer  than  the  line  ADC.  If 
A  B  =  B  C  =  1  mile,  and  B  D  =  300 
feet,  the  line  A  B  C  is  only  17  feet  longer 

than  ADC.     "  If  a  road  between  two  places  ten  miles  apart  were 
made  to  curve  so  that  the  eye  could  nowhere  see  more  than  a  quarter 


DISTANCE  43 


of  a  mile  of  it  at  once,  its  length  would  exceed  that  of  a  perfectly 
straight  road  between  the  same  points  by  only  about  one  hundred 
and  fifty  yards/* 

One  of  the  most  common  defects  of  ordinary  country  roads  is 
that  distance  has  been  saved  by  a  disregard  of  the  desirability  of 
easy  gradients.  The  curving  road  around  a  hill  may  often  be  no 
longer  than  the  straight  one  over  it.  The  latter  is  straight  only 
with  reference  to  the  horizontal  plane,  but  curved  as  to  the  vertical 
plane;  while  the  former  is  curved  as  to  the  horizontal  plane,  but 
straight  as  to  the  vertical  plane.  Both  lines  curve,  and  the  one 
passing  over  the  hill  is  called  straight  only  because  its  vertical  curva- 
ture is  less  apparent  to  the  eye. 

62.  Value  of  Saving  Distance.  Theoretically  the  value  of  a  dif- 
ference in  length  may  be  computed  by  determining  (1)  the  amount  of 
traffic,  (2)  the  cost  per  ton-mile,  and  (3)  the  total  coat  of  conducting 
the  traffic;  and  then  assuming  that  the  value  of  any  difference  of 
length  is  to  the  total  cost  of  transportation  as  the  difference  of  the 
length  is  to  the  total  length.  If  the  annual  cost  of  conducting  trans- 
portation over  a  given  road  is  known,  then  this  cost  divided  by  the 
length  of  the  road  gives  the  annual  interest  upon  the  sum  that  may 
be  reasonably  expended  in  shortening  the  road  1  mile,  i.  e.,  the  value 
of  a  saving  of  a  mile  of  distance;  and  of  course  dividing  this  sum  by 
the  number  of  feet  in  a  mile  will  give  the  value  of  saving  1  foot  of 
distance. 

Unfortunately  it  is  not  possible  to  determine  the  amount  of 
traffic  with  any  considerable  degree  of  accuracy.  At  some  railroad 
stations  the  sole  freight  shipped  out  is  agricultural  produce,  in  which 
case  the  traffic  over  any  particular  wagon  road  can  be  approximated 
by  distributing  the  shipments  according  to  'the  contributing  area. 
The  average  load  can  be  determined  with  sufficient  accuracy  by  con- 
sulting the  records  of  the  grain  dealers.  In  addition  to  the  above, 
which  may  be  called  the  heavy  freight  traffic,  there  is  a  considerable 
amount  of  light  freight  and  passenger  traffic  which  would  be  bene- 
fited by  a  saving  of  distance. 

For  the  sake  of  working  out  an  example,  it  will  be  assumed  that 
the  cost  of  transportation  is  10  cents  per  ton-mile.  This  cost  is 
made  up  of  the  cost  of  loading  and  unloading,  of  driving,  of  feed, 
and  of  wear  and  tear  of  horses,  wagon,  and  harness.  The  cost  of 
loading  and  unloading  is  independent  of  distance.  The  cost  of 
driving  nominally  varies  as  the  time,  i.  e.,  as  the  distance  (see  third 
paragraph  of  §  63).  The  cost  of  wear  and  tear  varies  as  the  distance; 


44  KOAD   LOCATION  [CHAP.   II 

but  the  cost  of  feed  does  not  so  vary.  It  is  impossible  to  assign 
reliable  values  to  these  several  factors  of  the  cost,  but  it  is  certain 
that  only  part  of  the  cost  of  transportation  varies  as  the  distance; 
and  for  the  sake  of  completing  the  illustration,  it  will  be  assumed  that 
8  cents  per  ton-mile  varies  as  the  distance.  This  sum  multiplied  by 
the  number  of  tons  passing  over  the  road  in  a  year  will  give  the  sum 
that  may  be  spent  annually  to  secure  a  saving  of  1  mile  of  distance. 
For  example,  a  road  leading  to  a  certain  village  was  originally 
laid  out  on  the  east  and  north  sides  of  a  quarter-section,  but  on 
account  of  low  ground  on  the  northeast  corner  another  road  was 
opened  on  the  south  and  west  sides.  The  quarter-section  was  one 
large  field.  How  much  expense  would  the  traffic  justify  in  order  to 
secure  a  road  diagonally  through  the  quarter-section.  The  heavy 
freight  traffic  was  approximately  3,000  loads  of  1  ton  each  per  annum. 
The  annual  value  of  saving  1  mile  would  then  be  8  cents  X  3,000  = 
$240.  The  saving  in  distance  by  going  through  the  quarter-section 
is  0.29  mile;  and  the  annual  value  of  saving  this  distance  is  $240  X  0.29 
=  $69.60.  The  diagonal  road  would  occupy  2\  acres  less  land 
than  the  longer  one;  and  as  the  land  rented  for  $3  per  acre,  this 
adds  $3  X  2  J  =  $7  per  annum  to  the  value  of  the  diagonal  road.  The 
annual  saving  from  these  two  items  is  then  $69.60  +  $7.00  =  $76.60. 
This  is  the  interest  at  5  per  cent  on  $1,532,  which  sum,  according  to 
the  above  computations,  could  be  borrowed,  and  used  to  secure  this 
improvement,  and  the  community  be  no  worse  off  financially. 

In  addition,  there  would  be  some  advantage  to  the  light  freight 
and  passenger  traffic  by  shortening  the  road,  but  it  was  difficult,  if  not 
impossible,  to  estimate  this  saving;  and  as  the  benefit  per  trip 
would  probably  be  less  than  for  the  heavy  freight  traffic,  it  was 
neglected.  There  would  be  a  slight  saving  in  the  cost  of  mainte- 
nance of  the  shorter  road,  as  in  this  case  the  soil  and  drainage  was 
as  good  on  one  line  as  on  the  other.  Further,  there  would  be  some 
saving  on  the  return  trip  by  the  shorter  road.  On  the  other  hand, 
it  is  probable  that  the  smaller  number  of  acres  required  for  the 
diagonal  road  would  cost  at  least  as  much  as  for  the  road  around 
the  quarter-section,  owing  to  the  farmers'  justifiable  dislike  for  non- 
rectangular  fields,  and  because  the  diagonal  road  would  divide  the 
quarter-section. 

63.  There  are  several  matters  that  materially  affect  the  relia- 
bility of  the  method  of  the  above  investigation.  In  the  first  place, 
the  cost  of  transportation  can  not  be  known  with  any  degree  of 
reliability.  The  farmers  concerned  would  stoutly  contend  that  the 


DISTANCE  45 


price  assumed  above  is  much  too  great;  while  freighters  would 
claim  that  it  was  too  low  (§  4-9). 

In  the  second  place,  not  all  of  the  computed  annual  saving  is 
available  for  making  the  improvement,  since  some  of  it  should  be 
set  aside  to  form  a  sinking  fund  to  be  used  ultimately  in  extinguish- 
ing the  debt.  It  is  not  the  part  of  wisdom  to  extend  the  debt  very 
far  into  the  future,  since  the  conditions  may  materially  change. 
For  example,  a  new  railroad  may  divert  the  traffic  from  this  par- 
ticular road,  or  improvements  in  the  condition  of  the  surface  of  the 
road  may  decrease  the  cost  of  transportation, — either  of  which 
would  decrease  the  value  of  the  proposed  improvement.  Of  course, 
certain  contingencies  may  increase  the  traffic  and  thereby  add  to 
the  value  of  the  improvement;  but  it  is  not  wise  to  incur  a  definite 
debt  for  an  equal  and  somewhat  problematic  saving.  Road  reformers 
sometimes  overlook  the  fact  that  interest  is  a  yearly  charge  and  that 
the  debt  must  finally  be  paid. 

In  the  third  place,  the  cost  of  transportation  does  not  necessarily 
vary  proportionally  to  the  distance,  as  was  assumed  above.  If  the 
difference  in  distance  is  sufficient  to  make  a  difference  of  one  trip 
per  day,  then  the  value  of  the  saving  in  distance  is  tangible;  but 
where  the  saving  in  length  is  insufficient  for  an  additional  trip,  the 
value  of  the  difference  in  distance  depends  upon  the  value,  for 
other  work,  of  the  small  portions  of  time  of  men  and  teams  which 
may  be  saved  by  the  shorter  route, — a  value  which  exists,  but  which 
is  difficult  to  estimate. 

Therefore  any  estimate  as  to  the  value  of  a  saving  of  distance  is 
necessarily  only  a  rough  approximation;  and  at  best  it  should  be 
used  only  as  a  guide  to  the  judgment. 

64.  The  problem  to  find  the  value  of  saving  distance  is  very  dif- 
ferent for  wagon  roads  than  for  railroads.     In  the  case  of  railroads 
the  cost  of  the  various  elements  has  been  carefully  investigated  for 
many  years,  and  the  transportation  is  all  conducted  under  a  single 
management  and  by  the  same  party  that  maintains  the  road  surface; 
while  in  the  case  of  wagon  roads,  a  multitude  of  private  parties 
conduct  the  transportation  under  various  conditions,  and  the  main- 
tenance of  the  road  is  in  the  hands  of  public  officials. 

65.  GRADE.    A  level  road  is  most  desirable;    but  as  it  can 
seldom  be  obtained,  we  must  investigate  the  effect  of  grades  upon 
the  cost  of  constructing  and  operating  the  road,  and  also  determine 
what  is  the  steepest  allowable  grade. 

The  grade  may  be  reduced  (1)  by  going  round  the  hill  or  by 


46  fcOAb  LOCATION  [CHAP,  n 

zigzagging  up  the  slope,  or  (2)  by  cutting  down  the  hill.  If  the 
slope  to  be  ascended  is  a  long  one,  the  first  method  must  be  em- 
ployed ;  but  if  the  grade  is  short,  the  second  is  usually  the  cheaper. 
Increasing  the  length  adds  to  the  cost  of  construction  and  of  trans- 
portation, while  cutting  down  the  hill  adds  only  to  the  cost  of  con- 
struction. The  maintenance  of  the  longer  and  flatter  line  may 
cost  either  more  or  less  than  the  shorter  and  steeper  one  according 
to  the  circumstances  of  the  case.  In  a  broken  or  rough  country, 
a  proper  adjustment  of  the  grades  is  the  most  important  part  of  the 
art  and  science  of  road  building,  and  the  better  the  road  surface  the 
more  necessary  is  such  an  adjustment. 

66.  All  grades  are  objectionable  for  two  distinct  reasons,  viz.: 
because  a  grade  increases  the  amount  of  power  required  to  move  a 
load  up  it,  and  because  a  grade  may  be  so  steep  as  to  limit  the  amount 
of  the  load  that  can  be  moved  over  the  road.     The  first  applies  to 
all  grades  whatever  their  rate  or  height;    while  the  latter  applies 
only  to  the  steepest  grade  on  the  road,  and  in  a  measure  is   inde- 
pendent of  its  height  and  depends  only  on  its  rate.     At  present  only 
the  first  objection  to  grades  will  be  considered;    and    subsequently 
the  second  objection  will  be  discussed  (§  74). 

67.  Ordinary   Effect   of   Grade.    Table  9,  page  24,  shows  the 
load  (in  terms  of  the  weight  of  the  horse)  which  a  horse  with  a  pull 
equal  to  one  tenth  of  its  weight  can  draw  up  various  grades  on  several 
road  surfaces.     To  emphasize  the  effect  of  the  grade  upon  the  load, 
the  same  data  are  presented  in  a  slightly  different  form  in  Table  10, 
page  25,  which  shows  at  a  glance  the  load  on  any  grade  in  terms  of 
the  load  on  the  level.     Tables  9  and  10  show  that  the  better  the  con- 
dition of  the  road  surface,  i.  e.,  the  less  the  rolling  resistance,  the 
more  objectionable  a  grade.     For  example,  according  to  Table  10, 
on  iron  rails  on  a  3  per  cent  grade  a  horse  can  draw  only  10  per  cent 
as  much  as  on  a  level;  while  on  a  water-bound  macadam  road  on  a 
3  per  cent  grade  it  can  draw  25  per  cent  as  much  as  on  a  level. 

A  horse  can  occasionally  and  for  a  short  time  exert  a  pull  equal 
to  more  than  one  tenth  of  its  weight.  If  the  grade  is  not  too  long,  a 
horse  can  safely  exert  a  force  equal  to  one  quarter  of  its  weight, 
and  in  emergencies  one  half. 

To  move  a  load  over  an  ordinary  earth  road  requires  a  tractive 
force  of  100  Ib.  per  ton  (see  Table  8,  page  21);  and  therefore  a  team 
of  1200-lb.  horses  exerting  a  force  equal  to  one  tenth  of  their  weight 
can  draw  2.4  tons  on  the  level.  The  reserve  power  to  take  the 
load  up  the  hill  is  (0.25  -  0.10)  X  1200  X  2  =  360  pounds.  The 


GRADE  47 


total  load  to  be  carried  up  the  grade  is  the  wagon  and  its  load  plus 
the  weight  of  the  team,  or  2.4  +  (1200  X  2  -f-  2000)  =  3.6  tons.  The 
grade  resistance  is  20  Ib.  per  ton  for  each  per  cent  of  inclination 
(§23);  and  the  grade  resistance  for  this  load  on  a  1  per  cent  grade 
is  3.6  X  20  =  72  Ib.  Therefore,  the  grade  up  which  a  pull  of  360  Ib. 
will  take  the  3.6  tons  is  360  -f-  72  =  5  per  cent,  which  is  the  maximum 
permissible  grade  for  an  earth  road  in  ordinary  condition.  The 
team  could  probably  pull  this  load  up  400  to  500  feet  of  such  a  grade. 

By  the  same  method  of  analysis,  the  load  for  the  same  team  on  a 
level,  muddy  earth  road  having  a  tractive  resistance  of  200  Ib.  per 
ton  is  1.2  tons,  and  the  maximum  permissible  grade  is  7.5  per  cent. 

For  a  water-bound  macadam  road  having  a  tractive  resistance  of 
33  Ib.  per  ton,  the  load  on  the  level  is  7.3  tons,  and  the  permissible 
maximum  grade  is  2.2  per  cent. 

68.  What  load  can  the  above  team  take  up  a  4  per  cent  maxi- 
mum grade  on  a  water-bound  macadam  road  having  a  tractive 
resistance  of  33  Ib.  per  ton?    The  grade  resistance  is  20  X  4  =  80  Ib. 
per  ton;  and  the  tractive  resistance  is  33  Ib.  per  ton;  therefore  the 
total  resistance  is  80  +  33  =  113  Ib.  per  ton.     The  maximum  tractive 
power  of  the  team  is  equal  to  one  quarter  of  its  weight,  or  600  Ib.; 
and  the  grade  resistance  for  the  weight  of  the  team  =  2400  -r-  2000  X 
80  =  96  Ib.;  therefore  the  net  tractive  power  of  the  team  is  600  — 
96  =  504  Ib.     Then  the  weight  of  the  wagon  and  the  load  which  the 
team  can  draw  up  this  grade  is  504  -f-  113  =  4.4  tons. 

69.  Rise  and  Fall.    By  rise  and  fall  is  meant  the  vertical  height 
through  which  the  load  must  be  lifted  in  passing  over  the  road  in  each 
direction.     One  foot  of  rise  and  fall  is  a  foot  of  ascent  with  its  cor- 
responding foot  of  descent.     In  passing  over  a  ridge  10  feet  high 
standing  in  the  middle  of  a  level  plain,  there  is  only  10  feet  of  rise 
and  fall;   and  not  10  feet  of  rise  plus  10  feet  of  fall.     If  the  road  is 
level,  Fig.  4,  then  an  elevation  or  depression  of,  say,  1  foot  produces 


FIG.  4.  Fio.  5. 

literally  1  foot  of  rise  and  a  corresponding  foot  of  fall;  but  if  the 
road  is  on  a  steep  grade,  Fig.  5,  an  elevation  of  1  foot  above  the  grade 
line  or  of  a  like  amount  below  the  grade  line,  literally  speaking, 
produces  no  rise  and  fall,  because  in  either  case  it  is  a  continuous 


48  ROAD   LOCATION  [CHAP.    II 

up  grade.  However,  as  far  as  operation  is  concerned,  the  two 
cases  are  exactly  alike,  and  each  has  a  foot  of  rise  and  fall. 

Rise  and  fall  is  measured  by  the  number  of  vertical  feet  of  rise, 
as  shown  by  the  differences  of  elevation  on  the  profile. 

70.  The  introduction  of  rise  and  fall  is  a  question  either  (1) 
between  the  increased  cost  of  operation  and  the  increased  cost  of 
construction  required  to  fill  up  the  hollow  or  to  cut  down  the  hill, 
or  (2)  between  the  cost  of  operation  of  the  rise  and  fall  and  of  the 
increased  distance  necessary  to  go  around  the  obstruction. 

The  following  example  is  often  cited  as  showing  the  improve- 
ment that  can  be  made  in  locating  roads.  "  An  old  road  in  Anglesea 
rose  and  fell  between  its  extremities,  24  miles  apart,  a  total  vertical 
amount  of  3,540  feet;  while  a  new  road  laid  out  by  Telford  between 
the  same  points,  rose  and  fell  only  2,257  feet;  so  that  1,283  feet  of 
vertical  height  is  now  done  away  with,  which  every  horse  passing 
over  the  road  had  previously  been  obliged  to  ascend  and  descend 
with  its  load.  The  new  road  is,  besides,  more  than  two  miles  shorter. 
Such  is  one  of  the  results  of  the  labors  of  a  skilful  road  maker." 
The  road  may  have  been  economically  re-located,  but  the  citation 
fails  to  show  whether  the  increased  cost  of  construction  to  eliminate 
rise  and  fall  was  justified  by  the  decreased  cost  of  operation. 

The  following  example  from  the  same  author,  also  frequently 
quoted,  shows  that  rise  and  fall  was  eliminated  by  increasing  the 
distance,  although  no  attempt  is  made  to  show  that  the  increased 
distance  was  more  economical  than  the  rise  and  fall  thereby  elimi- 
nated. "  A  plank  road,  laid  out  between  Cazenovia  and  Chitten- 
ango,  N.  Y.,  is  an  excellent  exemplification  of  the  true  principles  of 
road  making.  Both  these  villages  are  situated  on  the  Chittenango 
Creek,  the  former  being  800  feet  higher  than  the  latter.  The  most 
level  wagon  road  between  these  villages  rises  more  than  1,200  feet  in 
going  from  Chittenango  to  Cazenovia,  and  rises  more  than  400  feet  in 
going  from  Cazenovia  to  Chittenango,  in  spite  of  this  latter  place 
being  800  feet  lower.  It  thus  adds  one  half  to  the  ascent  and  labor 
going  in  one  direction;  and  in  the  other  direction  it  goes  up  hill  one 
half  the  height,  which  should  have  been  a  continuous  descent.  The 
line  of  the  plank  road  by  following  the  creek  (crossing  it  five  times) 
ascends  only  the  necessary  800  feet  in  one  direction,  and  has  no 
ascents  in  the  other,  with  two  or  three  trifling  exceptions  of  a  few 
feet  in  all,  admitted  in  order  to  save  expense.  There  is  a  nearly 
vertical  fall  in  the  creek  of  140  feet.  To  overcome  this,  it  was 
necessary  to  commence  far  below  the  falls,  to  climb  up  the  steep 


RISE   AND    FALL  49 


hillside,  following  up  the  sides  of  the  lateral  ravines  until  they  were 
narrow  enough  to  bridge,  and  then  turning  and  following  back  the 
opposite  sides  till  the  main  valley  was  again  reached.  The  extreme 
rise  is  at  the  rate  of  1  foot  to  the  rod  (1  in  16J),  and  this  only  for 
short  distances,  and  in  only  three  instances,  with  a  much  less  grade 
or  a  level  intervening." 

71.  Classes  of  Rise  and  Fall.     In  discussing  the  effect  of  rise  and 
fall  upon  the  operation  of  a  road,  a  distinction  must  be  made  be- 
tween three  classes  of  rise  and  fall,  as  follows: 

Class  A.  Rise  and  fall  on  grades  at  a  less  slope  than  the  angle 
of  repose  (the  grade  on  which  a  vehicle  by  its  own  weight  will  main- 
tain a  uniform  speed),  and  so  situated  as  not  to  require  any  addition 
to  the  total  power  required  to  move  a  load  over  the  road. 

Class  B.  Rise  and  fall  on  grades  so  steep  as  to  require  either  the 
holding  back  of  the  load  by  the  team  or  the  application  of  brakes. 

Class  C.     Rise  and  fall  on  the  maximum  grade. 

72.  An  example  of  the  first  class  of  rise  and  fall  is  shown  in 
Fig.  6.     The  team  is  relieved  on  the  down  grade  an  amount  exactly 

equal   to   the   extra   tax   upon   the   up     ^_ -^ 

grade,    and    the    only    effect   upon    the  ^^^ 

team  is  that  the  effort  is  concentrated 

on  the  up  grade  instead  of  being  uniformly  distributed  over  the  road ; 
but  as  the  slope  is  assumed  to  be  equal  to  or  less  than  the  angle  of 
repose,  the  maximum  effort  is  equal  to  or  less  than  twice  the  normal. 
If  the  grade  line  rises  above  the  level  instead  of  dipping  below  it, 
the  case  is  not  changed  except  that  the  rise  is  a  little  more  unfavor- 
able, since  the  team  has  no  relief  before  the  increase  in  effort  is 
required.  Therefore  this  class  of  rise  and  fall  costs  little  or  nothing. 

In  the  preceding  examples,  a  change  of  velocity  would  alter  the 
power  required  at  any  particular  instant;  but  in  wagon-road  traffic 
the  speed  is  always  small  and  consequently  the  effect  of  variations 
of  speed  are  quite  small,  and  may  be  entirely  neglected.  On  rail- 
roads a  variation  of  the  velocity  materially  affects  the  cost  of  rise 
and  fall. 

If  the  grade  is  greater  than  the  angle  of  repose,  the  team  in  descend- 
ing must  hold  back  the  load,  which  is  lost  energy,  or  brakes  must 
be  applied,  which  tend  to  destroy  the  road;  and  in  ascending,  the 
demand  upon  the  team  is  greater  than  twice  the  normal.  There- 
fore in  either  case  this  class  of  rise  and  fall  adds  to  the  cost  of  oper- 
ating the  road. 

If  the  grade  is  the  maximum,  it  may  be  sufficient  to  limit  the 


50  KOAD    LOCATION  [CHAP.    II 

amount  of  the  load  a  team  may  draw  over  the  more  level  portions 
of  the  road,  and  therefore  greatly  add  to  the  cost  of  transportation. 
As  a  chain  is  no  stronger  than  its  weakest  link,  so  a  road  is  no  better 
than  its  steepest  grade. 

73.  Cost  of  Rise  and  Fall.    What  does  it  cost  to    develop  the 
power  required  to  haul  a  load  up  a  grade  less  than  the  grade  of 
repose?     In  other  words,  what  is  the  cost  of  Class  A  rise  and  fall? 

The  cost  of  transportation  consists  chiefly  of  the  cost  of  driving, 
of  feed,  and  of  the  wear  and  tear  on  the  team.  Usually  the  cost  of 
driving  will  be  approximately  half  of  the  total  cost  of  transportation; 
and  as  a  team  can  draw  a  load  up  the  grade  of  repose  at  practically 
the  same  speed,  at  least  for  short  stretches,  as  upon  the  level,  there 
will  usually  be  no  material  increase  in  the  cost  of  driving.  Even 
though  the  team  may  travel  slower  because  of  the  grade,  the  cost 
of  the  increased  time  can  scarcely  be  computed  because  of  the  impos- 
sibility of  determining  the  value  of  fractions  of  time  for  other  pur- 
poses. The  cost  of  wear  and  tear  on  the  team  and  part  of  the  cost  of 
feed  must  vary  approximately  as  the  total  power  developed.  There- 
fore the  conclusion  may  be  drawn  that  rise  and  fall  belonging  to 
Class  A  will  not  add  appreciably  to  the  cost  of  transportation. 
This  conclusion  is  corroborated  by  the  popular  belief  that  a  gently 
undulating  road  is  less  fatiguing  to  horses  than  one  which  is  perfectly 
level.  The  argument  in  support  of  this  belief  is  that  alternations  of 
ascents,,  descents,  and  levels  call  into  play  different  muscles,  allowing 
some  to  rest  while  others  are  exerted,  and  thus  relieving  each  in  turn. 
The  argument  is  false,  and  probably  originated  in  the  prejudices  of 
man  in  his  quest  for  variety,  rather  than  in  the  anatomy  of  the  horse; 
but  the  above  theory  would  not  have  gained  its  wide  popularity 
if  a  gently  undulating  road  were  appreciably  more  fatiguing  to  a 
horse  than  a  perfectly  level  one.  A  perfectly  level  road  is  the  best 
for  ease  of  transportation. 

74.  Limiting  Effect  of  a  Grade.    If  the  grade  is  steeper  or  longer 
than  that  up  which  the  team  can  draw  the  normal  load  by  exerting 
twice  the  tractive  power  required  on  the  level,  i.  e.,  if  the  rise  and  fall 
belongs  to  Class  C,  then  the  grade  has  the  effect  of  limiting  the  load 
that  can  be  drawn  over  the  level  portion  of   the  road,  and  conse- 
quently increases  the  cost  of  transportation.     The  load  which  a  team 
can  draw  up  any  grade  can  be  approximately  computed  as  in  §  68. 
If  the  load  that  can  be  drawn  up  any  particular  grade  is,  for  example, 
three  fourths  of  the  normal  load  on  the  level;  then  it  will  cost  as  much 
to  haul  three  fourths  of  a  load  with  this  grade  as  a  full  load  without 


RISE    AND    FALL  51 


the  grade.  If  the  cost  with  a  grade  less  than  the  maximum  is  10 
cents  per  ton-mile  (§  4-7  and  §  12),  then  the  cost  with  the  maximum 
grade  will  bel0^f=13j  cents  per  ton-mile;  and  therefore  for 
each  ton  going  over  the  road,  the  maximum  grade  adds  3|  cents  per 
ton  mile.  In  determining  the  amount  of  traffic,  only  full  loads 
should  be  included;  but  notice  that  the  full  load  varies  with  the 
speed.  A  ton  may  be  a  full  load  at  3  miles  per  hour,  while  half  a 
ton  may  be  a  full  load  at  6  miles  per  hour. 

Knowing  the  load  on  the  maximum  grade  and  also  the  cost  per 
ton-mile  for  a  level  road  or  for  a  grade  less  than  the  maximum,  the 
justifiable  expenditure  to  reduce  the  maximum  grade  may  be  com- 
puted as  follows:  The  difference  in  cost  per  ton-mile  with  and  with- 
out the  maximum  grade  may  be  determined  as  in  the  preceding 
paragraph;  and  this  multiplied  by  the  number  of  loads  annually 
going  over  the  road  gives  the  sum  that  may  be  spent  annually  to 
reduce  the  maximum  grade  to  the  lesser  value.  This  sum  may  be 
used  to  pay  interest  on  the  cost  of  cutting  down  the  hill  or  of  filling 
up  the  hollow. 

The  data  are  so  uncertain  that  the  result  must  be  regarded  only 
as  a  rough  approximation;  and  yet  it  is  worth  while  to  make  an 
investigation  as  above  as  a  guide  to  the  judgment. 

75.  Class  B  rise  and  fall  is  intermediate  between  Class  A  and 
Class  C,  and  its  cost  is  even  more  difficult  to  compute  than  that  of 
Class  C.  The  chief  difficulty  is  in  determining  the  relative  cost  of 
developing  power  on  a  level  and  up  a  grade.  Only  an  estimate  can 
be  made,  and  the  estimate  will  vary  greatly  with  the  point  of  view. 
For  example,  farmers  usually  have  a  surplus  of  power  (horses)  as 
far  as  transportation  is  concerned,  and  therefore  they  would  con- 
sider a  slight  increase  in  the  demand  for  power  as  a  matter  of  small 
moment.  Again,  teamsters  differ  greatly  as  to  what  is  a  proper 
or  economical  load  for  a  horse,  and  also  as  to  the  effect  of  a  tem- 
porary over-load. 

There  are  two  methods  of  computing  the  cost  of  this  class  of 
rise  and  fall,  neither  of  which  is  more  than  roughly  approximate. 

1.  Assume  that  the  cost  of  Class  B  rise  and  fall  bears  the  same 
relation  to  that  of  Classes  A  and  C,  that  the  grade  of  B  bears  to  that 
of  A  and  C.     Then  if  the  grade  for  Class  B  is  only  a  little  greater 
than  the  angle  of  repose,  the  cost  is  only  a  trifle  greater  than  that 
of  Class  A ;   and  if  the  grade  is  nearly  a  maximum,  then  the  cost  of 
the  rise  and  fall  closely  approximates  that  of  Class  C. 

2.  Assume  that  the  energy  developed  on  a  grade  over  and  above 


52  ROAD   LOCATION  [CHAP.    II 

uhat  required  on  the  grade  of  repose,  costs  the  same  per  unit  as  that 
of  an  equal  amount  of  energy  developed  on  the  level.  For  example, 
assume  that  the  rise  is  1  foot  more  than  the  angle  of  repose;  and 
assume  that  the  cost  of  drawing  a  load  on  a  good  water-bound 
macadam  road  is  5  cents  per  ton-mile  (§  5-6),  and  that  the  tractive 
power  is  40  Ib.  per  ton.  Then,  moving  a  ton  1  mile  will  develop 
5,280  X  40  =  211,200  foot-pounds  of  energy,  which  will  cost  5  cents. 
The  cost  of  1  foot-pound  of  energy,  then,  is  5  4-  211,200  =  0.000,023,7 
cents.  Drawing  a  ton  over  a  rise  1  foot  high  develops  2,000  foot- 
pounds, the  cost  of  which  is  0.000,023,7  -f-  2  X  2,000  =  0.023,7 
cents.  In  going  up  the  above  grade,  the  team  must  develop  enough 
power  to  move  the  load  up  the  grade  of  repose  and  in  addition  must 
develop  enough  to  lift  the  load  1  foot  vertically.  Therefore  the 
cost  of  the  1  foot  of  rise  assumed  above  is  0.023,7  cents  for  each 
ton  going  over  the  road. 

It  was  assumed  above  that  the  load  is  retarded  in  the  descent  by 
the  application  of  brakes;  but  if  the  grade  in  question  is  situated  in 
a  flat  country  where  brakes 'are  not  usually  placed  upon  vehicles, 
the  team  must  hold  back  on  the  descent  an  amount  equal  to  the 
extra  energy  required  on  the  ascent,  and  therefore  the  cost  of  the 
foot  of  rise  and  fall  will  be,  almost  or  quite,  doubled. 

With  data  similar  to  the  above,  and  with  a  knowledge  of  the 
amount  of  traffic,  it  is  a  simple  arithmetical  process  to  compute  the 
sum  that  may  be  spent  annually  to  eliminate  one  or  more  feet  of 
rise  and  fall.  Notice  that  in  this  case  only  the  full  loads  should  be 
considered  (see  the  first  paragraph  of  §  74).  For  example,  assume 
that  a  water-bound  macadam  road  has  a  traffic  of  20  tons  per  day 
one  way  for  300  days  of  the  year,  or  an  annual  traffic  of  20  X  300 
=  6,000  tons.  The  cost  of  a  foot  of  rise  and  fall  per  ton  of  traffic 
is  0.023,7  cents,  and  the  annual  cost  on  this  particular  road  is  0.000,237 
X  6,000  =  $1.42.  This  is  the  amount  which,  according  to  the  above 
investigation,  can  be  spent  annually  to  cut  down  the  hill  or  to  fill 
up  the  hollow  sufficiently  to  eliminate  1  foot  of  rise  and  fall. 

Similarly,  for  an  earth  road  having  a  cost  of  15  cents  per  ton- 
mile  and  a  tractive  power  of  100  Ib.  per  ton,  1  foot  of  rise  costs 
0.028,4  cents,  and  the  foot  of  ascent  assumed  above  will  cost  0.028,4 
cents  for  each  ton  going  over  the  road.  If  this  road  has  a  traffic 
of  5  tons  one  way  for  300  days  of  the  year,  the  annual  cost  of  the 
foot  of  rise  and  fall  is  0.028,4  X  5  X  300  =  44.6  cents,  which  is 
the  sum  that  can  be  spent  annually  to  eliminate  the  foot  of  rise  and 
fall. 


RISE   AND    FALL  53 


From  the  point  of  view  of  the  last  solution,  it  appears  that  the 
cost  of  Class  A  rise  and  fall  increases  with  the  steepness  of  the 
grade,  that  is,  increases  as  the  rate  of  the  grade  approaches  the 
angle  of  repose.  In  all  probability  this  is  correct,  but  all  the  data 
involved  ^re  too  uncertain  to  warrant  any  further  discussion  of 
the  subject  here.  However,  the  engineer  should  bear  such  relations 
in  mind  in  solving  a  particular  problem. 

76.  Distance  vs.  Rise  and  Fall.     In  locating  a  road  the  question 
may  arise  between  the  relative  desirability  of  introducing  rise  and 
fall  and  of  increasing  the  length  of  the  road.     The  problem  then  is 
to  determine  the  relative  value  of  distance  and  of  rise  and  fall. 

If  the  conclusion  in  §  73'  is  correct,  that  the  cost  of  Class  A  rise 
and  fall  is  not  appreciable,  then  the  distance  should  not  be  increased 
at  all  to  eliminate  Class  A  rise  and  fall. 

77.  For  Class  B  rise  and  fall  an  approximate  solution  can  be 
obtained  by  assuming  that  it  costs  the  same  to  develop  a  certain 
amount  of  energy  in  overcoming  Class  B  rise  and  fall  as  to  develop 
a  like  amount  of  energy  in  moving  a  load  on  a  level  road.     This 
assumption  is  probably  reasonably  correct. 

For  example,  the  tractive  resistance  of  the  best  water-bound 
macadam  road  is  33  Ib.  per  ton,  and  the  work  necessary  to  raise  1  ton 
through  1  foot  of  rise  is  2,000  foot-pounds;  therefore  to  develop 
2,000  foot-pounds  of  work  on  a  level  water-bound  macadam  road, 
a  ton  must  be  moved  2,000  -*-  33  =  60  feet.  Hence  the  cost  of 
operating  60  feet  of  distance  on  this  road  may  be  considered  as  equiv- 
alent to  1  foot  of  rise  and  fall.  Therefore  to  eliminate  a  foot  of  rise 
and  fall  of  Class  B,  the  length  of  the  road  may  be  increased  60  feet. 
Table  14  gives  the  corresponding  distance  for  other  road  surfaces.* 

78.  Apparently  writers  on  roads  have  not  made  a  distinction 
between  the  several  classes  of  rise  and  fall.     Herschel  says:  f    "  To 
determine  whether  it  is  more  advisable  to  go  over  than  around  a 
hill,  all  other  considerations  being  equal,  we  have  this  rule:     Call  the 
difference  between  the  distance  around  on  a  level  and  that  over  the 
hill  d  (the  distance  around  being  taken  as  the  greater),  and  call  h 

*  The  above  relations  are  for  a  load  transported  on  wheels.  It  may  be  interesting  to  know 
the  corresponding  relations  for  pedestrians.  The  work  (energy)  required  of  a  man  in  walking 
is  practically  independent  of  the  nature  of  the  road  surface.  A  man  makes  progress  in  walking 
by  allowing  his  body  to  fall  through  a  small  space  and  then  raising  it  again  preparatory  to 
another  fall.  For  an  average  man,  the  energy  expended  in  walking  16  to  20  feet  horizontally 
is  sufficient  to  raise  his  body  through  1  foot  vertically.  Therefore,  for  pedestrians  1  foot  of 
rise  and  fall  is  equivalent  to,  say,  18  feet  of  horizontal  distance. 

t  Clemens  Herschel,  Science  of  Road  Making,  Prize  Essay  of  the  State  Board  of  Agriculture 
of  Massachusetts,  Boston,  1869,  p.  207-63;  revised  edition,  Engineering  News,  New  York, 
1890,  p.  9. 


54  ROAD   LOCATION  [CHAP.   II 

TABLE  14 
HORIZONTAL  DISTANCE  EQUIVALENT  TO  1  FOOT  OF  CLASS  B  RISE  AND  FALL 

Earth  roads,  muddy (tractive  resistance  200  Ib.  per  ton) ....     10  feet 

ordinary "  100  "           20    " 

dry  and  hard "  80  ....     25   " 

Stone-block  pavement,  best "  40  "          50" 

ordinary; "  80  "  ....     25   " 

Gravel,  best "  50  "  ....     40" 

"      ordinary "  80  . ...     25   " 

Water-bound  macadam  road,  best "  33  "  60" 

ordinary.,  "  50  "  ....     40'" 

Brick  on  concrete "  25  "          80" 

Sheet  asphalt "  20  "  ....  100   " 

Iron  rails,  clean "  10  "  ....  200   " 

the  height  of  the  hill.  Then  in  case  of  a  first-class  road,  we  go  round 
when  d  is  less  than  16ft;  and  in  case  of  a  second-class  road,  we  go 
round  when  d  is  less  than  10ft."  Although  not  specially  so  stated, 
the  above  rule  was  plainly  intended  for  water-bound  macadam 
roads. 

The  above  rule  (which  has  been  frequently  quoted)  recognizes 
no  distinction  between  the  several  classes  of  rise  and  fall.  It  makes 
the  avoidance  of  a  foot  of  rise  in  going  over  a  small  culvert  or  of  a 
foot  of  fall  in  crossing  an  open  ditch,  equally  as  important  as  the 
elimination  of  a  foot  of  rise  and  fall  on  the  maximum  grade.  It  is 
not  possible  to  draw  sharp  lines  between  the  several  classes  of  rise 
and  fall,  but  it  is  certain  that  there  is  a  great  difference  in  cost 
between  a  foot  of  rise  and  fall  on  a  flat  grade  and  the  same  quantity 
on  the  maximum  or  limiting  grade.  Notice  that  the  above  rule 
makes  the  horizontal  distance  equivalent  to  a  foot  o£  rise  much  less 
than  that  stated  in  Table  14. 

79.  Maximum  Grade.     The  fixing  of  the  proper  maximum  or 
ruling  grade  is  the  most  important  matter  connected  with  the  loca- 
tion of  a  road.     To  do  this  intelligently,  the  maximum  grade  must  be 
considered  both  as  an  ascent  and  as  a  descent.     Viewed  as  an 
ascent,  the  maximum  or  ruling  grade  chiefly  concerns  the  draught  of 
heavy  loads;   and  viewed  as  a  descent,  it  chiefly  concerns  the  safety 
of  rapid  traveling.     In  both  respects,   the  effect  of  the  grade  in 
limiting  the  load  depends  upon  its  rate,   and  is  practically  inde- 
pendent of  its  length. 

80.  As  an  Ascent.     The  load  which  a  team  can  draw  over  any 
road  is  determined  by  the  length  and  steepness  of  the  maximum 
grade;    or,  in  other  words,  the  length  and  rate  of  the  permissible 


MAXIMUM   GRADE  55 


maximum  grade  depends  upon  the  endurance  of  the  team.  TL 
method  of  computing  the  load  that  a  team  can  draw  up  any  grade 
was  explained  in  §  68,  page  47.  That  investigation  shows  that  the 
permissible  maximum  grade  varies  greatly  with  the  conditions  of 
the  surface;  and  that  the  better  the  surface  the  less  should  be  the 
ruling  grade.  In  other  words,  unless  the  maximum  grade  is  light, 
the  amount  that  can  be  hauled  on  a  water-bound  macadam  road 
does  not  differ  greatly  from  that  on  an  earth  road. 

A  team  could  probably  pull  the  maximum  load  up  a  stretch 
of  the  maximum  grade  400  to  500  feet  long;  and  if  the  maximum 
grade  does  not  occur  too  often,  it  could  probably  pull  the  load  up  a 
stretch  two  or  three  times  as  long.  On  long  maximum'grades,  it  is 
wise  to  provide  a  little  stretch  of  nearly  level  grade  upon  which  to 
let  the  team  rest.  In  the  above  computation,  the  team  is  assumed 
to  have  a  reserve  power  equal  to  that  exerted  on  the  maximum 
grade;  but  the  power  required  to  start  the  load  may  be  four  or 
five  times  the  normal  tractive  resistance,  and  hence  a  nearly  level 
resting  place  is  required,  so  that  the  team  may  readily  start  the 
load. 

81.  Many  books  on  roads  state  that  if  the  maximum  grade 
is  long,  the  slope  should  be  flattened  toward  the  summit  to  com- 
pensate for  the  decreased  strength  of  the  fatigued  horses.     This 
reasoning  is  incorrect  and  the  remedy  is  impracticable.     The  argu- 
ment is  incorrect  since  it  assumes  that  if  the  horse  is  to  develop 
energy  to  lift  the  load  up  the  incline,  it  should  not  work  at  a  uniform 
rate.     Universally  the  race  horse  goes  fastest  on  the  home  stretch; 
and  if  it  is  urged  to  its  utmost  speed  at  first,  it  is  sure  to  lose  the  race. 
The  recommendation  is  impracticable,  since  the  topography  would 
rarely  permit  the  flattening  of  the  grade  at  the  top  without  increased 
expense,  and  it  would  not  be  wise  to  incur  extra  cost  for  this  pui- 
pose, 

82.  If  the  loads  are  much  heavier  in  one  direction  than  in  the 
other,  it  is  permissible  to  oppose  the  lighter  traffic  with  the  steeper 
ruling  grade. 

83.  As  a  Descent.     Viewed  as  a  descent,  the  maximum  grade 
concerns  chiefly  the  safety  of  rapid  travel.     Many  writers  on  roads 
claim  that  the  descending  grade  should  not  exceed  the  angle  of 
repose,  i.  e.,   should  not  exceed  the  inclination  down  which  the 
vehicle  will  descend  by  its  own  weight.     This  limit  is  impracti- 
cable, since  the  angle  of  repose  varies  with  the  kind  of  vehicle, 
degree  of  lubrication,  amount  of  load,  size  of  wheels,  etc.     Besides, 


56  ROAD  LOCATION  [CHAP.   II 

this  limitation  is  unnecessary,  since  the  resistance  of  traction  in- 
creases as  the  speed,  and  in  going  down  it  is  only  necessary  to 
drive  faster  to  prevent  the  vehicle  from  unduly  crowding  upon 
the  team;  but  of  course  this  remedy  has  its  limitations.  Further, 
the  speed  in  descending  may  be  checked  by  the  application  of  the 
brake;  but  it  should  be  remembered  that  the  use  of  the  brake  is 
detrimental  to  the  road  surface,  particularly  on  the  maximum 
grade. 

Grades  twice  as  steep  as  the  angle  of  repose  are  operated  without 
inconvenience  or  danger.  In  Europe  it  is  usually  assumed  that  on  a 
good  water-bound  macadam  road,  of  which  the  angle  of  repose  is 
about  2  or  2j  per  cent,  a  5  per  cent  grade  is  the  maximum  that  can 
be  descended  safely  at  a  trot  without  brakes;  and,  if  the  stretch 
is  long,  3  per  cent  is  considered  the  maximum  for  safety.  On  moun- 
tain roads  having  a  water-bound  macadam  surface,  freight  wagons 
descend  12  per  cent  grades  by  the  use  of  brakes,  but  only  with 
expert  drivers. 

84.  Safety  at  Summit.    The  grade  each    side  of  the   summit 
should  be  such  that  two  automobiles  approaching  the  summit  should 
be  able  to  see  each  other  when  at  least  200  feet  apart,  which  is  prac- 
tically equivalent  to  limiting  the  grade  for  100  feet  on  each  side  of 
the  summit  to  5  per  cent,  or  in  other  words  to  limiting  the  sum  of 
the  grade  for  100  feet  either  side  of  the  summit  to  10  per  cent.     This 
is  particularly  important  on  a  road  having  only  a  one-track  improved 
surface. 

Where  a  highway  crosses  a  steam  or  electric  railway  on  the  same 
level,  and  where  the  highway  has  a  steep  grade  as  it  approaches  the 
crossing,  there  should  be  sufficient  level  road  at  the  top  of  the  grade, 
say  50  feet,  to  permit  a  wagon  or  an  automobile,  particularly  the 
latter,  to  stand  while  the  train  passes.  If  this  condition  does  not 
obtain,  an  automobilist  is  liable  to  kill  his  engine  at  the  top  of  the 
grade  just  as  he  is  starting  to  cross  the  track  and  just  as  a  train  is 
coming.  Automobilists  quite  frequently  encounter  such  dangerous 
crossings. 

The  crossing  should  be  wide  enough,  say  18  feet,  to  permit  two 
vehicles  to  meet  upon  the  crossing. 

85.  Table  15,  page  57,  are  the  limits  recommended  by  a  special 
committee  of  the  American  Society  of  Civil  Engineers.*     They  are 
presented  here  for  convenience  of  reference  and  comparison. 

*Proc.  Amer.  Soc.  of  C.  E.,  Vol.  42  (1916),  p.  1612. 


MAXIMUM    GRADE  57 


TABLE  15 
MAXIMUM  PERMISSIBLE  GRADE 


Kind  of  Road  Surface. 

Grade. 

Stone  block  with  bituminous  filler 

15 

Gravel                     

12 

Brick  with  bituminous  filler.          

12 

Water-bound  macadam          

12 

Stone  block  with  portland-cement  filler  

9 

g 

Bituminous  concrete 

g 

Portland-cement  concrete                                 .  . 

g 

Bituminous  carpet                                 

6 

Brick  with  portland-cement  filler  

6 

Sheet  asphalt                             

5 

Wook  block                                 

4 

86.  Minimum  Grade.  Considering  only  the  cost  of  transporta- 
tion, a  perfectly  level  road  is  the  best;  but  it  costs  less  to  maintain 
a  road  upon  a  slight  grade  than  one  perfectly  level.  All  roads 
should  be  higher  in  the  center  than  at  the  sides,  so  as  to  shed  the 
rain  to  the  side  ditches,  but  on  any  road  longitudinal  ruts  are  lia- 
ble to  form  and  interfere  with  the  surface  drainage;  and  therefore  if 
the  road  is  perfectly  level  in  its  longitudinal  direction,  its  surface 
can  not  be  kept  free  from  water  without  giving  it  so  great  an  incli- 
nation transversely  as  to  expose  vehicles  to  the  danger  of  overturn- 
ing or  skidding.  On  a  perfectly  level  road,  every  rut  will  hold  water, 
which  will  soak  into  the  road  and  soften  it  whether  it  be  earth  or 
broken  stone;  whereas  with  even  a  slight  longitudinal  grade,  every 
wheel  track  becomes  a  channel  to  carry  off  the  water.  It  is  a  com- 
mon observation  that  earth  roads  running  up  hill  and  down  dale 
have  surfaces  better  to  travel  upon  than  more  level  ones.  This  is 
largely  due  to  the  better  longitudinal  surface  drainage. 

The  harder  the  road  material  the  less  the  necessity  for  longitudi- 
nal drainage  of  the  surface.  An  earth  road  surface  is  certain  to 
wear  into  ruts,  and  hence  is  greatly  benefited  by  having  a  longi- 
tudinal slope.  Gravel  and  broken-stone  roads  are  liable  to  wear 
into  longitudinal  ruts,  and  hence  need  longitudinal  drainage.  Water- 
bound  macadam  roads  built  with  the  hardest  limestones  or  trap  are 
not  easily  worn  into  ruts,  and  therefore  the  necessity  for  a  longi- 
tudinal grade  is  less  with  this  class  of  construction. 

A  longitudinal  grade  decreases  the  cost  of  maintenance,  and  the 
advisability  of  introducing  a  grade  for  such  a  purpose  depends  upon 
the  relative  cost  of  constructing  it  and  upon  the  capitalized  value 


58  ROAD   LOCATION  [CHAP.    II 

of  the  cost  of  maintaining  it.  With  earth  roads  the  expenditures 
for  maintenance  are  ordinarily  too  slight  to  justify  much  expense  in 
securing  a  longitudinal  grade;  but  with  high  class  broken-stone 
roads,  which  naturally  have  a  heavy  traffic,  a  considerable  expense 
to  secure  a  slight  longitudinal  grade  is  usually  justifiable.  Engi- 
neers whose  experience  has  been  largely  upon  railroads  and  canals 
are  prone  to  spend  money  to  secure  an  absolutely  level  road,  where 
a  slight  grade  could  be  secured  at  less  expense.  In  filling  up  a 
hollow  or  cutting  down  a  hill,  the  employment  of  a  light  longitudinal 
grade  may  decrease  the  cost  of  construction  and  also  the  cost  of 
maintenance  without  increasing  the  cost  of  transportation  (§  71-73). 
The  important  principle  to  remember  is  that  a  slight  longitudinal 
grade  is  an  advantage;  although  over  a  long  stretch  of  level  country 
it  may  not  be  practicable  to  secure  it. 

The  following  is  the  minimum  grade  adopted  by  leading  engi- 
neers for  water-bound  macadam  roads:  in  England  1  in  80  or  Ij 
per  cent;  in  France,  by  the  Corps  des  Fonts  et  Chaussees,  1  in  125 
or  0.8  per  cent;  in  the  United  States  1  in  200  or  0.5  per  cent. 

87.  CURVES.     Theoretically   the   shortest   radius    of    curvature 
allowable  on  roads  depends  upon  the  width  of  the  road,  and  upon  the 
maximum  length  of  horse  teams  frequenting  the  road  or  upon  the 
speed  of  the  shorter  teams.     Since  the  length  of  a  four-horse  team  and 
vehicle  is  about  50  feet,  to  permit  such  a  team  to  keep  upon  a  12-foot 
roadway  would  require  a  radius  of  the  inside  of  the  curve  of  about 
100  feet;   on  a  16-foot  roadway  a  radius  of  about  J5  feet  would  be 
required;    and  on  an  18-foot  roadway,  a  radius  of  about  66  feet. 
In  France  the  minimum  radius  is  as  follows:    on  main  and  depart- 
mental roads  of  which  the  trackway  is  20  to  22  feet  wide,  165,  and 
in  extreme  cases  100  feet;    on  principal  country  roads  which  are 
20  feet  wide,  50.     In  Saxony  the  minimum  radius  on  principal  roads 
is  82  feet,  and  on  ordinary  country  roads  it  is  40  feet. 

"  On  mountain  roads  with  grades  of  1  or  2  per  cent,  heavy  teams 
require  curves  of  40  feet  radius,  and  light  ones  30  feet;  and  with 
grades  of  3  or  4  per  cent,  heavy  teams  require  65  and  light  ones  50 
feet."  "  In  extreme  cases  on  mountain  roads  four-  and  six-horse 
teams  haul  maximum  loads  over  16-foot  roads  having  a  radius  at 
their  outer  edge  of  30  feet."  However,  in  this  case  the  roads  on 
the  curves  must  be  level,  as  the  rear  team  is  expected  to  do  all  of 
the  pulling  on  the  curve. 

88.  For  safety  of  automobile-  travel,  curves  should  be  so  flat 
that  two  automobiles  in  approaching  will  be  able  to  see  each  other 


CtJRVES 


when  at  least  200  feet  apart;  and  where  this  is  not  feasible,  a  con- 
spicuous sign  should  be  placed.  When  the  curve  is  located  on  a  grade, 
the  tadius  of  the  curve  should  be  not  less  than  300  or  400  feet,  even 
if  the  view  is  unobstructed. 

At  the  corners  or  intersections  of  roads,  the  hedges,  trees,  etc., 
should  be  removed  so  that  automobilists  approaching  the  corner 
can  have  an  unobstructed  view  of  the  side  road  for  200  or  300 
.  feet. 

89.  90°    Curves.     There    are    many    90°    curves    in    highways, 
especially  in  that  part  of  the  country  where  the  land  was  surveyed 
according  to  the  U.  S.  public  land  system.     If  a  pavement  15  feet 
wide   is    constructed    in 
the  middle  of  a  50-foot 
right-of-way,  and  if   the 
improvement   is    to    be 
kept  within  the  right-of- 
way  at  the  corner,  the  ~t 
radius  of  the  center  line 
of  the  curve  can  be  only 
52.9  feet.     But  by  pur- 
chasing a  comparatively 
small  area  on  the  corner, 
the  length  of  the  radius 
can  be  greatly  increased. 
Fig.  7  shows  a  solution 
of  thi's  problem.*    "  The 
piece  of  land  L  K  M  N 

Contains  Only  0.055  acres,  FIG.  7.— CURVE  OF  PAVED    WAY  AT  90°  CORNER. 

and  often  the  saving  in 

the  decreased  amount  of  paving  will  more  than  pay  for  the  extra 
land.  In  addition  the  right-of-way  is  not  contracted  at  the  corner 
as  it  is  by  the  shorter  radius  curve,  so  that  the  17.5-foot  margin 
between  the  inner  edge  of  the  pavement  and  the  proper  tyline  is  pre- 
served for  use  as  an  earth  road  around  the  corner  as  wfcll  as  on  tan- 
gents." 

Fig.  8,  page  60,  shows  the  solution  of  the  above  problem  at  the 
intersection   of   two   paved   roads,  f     "  Sections   like  GHJKLPQR 

*  H.  E.  Bilger,  Road  Engineer,  Illinois  Highway  Department,  Illinois  Highways,  January, 
1917,  p.  5. 

t  H.  E.  Bilger,  Road  Engineer,  Illinois  Highway  Department,  in  Engineering  News- 
Record,  Vol.  79  (1917),  p.  134. 


60 


ROAD   LOCATION 


[CHAP,  ii 


should  be  built  monolithic  with  the  usual  convexity  of  surface  at 
JK,  although  J  is  depressed.  Areas  like  HJK  will  come  out  warped 
surfaces,  but  are  easily  built  by  an  experienced  contractor.  The 


FIG.  8.  —  CURVES  AT  INTERSECTION  OF  Two  PAVED  ROADS. 


ten  construction  joints  shown  should  be  nothing  more  than  planes 
of  cleavage.  Sections  like  KEFJ,  which  are  built  last,  have,  their 
corner  elevations  fixed  by  the  main  pavement.  Therefore  the  usual 
convexity  of  surface  is  preserved;  and  the  inner  edge  FJ  is  depressed 
to  meet  the  required  elevation.  In  areas  like  KLE  the  surface  of 
the  ground  should  be  kept  about  1  inch  below  that  of  the  surface 
of  the  pavement  adjacent.  The  catch  basins  and  drains  will  keep 
the  ground  dry." 

90.  Super-elevation.  It  is  natural  for  vehicles  to  keep  to  the 
inside  of  the  curve,  partly  to  save  distance  and  partly  to  get  the 
benefit  of  the  crown  of  the  road  to  prevent  tipping  outward.  If 
the  curve  has  no  super-elevation  on  the  outside,  the  slew  of  the 
vehicle,  particularly  a  fast-moving  motor-driven  one,  will  materially 
grind  out  the  surface  of  the  road. 

The  theoretically  perfect  super-elevation  is  given  by  the  formula 


E  = 


W2S2 
32.2  R' 


(1) 


CURVES  61 


in  which  E  is  the  elevation  in  feet,  W  the  width  of  the  road  in  feet, 
S  the  speed  in  miles  per  hour,  R  the  radius  of  the  curve  in  feet. 
However,  the  maximum  super-elevation  is  limited  by  the  transverse 
slope  suitable  for  horse-drawn  traffic. 

The  method  adopted  by  the  Illinois  Highway  Department  is 
very  simple  and  effective.  It  is  as  follows:  "  Whatever  the  char- 
acter of  the  road  surface,  the  inner  half  of  the  curve  is  carried  around 
on  the  level;  and  the  outer  half  of  the  curved  roadway  is  elevated 
so  that  the  surface  is  a  right  line  from  inside  to  outside  on  any 
radial  line.  For  example,  if  the  road  surface  is  concrete  16  feet  wide 
with  a  2-inch  crown,  then  the  outer  edge  of  the  outer  half  will  be 
elevated  2  inches,  and  the  super-elevation  of  the  curve  proper  will  be 
4  inches;  and  including  the  slope  of  the  extra  width  (§97),  the  total 
super-elevation  will  be  nearly  5  inches." 

The  California  Highway  Department  employs  the  following 
method:  The  super-elevation  on  all  curves  is  f  inch  per  foot,  which 
on  a  300-foot  radius  is  perfect  compensation  for  a  speed  of  17  miles 
per  hour  and  on  a  200-foot  radius  for  13  miles  per  hour. 

When  curves  have  the  proper  super-elevation,  the  tendency 
to  keep  to  the  inside  of  the  curve  will  be  less,  and  the  damage  due 
to  slewing  will  be  nearly  or  wholly  eliminated.  For  the  best  results 
the  super-elevation  should  begin  a  short  distance  before  the  tangent 
point  and  not  reach  its  full  amount  until  an  equal  distance  past  the 
tangent  point. 

91.  Aesthetic  Value  of  Curves.     On  a  curved  road  there  is  a 
constantly  changing  panorama  or  vista  before  the  traveler,  rather 
than  the  constant  and  uninteresting  vanishing  point  on  a  straight 
road.    However,  in  most  cases  the  location  of  buildings  and  the  tillage 
of  fields  have  fixed  the  location  of  the  road  within  narrow  limits;  and 
hence  there  is  but  little  opportunity  to  consider  the  aesthetic  or 
artistic  features  of  the  location  of  the  ordinary  highway.     In  the  loca- 
tion of  park  drives  the  artistic  feature  is  the  controlling  element. 

92.  WIDTH.     Under  this  head  will  be  considered  the  width  of 
the  right-of-way  and  also  the  width  of  the  improved  portion. 

93.  Width  of  Right-of-Way.     The  legal  width  of  right-of-way 
varies  greatly  in  different  states.     In  an  early  day,  before  any  attempt 
was  made  to  improve  the  wheel  way,  the  legal  width  was  often  100 
feet,  and  sometimes  10  rods  (165  feet).     In  some  of  the  states  where 
land  is  cheap,  the  former  width  to  some  extent  still  prevails.     In 
most  of  the  states  of  the  Mississippi  Valley,  particularly  those  in 
which  the  land  was  divided  according  to  the  system  of  U.  S.  public 


62  ROAD   LOCATION  [CHAP.    II 

land  survey,  the  legal  width  of  right-of-way  is  usually  66  feet.  A 
few  of  these  states  classify  the  roads,  making  the  less  frequented 
ones  narrower;  for  example,  in  Texas  the  widths  of  first,  second,  and 
third  class  roads  are  60,  30,  and  20  feet,  respectively.  In  the  earlier 
settled  states  along  the  Atlantic  coast,  3  rods  (49J  feet)  is  a  common 
width,  although  some  of  the  less  frequented  roads  are  only  2  rods 
(33  feet)  wide. 

If  the  surface  is  loam  or  clay,  a  considerable  width  of  traveled 
way  is  required  that  the  traffic  may  not  cut  the  surface  up  so  badly 
when  it  is  soft.  This  is  probably  the  explanation  of  the  60  or  66 
feet  so  common  in  the  Mississippi  Valley.  In  some  of  the  states, 
for  example,  Illinois,  the  law  specifies  that,  "  if  possible,"  a  strip 
equal  in  width  to  one  tenth  of  the  right-of-way  shall  be  reserved  for 
pedestrians  on  each  side  between  the  property  line  and  the  ditch. 
This  leaves  53  feet  for  the  wheelway  and  ditches,  which  is  probably 
none  too  much  for  a  loam  or  clay  road.  If  the  ditches  are  deep  and 
consequently  wide,  the  sidewalk  is  usually  curtailed  rather  than  the 
wheelway. 

In  Massachusetts  the  roads  improved  by  state  aid  usually  have  a 
right-of-way  of  50  feet  wide,  and  in  localities  where  there  was  a 
possibility  of  space  being  required  by  an  electric  road,  they  are  60 
feet,  the  latter  being  considered  sufficient  to  accommodate  a  double- 
track  electric  road,  wagon  ways,  and  sidewalks. 

94.  In  England  the  principal  roads,  especially  those  near  popu- 
lous cities,  are  laid  out  66  feet  wide,  20  or  22  feet  being  covered 
with  broken  stone. 

In  Holland  the  usual  width  is  38  feet,  of  which  14  feet  is 
improved. 

In  France  the  standard  widths  are  to  the  nearest  foot  as  follows: 

Class  of  Road.  Right-of-Way.  Width  Improved. 

National  roads 66  feet  22  feet 

Departmental  roads 40    "  20    " 

Provincial  "    33    "  20    " 

Neighborhood      " 26    "  16    " 

95.  Width  of  Improved  Portion.    In  view  of  the  cost  of  improv- 
ing or  paving  the  roadway,  it  is  important  to  determine  the  proper 
or  best  width  of  the  improved  portion.     The  best  or  economic  width 
of!  the  improved  portion  depends  upon  (1)  the  cost  of  the  paved 
portion,  (2)  the  cost  of  constructing  the  shoulders,  i.  e.,  of  partially 
improving  or  hardening  the  natural  soil  at  the  edges  of  the  improved 


WIDTH  63 


portion,  (3)  the  amount  of  travel,  and  (4)  the  proportion  of  motor- 
driven  vehicles. 

Except  for  cost,  the  wider  the  improved  way  the  better;  but  length 
is  more  valuable  than  width,  and  it  is  often  difficult  to  get  an  improved 
road  because  of  the  expense.  Hence  it  is  wise  to  make  the  paved  way 
only  wide  enough  to  accommodate  the  travel  reasonably  well. 

The  width  necessary  for  ordinary  rural  traffic  is  often  over- 
estimated. Two  wagons  having  a  width  of  wheel  base  of  5  feet  and 
a  width  of  load  of  9  feet  can  pass  on  a  16-foot  roadway  and  leave 
6  inches  between  the  outer  wheel  and  the  edge  of  the  paved  way 
and  a  clearance  of  1  foot  between  the  inner  edges  of  the  loads.  This 
extreme  case  will  rarely  occur,  and  hence  a  width  of  16  feet  will 
certainly  be  enough  unless  there  is  considerable  rapid  traffic. 

The  Massachusetts  Highway  Commission  carefully  measured 
the  width  of  traveled  way  on  numerous  crushed-stone  roads,  and 
found  that  with  an  improved  width  of  15  to  24  feet, — the  average 
being  16.1  feet, — the  maximum  width  of  traveled  way  averaged 
14.92  feet  and  the  width  commonly  traveled  averaged  11.05  feet.* 
On  this  evidence  the  Commission  concludes  that  "  a  width  of  15 
feet  is  ample  except  in  the  vicinity  of  the  larger  towns,  and  that 
12  feet  is  sufficient  for  the  lighter  traveled  ways,  bat  that  10  feet 
is  too  narrow  unless  good  gravel  can  be  obtained  for  the  shoulders." 
The  average  width  commonly  traveled  on  forty-six  of  the  15-foot 
roads  was  9.58  feet. 

In  New  Jersey  the  improved  width  for  state-aid  roads  is  9  to 
16  feet,  mostly  10  to  12  feet.  The  improved  width  of  French  roads 
varies  from  16  to  22  feet  (§  94);  in  Austria,  from  14  to  26  feet;  and 
in  Belgium  there  are  many  roads  surfaced  only  8|  feet  wide. 

96.  The  preceding  data  for  the  width  of  the  improved  portion  were 
fixed  before  automobiles  became  numerous.  Naturally  provision 
should  be  made  to  permit  automobiles  to  pass  safely  at  considerable 
speed ;  and  hence  the  widths  stated  above  are  too  small.  Two  auto- 
mobiles can  not  safely  pass  at  low  speed  upon  less  than  12  feet,  and 
usually  it  is  considered  that  a  road  having  any  considerable  motor 
travel  should  have  a  width  of  14  or  16  feet.  The  Massachusetts 
Highway  Commission  once  built  double-track  roads  18  feet  wide; 
but  in  consideration  of  the  large  number  of  wheels  that  went  off 
the  side  of  the  improved  way,  increased  the  width  to  19|  feet,  after 
which  few,  if  any,  wheels  went  off  the  side. 

*  Report  of  the  Massachusetts  Highway  Commission  for  1897,  p.  31.     For  a  summary  of 
similar  data  for  each  township  for  five  years,  see  Report  for  1901,  p.  47-55. 


64  ROAD  LOCATION  ICHAP.  n 

97.  Width  on  Curves.    If  the  deflection  angle  is  more  than  about 
30°,  the  traveled  way  should  be  widened  on  the  curve.     If  there  is 
likely  to  be  much  motor-driven  traffic,  the  width  at  the  center  of  the 
curve  should  be  increased  30  to  40  per  cent,  the  increase  tapering 
to  nothing  at  the  tangent  points. 

The  slope  of  the  inner  half  of  the  curve  should  be  continued  over 
this  extra  width. 

If  the  improved  way  is  so  narrow  that  a  considerable  number 
of  vehicles  turn  off  upon  the  shoulders,  then  the  proper  construction 
of  the  shoulders  becomes  a  considerable  item;  and  it  may  be  wiser 
to  improve  a  wider  portion  and  spend  less  money  upon  the  shoulders. 
Obviously  the  best  width  depends  upon  the  amount  of  travel,  the 
relative  cost  of  the  pavement  and  of  improving  the  shoulders.  It 
has  been  said  that  if  a  vehicle  is  compelled  to  turn  off  on  the 
shoulder  more  than  five  times  in  going  a  mile,  the  improved  portion 
should  be  widened. 

Since  earth  roads  have  the  same  material  in  the  shoulders  as  in 
the  traveled  way,  and  since  the  cost  of  an  improved  earth  road  is  so 
small,  the  whole  width  between  the  side  ditches  should  be  improved. 
Since  gravel  roads  are  comparatively  cheap  to  construct,  and  since 
there  is  only  a  little  difference  between  the  cost  of  the  improved  way 
and  that  of  the  shoulders,  gravel  roads  can  appropriately  be  wider 
than  roads  of  higher  unit  cost.  For  current  practice  concerning  the 
width  of  gravel  roads,  see  Figs.  44  and  45,  page  170.  For  examples 
of  the  way  in  which  these  principles  have  been  applied  in  water- 
bound  macadam,  see  Figs.  48-56,  pages  197-99;  and  for  concrete 
roads,  see  Fig.  75,  page  243. 

98.  Location  of  the  Wheelway.    The  improved  portion  is  some- 
times placed  in  the  middle  of  the  traveled  way,  and  sometimes  at 
one  side.     Apparently  the  natural  position  is  in  the  middle  with  an 
earth  track  on  each  side;  but  in  this  case,  if  the  pavement  is  crowned, 
as  is  usual,  one  half  of  the  storm  water  falling  on  it  is  discharged 
upon  the  shoulder  at  each  side  of  the  pavement,  i.  e.,  upon  that  por- 
tion of  the  road  the  harder  to  keep  in  proper  condition.     On  the 
other  hand,  if  the  improved  portion  is  placed  at  one  side  of  the  trav- 
eled way,  and  if  at  the  same  time  it  is  given  a  uniform  slope  toward 
the  nearer  side  ditch,  all  of  the  storm  water  falling  upon  the  im- 
proved portion  will  be  discharged  upon  the  unused  shoulder  next  to 
the  side  ditch  and  therefore  do  no  harm.     Further,  in  many  cases 
grass  will  grow  upon  the  unused  shoulder  and  protect  it,  so  no  harm 
will  be  done  if  an  occasional  wheel  does  turn  off  onto  this  shoulder. 


PLACING   THE   LINE 


65 


The  heavier  loads  usually  go  toward  town;  and  therefore  if 
the  single-track  improved  portion  is  placed  upon  the  right-hand  side 
going  toward  town,  the  heavier  loads  will  have  the  right-of-way  (in 
the  United  States  at  least),  and  will  not  turn  off  from  the  paved 
portion. 

99.  CROSS  SECTION.  The  cross  section  of  a  road  or  pavement 
depends  upon  the  material  of  the  road  surface,  and  hence  will  be 
considered  in  the  respective  chapters  following. 

However,  the  data  in  Table  16  on  the  crown  or  transverse  slope 
of  the  road  surface  are  given  here  for  convenience  of  reference  and 
comparison.  These  values  were  recommended  by  a  special  com- 
mittee of  the  American  Society  of  Civil  Engineers.* 

TABLE  16 
CROWN  OF  ROADWAY 


MATERIAL  OF  ROADWAY. 

TRANSVERSE  SLOPE, 
Inches  per  Foot. 

Maximum. 

Minimum. 

Earth                        

1 
1 

1 
I 

1 
i 

| 

4~ 

1 

4 

1 
4 

! 

Gravel                                    

\Vater-bound  IVIacadam               

Bituminous  IVIacadarn                

Bituminous  Concrete                

Bituminous  Carpet                

Stone-block                          

Portland-Cement  Concrete 

Brick 

Wood  Block                                                

Sheet  Asphalt                                           

100.  PLACING  THE  LINE.  The  controlling  points  of  a  line  are 
certain  points  at  which  the  position  of  the  road  is  restricted  within 
narrow  limits  and  is  not  subject  to  change.  These  may  be  points 
where  the  location  is  governed  by  the  necessity  of  providing  an  out- 
let for  the  traffic,  or  points  where  the  position  of  the  line  is  restricted 
by  topographical  considerations — such  as  a  summit  over  which  the 
road  must  pass,  or  a  suitable  location  for  a  bridge. 

After  the  reconnoissance  of  the  locality  is  completed  and  the 


Proc.  Amer.  Soc.  of  Civil  Engr's,  Vol.  42  (1916),  p.  1615. 


86  ROAD   LOCATION  [CHAP.    II 

position  and  elevation  of  the  controlling  points  are  known,  the  line 
must  be  marked  upon  the  ground.  For  example,  assume  that  it  is 
desired  to  run  a  road  from  A  to  D,  Fig.  9,  page  67,  D  being  a  pass 
over  the  ridge.  If  the  road  follows  the  line  A  B  C  D,  it  will  have 
the  profile  shown  near  the  bottom  of  Fig.  9.  The  average  grade 
from  A  to  B  is  1  per  cent,  and  from  B  to  C  5  per  cent.  If  it  is  de- 
sired to  locate  a  road  that  shall  have  a  grade  no  steeper  than  5  per 
cent,  we  may  begin  at  D  and  locate  a  line  having  an  uniform  5  per 
cent  grade.  It  is  best  to  commence  the  location  from  D,  since 
usually  the  slopes  nearer  the  foot  of  the  hills  are  flatter  than  those 
at  the  summit,  and  consequently  there  is  more  choice  of  position  of 
the  line  there  than  at  the  summit.  Frequently  in  rough  country, 
the  only  controlling  point  fixed  before  beginning  the  location  survey 
is  the  lowest  pass  over  a  ridge  or  mountain  range. 

Beginning  at  D,  a  line  may  be  located  either  (1)  by  setting  off 
the  angle  of  the  gradient  on  the  vertical  circle  of  a  transit  or  on  a 
gradienter,  and  sighting  upon  a  rod  which  is  moved  until  the  line 
of  sight  strikes  it  at  the  same  height  from  the  ground  that  the  instru- 
ment is  above  grade;  or  (2)  the  points  for  the  line  may  be  found 
by  running  a  line  of  levels  ahead  of  the  transit,  and  measuring  the 
distances  by  which  to  reckon  the  rate  of  the  grade.  The  line  DEC, 
Fig.  9,  has  a  uniform  gradient  of  5  per  cent. 

If  a  contour  map  is  at  hand,  the  line  can  be  located  approxi- 
mately by  opening  a  pair  of  dividers  until  the  distance  between  the 
points  corresponds  to  100  feet,  setting  one  point  on  the  place  of 
beginning  and  the  other  on  the  next  lower  contour,  which  gives  a 
line  100  feet  long  with  a  grade  equal  to  the  distance  between  con- 
tours— in  Fig.  9,  5  feet. 

The  line  D  F  G  has  a  uniform  grade  of  5  per  cent.  From  H  to  A 
the  road  will  have  considerably  less  grade  than  5  per  cent,  and  can 
have  a  comparatively  wide  range  of  position. 

The  average  grade  from  A  to  D  is  a  little  less  than  5  per  cent,  but 
the  slopes  are  so  steep  between  D  and  C  that  it  is  impossible,  within 
the  limits  of  the  map,  to  locate  such  a  line.  If  such  a  gradient  is 
located  from  D  toward  A,  it  will  necessarily  -make  a  number  of  short 
turns  on  itself,  which,  although  undesirable,  are  sometimes  un- 
avoidable. These  short  turns  seriously  impede  traffic,  since  vehi- 
cles can  not  easily  pass  each  other  on  such  short  curves — particu- 
larly if  each  is  drawn  by  a  long  team.  Short  turns  are  also  danger- 
ous in  descending,  in  case  control  of  the  vehicle  is  lost  or  the  team 
runs  away. 


PLACING   THE   LINE 


67 


101.  The  line  A  B  C  D  may  be  considered  as  an  old  road  which  it 
is  proposed  to  improve  by  reducing  the  grades.  Substituting  the 
line  C  E  D  for  CD  changes  the  maximum  grade  from  10  to  5  per 
cent. 


68  ROAD   LOCATION  [CHAP.   H 

102.  In  placing  the  line  attention  should  be  given  to  the  nature 
of  the  soil  on  alternative  lines,  since  on  one  side  of  the  valley  the 
surface  may  be  clay,  upon  the  opposite  gravel;  in  the  bottom  of 
the  valley  the  soil  is  usually  alluvial,  while  higher  up  it  is  generally 
better  for  road  purposes.  It  should  be  remembered  that  in  almost 
all  steep  slopes  covered  with  loose  material,  the  debris  is  either  slowly 
moving  down  the  slope  or  has  attained  a  state  of  repose  so  deli- 
cately adjusted  that  an  excavation  for  a  road-bed  on  the  inclined 
surface  will  again  set  the  mass  in  motion.  Such  movements  are 
particularly  common  in  loose  materials  in  countries  where  the  frost 
penetrates  deeply  and  the  ground  becomes  very  soft  when  thawing, 
and  frequently  entail  long-continued  and  serious  expense  in  main- 
tenance. 

If  the  road  is  to  have  a  surface  of  gravel  or  broken  stone,  the 
relative  proximity  of  the  materials  for  the  original  construction  as 
well  as  for  repairs  should  be  considered  in  deciding  between  possible 
locations.  However,  it  should  be  remembered  that  after  the  road 
is  completed,  the  amount  of  hauling  required  to  supply  materials 
for  maintenance  must  of  necessity  be  small  in  comparison  with  the 
ordinary  traffic  over  the  road;  and  hence  this  consideration  should 
not  have  undue  weight. 

Attention  should  also  be  given  to  the  disposal  of  the  drainage 
water,  and  to  the  question  of  danger  from  high  water  in  streams. 
For  example,  in  Fig.  9  it  is  possible  to  locate  a  line  on  the  upper  side 
of  the  map  with  an  uniform  grade  of  4  per  cent,  but  such  a  line  will 
lie  so  near  the  branch  entering  the  main  stream  at  B  as  to  be  in 
danger  from  floods.  The  matter  of  crossing  streams  should  receive 
most  careful  study.  Bridges  are  comparatively  expensive  to  build 
and  to  maintain. 

It  may  be  cheaper  to  carry  the  road  across  the  gully  on  an  em- 
bankment or  a  trestle  than  to  make  a  detour  around  the  head  of  the 
valley.  This  question  can  be  determined  by  comparing  the  greater 
cost  of  construction  of  the  shorter  line  with  the  capitalized  value 
of  the  greater  cost  of  operating  the  longer  line. 

In  some  localities  the  protection  of  the  road  against  snow  is  an 
important  matter.  Deep  cuts  almost  always  catch  snow;  and  for 
this  reason  it  is  sometimes  better  to  go  around  a  point  by  a  sup- 
ported grade  than  to  cut  through  it.  In  a  snow  country,  roads 
should  be  located  on  slopes  facing  south  and  east  in  preference  to 
slopes  facing  north  and  west,  as  the  sun  has  greater  power  on  the 
former  to  melt  the  snow. 


EXAMPLE    OF   RE-LOCATION 


69 


"  Nothing  pays  like  first  cost  in  road  building,"  i.  e.,  money 
expended  in  intelligent  study  of  the  location  is  the  most  economical 
expenditure  in  the  construction  of  a  road. 

103.  EXAMPLE  OF  RE-LOCATION.  Fig.  10  shows  the  old  and 
the  new  location  of  a  road.  The  old  location,  in  the  back-ground, 


FIG.  10. — RE-LOCATION  OF  ROAD. 

had  many  sharp  curves,  an  undulating  profile,  and  two  stream 
crossings;  while  the  new  location  has  easy  curves,  no  needless  rise 
and  fall,  and  no  stream  crossings. 

104.  ESTABLISHING  THE  GRADE  LINE.  After  placing  the  center 
line,  the  topography  should  be  taken  on  each  side  of  the  line  for 
some  distance — the  distance  depending  upon  the  lay  of  the  land; — 
and  then  a  map  should  be  drawn  showing  the  center  line  and  the  con- 
tours. This  will  serve  to  show  whether  the  line  is  placed  to  the  best 
advantage,  and  whether  any  changes  are  desirable.  This  is  especially 
necessary  over  rough  ground  or  where  the  line  is  on  a  maximum 
grade. 

The  center  line  for  a  final  location  should  be  carefully  run  and 
permanently  marked,  so  that  it  may  be  re-located  if  necessary.  A 
line  of  levels  should  be  run  and  a  profile  drawn,  upon  which  the 
grades  may  be  established  and  from  which  the  earthwork  may  be 
estimated  (§  138). 


CHAPTER  III 
EARTH  ROADS 

106.  In  1915  the  surface  of  87  per  cent  of  the  roads  of  the  United 
States  was  the  native  earth  (Table  12,  page  33);   and  in  all  prob- 
ability 70  to  80  per  cent  of  these  roads  will  always  remain  earth 
roads. 

The  earth  road  is  the  cheapest  road  in  first  cost.  It  is  a  light- 
traffic  road,  and  only  when  the  travel  becomes  considerable  is  it 
possible  to  procure  the  money  with  which  to  improve  the  surface  by 
the  use  of  some  foreign  material,  as  gravel  or  broken  stone.  For- 
tunately, the  best  form  for  the  earth  road  is  also  the  best  preparation 
for  any  improved  surface.  This  surface,  whatever  its  nature,  is 
only  a  roof  to  protect  the  earth  from  the  effects  of  weather  and  travel, 
and  any  preparation  that  will  enable  the  native  soil  when  unprotected 
to  resist  these  elements  will  enable  it  the  better  to  serve  as  a  founda- 
tion for  the  improved  surface.  Because  of  the  importance  of  earth 
roads  as  a  means  of  transportation  and  also  because  of  the  importance 
of  a  properly  formed  and  well-drained  road-bed  for  all  improved  road 
surfaces,  earth  roads  will  be  considered  somewhat  fully. 

107.  The  term  earth  road  will  be  used  as  applying  to  roads  whose 
surface  consists  of  the  native  soil;    and,  unless  otherwise  stated,  it 
will  be  understood  as  meaning  a  road  whose  surface  is  loam  or  clay. 

Roads  on  loam  and  clay  will  be  discussed  in  this  chapter;  and 
roads  on  sand  or  sand  and  clay  mixed  will  be  considered  in  the  next 
chapter. 

AKT.  1.     CONSTRUCTION 

108.  WIDTH.     The    width    of   the    right-of-way   varies   greatly 
but  is  usually  between  40  and  66  feet  (§93).    With  a  66-foot  right- 
of-way  it  is  customary  to  reserve  about  6  feet  outside  of  the  ditch 
on  each  side  for  a  foot-way,  and  to  grade  the  remaining  54  feet. 
With  a  40-foot  right-of-way  it  is  customary  to  reserve  6  feet  on  each 
side  for  a  foot-way,  thus  leaving  28  feet  for  ditches  and  wheelways. 

70 


ART.    1]  CONSTRUCTION  71 

For  equally  good  surface  drainage,  the  greater  width  requires  deeper 
ditches  and  more  cost  in  construction;  but  permits  a  wider  distribu- 
tion of  the  travel  which  is  an  advantage  when  the  roads  are  muddy 
or  rough.  The  deep  ditches  are  harder  to  maintain,  and  as  a  rule 
the  native  soil  from  the  bottom  of  deep  ditches  is  not  so  good  for  road 
building  purposes  as  that  nearer  the  surface.  The  cost  of  main- 
taining the  road  varies  with  the  amount  of  travel,  and  is  practically 
independent  of  the  width.  Therefore  the  width  to  be  improved 
depends  chiefly  upon  the  width  of  the  right-of-way,  the  character  of 
the  soil,  the  climate,  and  the  first  cost.  In  a  wet  climate,  with  soil 
easily  working  into  mud,  a  wide  wheelway  is  desirable;  while  in  a 
dry  climate,  or  with  a  soil  not  readily  forming  mud,  a  narrow  wheel- 
way  is  satisfactory. 

109.  Width  on  Curves.     For  a  rule  for  widening  the  wheel- 
way  on  curves,  see  §  97.     This  rule  hardly  applies  to  earth  roads, 
but  it  is  well  to  bear  it  in  mind  in  locating  or  improving  earth  roads 
that  may  ultimately  have  a  hard-surfaced  wheelway. 

110.  CROSS  SECTION.     The  cross  section  or  transverse  contour 
of  an  earth  road  is  an  important  matter  with  reference  to  the  cost  of 
construction  and  maintenance,  and  depends  mainly  upon  the  tools 
or  machinery  used  in  construction  and  maintenance  and  upon  the 
form   required   for   drainage.     The   subject   is   discussed   fully   in 
§  129-31. 

111.  Super-elevation  on  Curves.    For  a  discussion  of  the  super- 
elevation of  the  outer  edge  of  the  wheelway  on  curves,  see  §  90. 

112.  GRADES.     For  a  general  discussion  of  the  effect  of  both 
maximum  and  minimum  grades  upon  the  use  and  maintenance  of 
a  road,  see  §  79-86. 

The  principal  problem  in  reference  to  grades  is  the  determina- 
tion of  the  maximum  grade  permissible.  This  problem  does  not  admit 
of  exact  mathematical  determination;  and  therefore  recourse  must 
be  had  to  experience.  For  obvious  reasons  there  are  not  many 
definite  data  under  this  head  on  record.  In  hilly  country  short 
grades  of  1  in  3  (33%)  are  occasionally  found — particularly  in  a 
newly  settled  country, — and  grades  of  1  in  4  (25%)  are  somewhat 
common.  In  a  comparatively  flat  country,  grades  of  1  in  8  (12J%) 
are  not  infrequent. 

In  improving  the  celebrated  Holy  head  road,  Telford  found  in 
old  roads  many  grades  of  1  in  6  and  1  in  7.  A  number  of  roads 
improved  by  state  aid  in  New  Jersey  originally  had  grades  of  14 
per  cent.  Of  course  only  the  roads  having  the  most  traffic  were 


72  EAKTH   ROADS  [CHAP.    Ill 

improved;  and  probably  less  frequented  roads  in  each  locality  have 
much  greater  grades. 

For  mountain  roads,  where  the  bulk  of  the  traffic  is  usually 
down  hill,  the  maximum  grade  is  often  8  per  cent  and  sometimes 
as  much  as  12  per  cent.  "  Experience  in  heavy  freighting  has  shown 
that  wagons  can  be  satisfactorily  controlled  in  all  weather  on 
12  per  cent  grades,  but  they  can  not  be  safely  controlled  on  steeper 
grade." 

113.  DRAINAGE.     Drainage   is  the  most  important  matter  to 
be  considered  in  the  construction  of  roads,  since  no  road,  whether 
earth  or  stone,  can  long  remain  good  without  it. 

A  perfectly  drained  road  will  have  three  systems  of  drainage, 
each  of  which  must  receive  special  attention  if  the  best  results  are  to 
be  obtained.  This  is  true  whether  the  trackway  be  iron,  broken 
stone,  gravel,  or  earth,  and  it  is  emphatically  true  of  earth.  These 
three  systems  are  underdrainage,  side  ditches,  and  surface  drainage. 

114.  Underdrainage.    Any  soil  in  which  the  standing  water  in 
the  ground  comes  at  any  season  of  the  year  within  3 "feet  of  the 
surface  will  be  benefited  by  drainage;   that  is,  if  the  soil  does  not 
have  a  natural  underdrainage,  it  will  be  improved  for  road  purposes 
by  artificial  subsurface  drainage.     It  is  the  universal  observation 
that  roads  in  low  places  which  are  thoroughly  underdrained  dry  out 
sooner  than  undrained  roads  on  high  land.     Underdrained  roads  never 
get  as  bad  as  do  those  not  so  drained.     Underdrainage  without 
grading   is  better  than  grading  without  drainage;   and,  in  general, 
it  may  be  said  that  where  the  soil  does  not  have  natural    under- 
drainage, there  is  no  way  in  which  road  taxes  can  be  spent  to  better 
advantage  than  in  subsurface  drainage.     Underdrainage  is  the  very 
best  preparation  for  a  gravel  or  stone  road.     Gravel  or  broken 
stone  placed  upon  an  undrained  foundation  is  almost  sure  to  sink 
(perhaps  slowly,  but  none  the  less  surely),  whatever  its  thickness; 
whereas  a  thinner  layer  upon  a  drained  road-bed  will  give  much 
better  service. 

115.  The  Object.    The  opinion  is  quite  general  that  the  sole 
object  of  underdrainage  is  to  remove  the  surface  water,  but  this  is 
only  a  small  part  of  the  advantages  of  the  underdrainage  of  roads. 
There  are  three  distinct  objects  to  be  gained  by  the  artificial  under- 
drainage of  a  wagon  road. 

1.  The  most  important  object  is  to  lower  the  water  level  in  the 
soil.  The  action  of  the  sun  'and  the  wind  will  finally  dry  the  surface 
of  the  road;  but  if  the  foundation  is  wet?  it  will  be  soft  a.nd  spongy. 


ART.    1]  CONSTRUCTION  73 

the  wheels  will  wear  ruts,  and  the  horses'  feet  will  make  depressions 
between  the  ruts.  The  first  shower  will  fill  these  depressions  with 
water,  and  the  road  will  soon  be  a  mass  of  mud.  A  good  road  can 
not  be  maintained  without  a  good  foundation,  and  an  undrained  soil 
is  a  poor  foundation,  while  a  dry  subsoil  can  support  almost  any 
load. 

2.  A  second  object  of  underdrainage  is  to  dry  the  ground  quickly 
after  a  freeze.     When  the  frost  comes  out  of  the  ground  in  spring, 
the  thawing  is  quite  as  much  from  the  bottom  as  from  the  top.     If 
the  land  is  underdrained,  the  water  when  released  by  thawing  from 
below  will  be  immediately  carried  away.     This  is  particularly  im- 
portant in  road  drainage,  since  the  foundation  will  then  remain  solid 
and  the  road  itself  will  not  be  cut  up.     Underdrainage  will  usually 
prevent  the  "  bottom  dropping  out "  when  the  frost  goes  out  of  the 
ground. 

3.  A  third,  and  sometimes  a  very  important,  object  of  subdrainage 
is  to  remove  what  may  be  called  the  underflow.     In  some  places 
where  the  ground  is  comparatively  dry  when  it  freezes  in  the  fall,  it 
will  be  very  wet  in  the  spring  when  the  frost  comes  out — surpris- 
ingly so  considering  the  dryness  before  freezing.     The  explanation 
is  that  after  the  ground  freezes,  water  rises  slowly  in  the  soil  by 
the  hydrostatic  pressure  of  water  in  higher  places;   and  if  it  is  not 
drawn  off  by  underdrainage  it  saturates  the  subsoil  and  rises  a. 
the  frost  goes  out,  so  that  the  ground  which  was  comparatively  dry 
when  it  froze  is  practically  saturated  when  it  thaws. 

116.  The  underdrainage  of  a  road  not  only  removes  the  water, 
but  prevents,  or  greatly  reduces,  the  destructive  effect  of  frost. 
The  injurious  effect  of  frost  is  caused  entirely  by  the  presence  of 
water;  and  the  more  water  there  is  in  the  road-bed  the  greater  the 
injury  to  the  road.  The  water  expands  on  freezing,  the  surface  of 
the  road  is  upheaved,  and  the  soil  is  made  porous;  when  thawing 
takes  place,  the  ground  is  left  honeycombed  and  spongy,  ready  to 
settle  and  sink,  and  under  traffic  the  road  "  breaks  up."  If  the 
road  is  kept  dry,  it  will  not  break  up.  Underdrainage  can  not  pre- 
vent the  surface  of  the  road  from  becoming  saturated  with  water 
during  a  rain,  but  it  is  the  best  means  of  removing  surplus  water, 
thus  allowing  the  surface  to  dry  and  preventing  the  subsequent 
heaving  by  frost. 

That  frost  is  harmless  where  there  is  no  moisture,  is  shown  on  a 
large  scale  in  the  semi-arid  regions  west  of  the  Mississippi  river. 
The  ground  there  is  normally  so  dry  that  during  the  winter,  when 


74  EARTH   ROADS  [CHAP.    Ill 

it  is  frozen,  cracks  half  an  inch  or  more  wide  form,  owing  to  the  dry- 
ing and  consequent  contraction  of  the  soil,  which  shows  that  there 
is  no  expansion  by  the  freezing  of  water  in  the  soil;  and  therefore 
in  this  region  there  is  no  heaving  or  disturbance  by  frost. 

117.  The  Tile.  The  best  and  cheapest  method  of  securing  under- 
drainage  is  to  lay  a  line  of  farm  tile  3  or  4  feet  deep  on  one  or  both 
sides  of  the  roadway.  The  ordinary  farm  tile  is  entirely  satis- 
factory for  road  drainage.  It  should  be  uniformly  burned,  straight, 
round  in  cross  section,  smooth  inside,  and  have  the  ends  cut  off 
square.  Tile  may  be  had  from  3  to  30  inches  in  diameter.  The 
smaller  sizes  are  usually  a  little  over  a  foot  long, — the  excess  length 
being  designed  to  compensate  for  breakage;  and  the  larger  sizes 
are  nominally  2  or  2J  feet  long,  but  usually  a  little  longer.  The 
cost  of  tile  free  on  board  at  the  factory  is  usually  about  as  in  Table 
17,  page  75.  Y's  for  connections  can  be  had  at  most  factories,  but 
they  cost  four  or  five  times  as  much  as  an  ordinary  tile.  With 
patience  and  a  little  experience  ordinary  tile  can  be  cut  to  make 
fairly  good  connections. 

Before  the  introduction  of  tile  for  agricultural  drainage,  it  was 
customary  to  secure  underdrainage  by  digging  a  trench  and  deposit- 
ing in  the  bottom  of  it  logs  or  bundles  of  brush,  or  a  layer  of  stone ; 
or  a  channel  for  the  water  was  formed  by  setting  a  line  of  stones 
on  each  side  of  the  trench  and  joining  the  two  with  a  third  line 
resting  on  these  two.  Apparently  it  is  still  the  practice  in  some 
localities  to  use  such  substitutes  for  ordinary  drain  tile.  Tiles  are 
better,  since  they  are  more  easily  laid  and  are  less  liable  to  get 
clogged.  Tiles  are  cheaper  in  first  cost,  even  when  shipped  consid- 
erable distances  by  rail,  than  any  substitute;  and  the  drains  are 
much  more  durable. 

Tiles  are  laid  simply  with  their  ends  in  contact,  care  being  taken 
to  turn  them  until  the  ends  fit  reasonably  close.  In  some  localities 
there  is  apparently  fear  that  the  tile  will  become  stopped  by  fine 
particles  of  soil  entering  at  the  joints,  and  consequently  it  is  specified 
that  the  joints  shall  be  covered  with  tarred  paper  or  something  of  the 
sort;  but  in  the  Mississippi  Valley,  where  immense  quantities  of  tile 
have  been  laid,  no  such  difficulty  has  been  encountered.  With  a 
very  slight  fall  or  even  no  fall  at  all,  tiles  will  keep  clean,  if  a  free 
outlet  is  provided,  and  they  are  not  obstructed  by  roots  of  trees — 
particularly  willow. 

In  some  localities  it  is  apparently  customary  to  use  collars 
around  the  ends  of  the  tile  to  keep  them  in  line.  If  the  bottom  of 


ART.    1] 


CONSTRUCTION 


75 


the  trench  is  made  but  little  wider  than  the  diameter  of  the  tile,  or 
if  a  groove  is  scooped  out  in  the  bottom  of  the  trench  to  fit  the  tile, 
no  difficulty  need  be  apprehended  from  this  source. 


TABLE  17 
COST  AND  WEIGHT  OF  DRAIN  TILE 


Inside 
Diameter. 

Price  per  1000  Ft. 
f.o.b.  Factory. 

Weight 
per  Foot. 

Number  of  Feet 
in  a  Car  Load. 

3  inches 

$10.00 

51b. 

7000 

4 

15.00 

7 

6500 

5 

20.00 

9 

5000 

6 

27.00 

12 

4000 

7 

35.00 

14 

3000 

8 

45.00 

18 

2500 

9 

55.00 

21 

1800 

10 

65.00 

25 

1600 

12 

90.00 

33 

1000 

14 

120.00 

43 

800 

16 

150.00 

50 

600 

18 

240.00 

70 

400 

20 

300.00 

83 

330 

24 

360.00 

112 

300 

118.  The  Fall.  There  is  no  danger  of  the  grade  of  the  tile  being 
too  great,  and  the  only  problem  is  to  secure  sufficient  fall.  A  num- 
ber of  authorities  on  farm  drainage  and  also  several  engineering 
manuals  assert  that  a  fall  of  2|  or  3  inches  per  100  feet  is  the  lowest 
limit  that  should  be  attempted  under  the  most  favorable  conditions; 
but  practical  experience  has  abundantly  proved  that  a  much  smaller 
fall  will  give  good  drainage.  In  central  Illinois  and  northern 
Indiana  there  are  many  lines  of  tile  having  falls  of  only  £  to  J  of  an 
inch  per  100  feet  which  are  giving  satisfactory  drainage;  and  not 
infrequently  ordinary  tile  laid  absolutely  level  directly  upon  the 
earth  in  the  bottom  of  the  trench,  without  collars  or  other  covering 
over  the  joints,  has  given  fairly  good  drainage  without  trouble  from 
the  deposit  of  sediment.  Of  course,  extremely  flat  grades  are  less 
desirable  than  steeper  ones,  since  larger  tile  must  be  used,  and 
greater  care  must  be  exercised  in  laying  them,  and  since  there  is 
more  risk  of  the  drain's  becoming  obstructed;  but  these  extremely 
flat  grades  are  sometimes  all  that  can  be  obtained,  and  even  such 
drains  abundantly  justify  the  expense  of  their  construction. 

If  possible  at  reasonable  expense,  the  grade  should  be  at  least 
2  inches  per  100  feet;  and  unless  absolutely  necessary  should  never 


76  EARTH   ROADS  [CHAP.    Ill 

be  less  than  J  inch  per  100  feet.  On  level  or  nearly  level  ground 
the  fall  may  be  increased  by  laying  the  tile  at  the  upper  end  shallower 
than  at  the  lower. 

119.  Size  of  Tile.     The  following  formula  has  frequently  been 
employed  to  determine  the  size  of  tile. : 


*»L0;y£*, (i) 

in  which  A  is  the  number  of  acres  for  which  a  tile  having  a  diameter 
of  d  inches  and  a  fall  of  /  feet  in  a  length  of  I  feet  will  remove  1 
inch  in  depth  of  water  in  24  hours. 

Equation  (1)  is  based  on  the  formula  ordinarily  employed  for 
the  flow  of  water  through  smooth  cast  iron  pipe,  and  is  only  roughly 
applicable  to  tile.  It  probably  gives  too  great  a  capacity  Tor  tile. 
However,  all  the  factors  of  the  problem  are  too  uncertain  to  justify 
an  attempt  at  mathematical  accuracy.  For  example,  we  can  not 
know  with  any  certainty  the  maximum  rate  of  rainfall,  the  duration 
of  the  maximum  rate,  the  permeability  of  the  soil,  the  amount  of 
water  retained  by  the  soil,  the  effect  of  surface  water  flowing  onto  the 
road  from  higher  ground,  the  area  to  be  drained,  etc.  The  above 
formula  is  useful  only  in  a  locality  where  there  is  no  local  experience 
with  tile;  and  its  chief  value  consists  in  showing  the  relation  between 
capacity  and  grade,  and  the  effect  of  a  variation  in  the  diameter  of 
the  tile. 

The  object  of  under  draining  a  road  is  to  prevent  the  plane 
of  saturation  from  rising  so  near  the  surface  as  to  soften  the 
foundation  of  the  road  even  during  a  wet  time,  and  therefore  the 
provision  for  underdrainage  should  be  liberal;  but  what  will  be 
adequate  in  any  particular  case  depends  upon  the  amount  of  traffic, 
the  local  topographic  conditions,  the  character  of  the  soil,  etc.  The 
best  practice  in  agricultural  drainage  provides  for  the  removal  of 
0.5  to  1  inch  of  water  per  day;  but  since  the  side  ditches  will  assist 
in  removing  rain  water  from  the  road,  it  is  probable  that  a  provision 
for  the  removal  of  half  an  inch  per  day  is  sufficient  for  the  under- 
drainage of  a  road.  If  there  is  an  underflow  of  water  from  higher 
ground,  or  if  the  ground  is  "  springy,"  then  the  ordinary  provisions 
for  underdrainage  should  be  increased. 

120.  It  is  not  wise  to  lay  a  smaller  tile  than  a  4-inch  one,  and 
probably  not  smaller  than  a  5-inch.  Tile  can  not  be  laid  in  exact 
line,  and  any  tilting  up  of  one  end  reduces  the  cross  section.  Again, 


ART.   1]  CONSTRUCTION  77 

if  there  is  a  sag  in  the  line  equal  to  the  inside  diameter,  the  tile  will 
shortly  become  entirely  stopped  by  the  deposit  of  silt  in  the  depres- 
sion. 

It  is  sometimes  wiser  to  lay  a  larger  tile  than  to  increase  the  fall. 
Ordinarily,  the  deeper  the  tile  the  better  the  drainage,  although  3J 
or  4  feet  deep  is  usually  sufficient. 

121.  Laying  the  Tile.     It  is  unwise  to  enter  upon  any  entailed 
discussion  of  the  art  of  laying  tile.     The  individual  tiles  should 
be  laid  in  line  both  vertically  and  horizontally,  with  as  small  joints 
at  the  end  as  practicable.     Care  should  also  be  taken  that  the  tile 
is  laid  to  a  true  grade,  particularly  if  the  fall  is  small,  for  if  there  is 
a  sag  it  will  become  filled  with  sediment,  or  if  there  is  a  crest  silt 
will  be  deposited  just  above  it.     The  drain  should  have  a  free  and 
adequate  outlet.     The  end  of  the  line  of  tile  should  be  protected 
by  masonry,  by  plank  nailed  to  posts,  or  by  replacing  three  or  four 
tiles  at  the  lower  end  by  an  iron  pipe  or  a  wooden  box. 

122.  Cost  of  Laying  Tile.     On  the  basis  of  15  cents  an  hour  for 
common  labor,  the  prevailing  cost  of  laying  tile  in  loam  with  clay 
subsoil  is  about  as  follows:   for  8-inch  tile  or  less,  10  cents  per  rod 
for  each  foot  of  depth;   for  9-inch,  11  cents;   for  12-inch,  14  cents; 
for  15-inch,  17  cents;  and  for  16-inch,  18  cents.     To  aid  in  remem- 
bering the  above  data,  notice  that  the  price  is  10  cents  per  rod 
per  foot  of  depth  for  8-inch  tile  or  less,  with  an  increase  of  1  cent 
for  each  additional  inch  of  diameter. 

The  cost  of  a  mile  of  5-inch  tile  drain  is  usually  from  $200  to 
$250,  exclusive  of  freight  on  the  tile.  If  there  is  any  considerable 
amount  of  tiling,  the  above  prices  for  the  smaller  tile  can  be  reduced 
10  to  20  per  cent;  and  often  there  is  enough  discount  on  the  prices 
given  in  Table  7,  page  75,  to  cover  the  railroad  freight-charges. 
A  tile  drain  is  a  permanent  improvement  with  no  expense  for  main- 
tenance, the  benefit  being  immediate  and  certain;  and  therefore  it 
is  doubtful  if  money  can  be  spent  on  earth  roads  to  better  advan- 
tage than  in  laying  tile. 

123.  One  vs.  Two  Lines.    Usually  a  line  of  tile  2J  to  3  feet  deep 
under  the  ditch  at  one  side  of  the  road  will  give  sufficient  drainage. 
In  case  of  doubt  as  to  whether  one  or  two  lines  of  tile  are  needed, 
put  in  one  and  watch  the  results.     If  both  sides  of  the  road  are 
equally  good,  another  tile  drain  is  not  needed.     In  making  these 
observations  care  should  be  taken  not  to  overlook  any  of  the  con- 
tingent factors,  as,  for  example,  the  difference  in  the  effect  of  the  sun 
upon  the  south  and  the  north  sides  of  the  road,  the  effect  of  shade 


78  EARTH   ROADS  [CHAP.    Ill 

or  of  seepage  water,  the  transverse   slopes   of  the   surface  of   the 
road,  etc. 

124.  Location  of   Tile.     Some   writers   on   roads  recommend   a 
line  of  tile  under  the  middle  of  the  traveled  portion.     With  the 
same  depth  of  digging,  a  tile  under  the  side  ditch  is  more  effective 
than  one  under  the  center  of  the  road.     Further,  if  the  tile  is  under 
the  center,  there  is  liability  of  the  settling  of  the  soil  in  the  trench, 
which  will  make  a  depression  and  probably  a  mud  hole;   and  if  the 
tile  becomes  stopped,  it  is  expensive  to  dig  it  up,  and  the  doing  so 
interferes  with  traffic.     Finally,   if  the  road  is  ever  graveled  or 
macadamized,  the  disadvantage  of  having  the  tile  drain  under  the 
center  of  the  road  is  materially  increased. 

Some  writers  advocate  the  use  of  a  line  of  tile  near  the  surface, 
on  each  side  of  the  trackway.  The  object  of  placing  the  tile  in  this 
position  is  to  secure  a  rapid  drainage  of  the  surface;  but  very  little, 
if  any,  water  from  the  surface  will  ever  reach  a  tile  so  placed,  since 
the  road  surface  when  wet  is  puddled  by  the  traffic,  which  pre- 
vents the  water's  percolating  through  the  soil.  It  is  certain  that 
in  clay  or  loam  the  drainage  thus  obtained  is  of  no  practical  value. 
Many  farmers  have  tried  to  drain  their  barn-yards  by  laying  tile 
near  the  surface,  but  always  without  appreciable  effect.  The 
deeper  the  tile  the  better  the  drainage. 

The  .rapid  surface  drainage  sought  by  putting  a  tile  or  its  equiva- 
lent near  the  surface,  can  best  be  secured  by  giving  the  surface  of 
the  road  a  proper  crown  and  keeping  it  free  from  ruts  and  holes 
(§  205.) 

While  a  line  of  tile  on  one  side  of  the  road  is  usually  sufficient, 
there  is  often  a  great  difference  as  to  the  side  on  which  it  should  be 
laid.  If  one  side  of  the  road  is  higher  than  the  other,  the  tile  should 
be  on  the  high  side  to  intercept  the  ground  water  flowing  down  the 
slope  under  the  surface.  Sometimes  a  piece  of  road  is  wet  because 
of  a  spring  in  the  vicinity,  or  perhaps  the  road  is  muddy  because 
of  a  stratum  which  brings  the  water  to  the  road  from  higher  ground ; 
in  either  case,  the  source  of  supply  should  be  tapped  with  a  line 
of  tile  instead  of  trying  to  improve  the  road  by  piling  up  earth. 

125.  Side  Ditches.     The  side  ditches  are  to  receive  the   water 
from  the  surface  of  the  traveled  way,  and  should  carry  it  rapidly  and 
entirely  away  from  the  roadside.     They  are  useful,  also,  to  inter- 
cept and  carry  off  water  that  would  otherwise  flow  from  the  side 
hills  upon  the  road.     Ordinarily  they  need  not  be  deep;    but,  if 
possible,  should  have  a  broad,  flaring  side  toward  the  traveled  way, 


ART.    1]  CONSTRUCTION  79 

to  prevent  accident  if  a  vehicle  should  be  crowded  off  the  side  of 
the  roadway.  The  outside  bank  should  be  flat  enough  to  prevent 
caving. 

If  the  road  is  tiled  as  above  recommended,  the  side  ditch  need 
not  be  very  large;  but  it  should  be  of  such  a  form  as  to  permit  its 
construction  with  the  road  machine  or  scraping  grader  (§  155)  or 
with  a  drag  scraper  (§  150),  instead  of  by  hand.  On  comparatively 
level  ground,  the  proper  form  of  side  ditch  is  readily  and  cheaply 
made  with  the  usual  road  machine.  Fig.  11,  page  82,  shows  a  shal- 
low ditch  of  the  proper  form;  and  Fig.  12  shows  a  deeper  one  of  the 
same  general  form.  If  a  larger  ditch  is  needed,  it  should  be  of  such 
a  form  as  to  be  made  chiefly  with  the  drag-scoop  scraper. 

A  deep  narrow  ditch  is  expensive  to  maintain,  since  it  is  easily 
obstructed  by  caving  banks,  by  weeds,  and  by  floating  trash.  For- 
tunately the  shallow  ditch  is  easy  and  cheap  to  construct  and  also 
to  maintain.  If  it  is  necessary  to  carry  water  along  the  side  of  the 
road  through  a  rise  in  the  ground,  it  is  much  better  to  lay  a  line 
of  tile  and  nearly  fill  the  ditch  than  to  attempt  to  maintain  a  narrow 
deep  ditch.  A  tile  is  much  more  effective  per  unit  of  cross  section 
than  most  open  ditches. 

126.  The  side  ditch  should  have  a  uniform  grade  and  a  free  out- 
let into  some  stream,  so  as  to  carry  the  water  entirely  away  from 
the  road.  No  good  road  can  be  obtained  with  side  ditches  that 
hold  the  water  until  it  evaporates.  For  this  reason  much  ostensible 
road  work  is  a  positive  damage.  Piling  up  the  earth  in  the  middle 
of  the  road  is  perhaps  in  itself  well  enough,  but  leaving  undrained 
holes  at  the  side  probably  more  than  counterbalances  the  benefits 
of  the  embankment.  A  road  between  long  artificial  ponds  is  always 
inferior  and  is  often  impassable.  It  is  cheaper  and  better  to  make 
a  lower  embankment,  and  to  drain  thoroughly  the  holes  at  the  side 
of  the  road.  Public  funds  often  can  be  more  widely  used  in  making 
ditches  in  adjoining  private  lands  than  in  making  ponds  at  the 
roadside  in  an  attempt  to  improve  the  road  by  raising  the  surface. 
It  is  cheaper  and  better  to  allow  the  water  to  run  away  from  the  road 
than  to  try  to  lift  the  road  out  of  the  water. 

When  the  road  is  in  an  excavation,  great  care  should  be  taken 
that  a  ditch  is  provided  on  each  side  to  carry  away  the  water  so  that 
it  shall  not  run  down  the  middle  of  the  road.  Every  road  should 
have  side  ditches,  even  one  that  runs  straight  down  the  side  of  a 
hill.  Indeed,  although  it  often  has  none,  the  steepest  road  needs  the 
side  ditch  most.  Frequently  the  water  runs  down  the  middle  of 


80  EARTH    ROADS  [CHAP.    Ill 

the  road  on  a  side  hill  and  wears  it  into  gullies,  which  are  a  discom- 
fort, and  often  dangerous,  in  both  wet  weather  and  dry. 

In  a  slightly  rolling  country,  the  side  ditch  frequently  has  no 
outlet,  and  the  water  is  allowed  to  accumulate  at  the  foot  of  the 
slope  and  there  remain  until  it  is  absorbed  by  the  ground  or  seeps 
into  the  tile  drain.  The  water  seeps  away  very  slowly  because  the 
fine  silt  carried  down  by  the  water  fills  up  the  pores  of  the  native 
soil  and  renders  it  nearly  impervious.  The  difficulty  could  be 
remedied  by  providing  an  inlet  from  the  open  ditch  to  the  tile.  This 
may  be  a  well,  walled  with  plank  or  masonry  without  mortar  (except 
near  the  topj  and  having  a  grating  in  the  side  or  top  through  which 
the  water  may  pass.  The  well  should  be  large  enough  to  allow  a 
man  to  enter  it  to  clean  it,  and  should  extend  a  foot  or  more  below 
the  bottom  of  the  tile.  Earth  roads  in  villages  and  towns  are  usually 
better  provided  with  such  inlets  than  country  roads,  but  both  could 
be  materially  improved  at  comparatively  small  expense  by  pro- 
viding inlets  from  the  side  ditch  into  the  tile. 

127.  If  it  can  be  prevented,  no  attempt  should  be  made  to  carry 
water  long  distances  in  side  ditches;  for  large  bodies  of  water  are 
hard  to  handle,  and  are  liable  to  become  very  destructive.  Side 
ditches  should  discharge  frequently  into  the  natural  watercourses, 
though  to  compass  this,  it  may  in  some  cases  be  necessary  to  carry 
the  water  from  the  high  side  to  the  low  side  of  the  road.  This  is 
sometimes  done  by  digging  a  gutter  or  by  building  a  dam  diagonally 
across  the  road,  but  both  are  very  objectionable.  A  better  way 
is  to  lay  a  tile  or  put  in  a  culvert  (Fig.  53,  page  198),  the  amount 
of  water  determining  which  shall  be  done. 

It  is  sometimes  necessary  to  carry  water  a  considerable  distance 
in  the  side  ditches,  as,  for  eaxmple,  when  the  road  is  in  excavation. 
This  requires  deep  ditches,  which  are  undesirable  and  dangerous; 
and  if  the  grade  is  considerable,  the  ditches  wash  rapidly.  In  such 
cases,  it  is  wise  to  lay  a  line  of  tile  under  the  side  ditch,  and  at 
intervals  turn  the  water  from  the  surface  ditch  into  the  tile  drain. 
This  can  be  accomplished  readily  by  inserting  in  the  line  of  porous 
tile  a  Y  section  of  vitrified  pipe,  with  the  short  arm  opening  up  hill. 
Of  course,  the  short  arm,  i.  e.,  the  vertical  arm,  need  not  be  as  large 
as  the  body.  If  necessary,  two  or  three  lengths  of  porous  tile  may 
be  added  at  the  upper  end  of  the  Y  to  make  connections  with  the 
bottom  of  the  open  ditch.  Earth,  sods,  or  stones,  can  be  piled 
around  the  upper  end  of  the  tile  to  make  a  dam  and  to  hold  the  tile 
in  place. 


ART.    1]  CONSTRUCTION  81 

Some  road  engineers  lay  a  line  of  tile  under  the  side  ditch,  and 
fill  the  trench  with  broken  stone,  thus  making  the  tile  carry  both 
the  surface  water  and  the  underdrainage.  This  practice  probably 
affords  better  surface  drainage,  but  it  costs  more  than  to  allow  the 
surface  water  to  flow  away  in  the  side  ditches.  This  construction 
is  sometimes  defended  on  the  ground  that  the  broken  stone  prevents 
the  wheels  from  striking  the  tile  when  vehicles  in  passing  are  forced 
into  the  ditches.  This  danger  does  not  seem  very  great,  and  would 
not  occur  at  all  if  the  tile  were  laid  at  the  proper  depth;  but  this  is 
sometimes  impossible  owing  to  a  hard  substratum. 

128.  As  a  rule  side  ditches  will  not  have  too  much  fall;    but 
sometimes  a  ditch  straight  down  a  hill  will  have  so  much  as  to  wash 
rapidly,  in  which  case  it  is  an  advantage  to  put  in  an  obstruction  of 
stone  or  brush.     In  extreme  cases  the  bottom  of  the  ditch  is  paved 
with  stones. 

129.  Surface  Drainage.    The  drainage  of  the  surface  of  a  road 
is  very  important,  and  is  provided  for  by  crowning  the  surface 
and  keeping  it  smooth.     It  should  be  remembered  that  water  upon 
the  surface  of  the  road  can  not  be  carried  away  by  the  underdrains, 
since  the  water  can  reach  them  only  after  it  has  penetrated  and 
softened  the  road  surface.     The  slope  from  the  center  to  the  side 
should  be  enough  to  carry  the  water  freely  and  quickly  to  the  side 
ditch;  and  if  the  surface  is  kept  free  from  ruts  and  holes,  less  crown 
will  suffice  than  if  no  attention  is  given  to  keeping  the  surface  smooth. 
If  there  is  not  enough  crown,  the  water  can  not  easily  reach  the 
side  ditches;   and  hence  the  road  soon  becomes  watersoaked. 

On  the  other  hand,  the  crown  may  be  too  great.  If  the  side 
slopes  are  so  steep  that  traffic  keeps  continually  in  the  middle,  the 
road  will  be  worn  hollow  and  retain  the  water  instead  of  shedding  it 
promptly  to  the  side  ditches.  If  the  crown  is  too  great,  it  is  difficult 
for  vehicles  to  turn  out  in  passing  each  other.  Again,  if  the  earth 
is  piled  too  high  in  the  middle,  the  side  slopes  will  be  washed  into 
the  side  ditches,  which  not  only  damages  the  road  but  also  fills  up 
the  ditches.  Further,  if  the  side  slopes  are  steep,  the  top  of  the 
wheel  will  be  further  from  the  center  of  the  road  than  the  bottom; 
and  the  mud  picked  up  by  the  bottom  of  the  wheel  will  be  carried  to 
the  top  of  the  wheel  and  then  dropped  farther  from  the  center  of  the 
road  than  it  was  before,  each  passing  vehicle  moving  the  earth 
from  the  center  toward  the  side  of  the  road.  Finally  with  the 
ordinary  method  of  caring  for  earth  roads,  more  water  stands  on  a 
very  convex  road  than  on  a  flatter  one. 


82 


EARTH    ROADS 


[CHAP,  in 


The  slope  from  the  center  to  the  side  should  be  at  least  half  an 
inch  to  a  foot,  or  1  foot  in  24  feet;  and  it  should  not  be  more  than  1 
inch  to  a  foot,  or  1  foot  in  12  feet.  If  the  surface  is  well  cared  for, 
the  former  is  better  than  the  latter;  but  in  no  case  is  it  wise  to 
exceed  the  latter  slope. 

Some  claim  that  theoretically  the  cross  section  of  the  surface 
should  be  the  arc  of  a  circle,  and  others  that  it  should  consist  of 
two  planes  meeting  at  the  center  and  having  their  junction  rounded 
off  with  a  short  curve.  Great  refinement  in  this  matter  is  neither 
possible  nor  important.  Two  examples  of  a  properly  crowned 
road  are  shown  in  Figs.  11  and  12.  The  crown  can  be  easily  and 
cheaply  constructed  with  the  scraping  grader  (§  155). 


FIG.  11. — ROAD  SURFACE  AN  ARC.     SHALLOW  SIDE  DITCH. 

The  drainage  of  the  surface  of  a  road  is  chiefly  a  matter  of  main- 
tenance (see  Art.  2  of  this  chapter);  and  one  of  the  most  common 
defects  of  maintenance  is  the  failure  to  fill  ruts  and  keep  the  surface 
smooth  so  that  the  water  will  .be  promptly  discharged  into  the  side 
ditches.  A  comparatively  shallow  rut  will  nullify  the  effect  of  any 


f — en 


0.5V/?.  Tile 

FIG.  12. — ROAD  SURFACE  AN  ARC.     DEEP  SIDE  DITCH. 

reasonable  amount  of  crown.  Seldom  is  a  mile  of  road  seen  which 
does  not  have  a  number  of  ruts  and  saucer-like  depressions  which 
catch  and  hold  the  water.  On  undulating  roads,  ruts  and  holes  are 
naturally  drained;  and  this  is  the  reason  why  undulating  roads  are 
better  than  perfectly  flat  ones  (see  Minimum  Grade,  §86). 

Fig.  13  shows  a  form  of  cross  section  sometimes  adopted  for 
earth  roads  in  villages  and  towns.  The  gutter  usually  is  made  next 
to  the  sidewalk,  which  is  objectionable,  since  horses  must  stand 
in  the  mud  and  water  when  hitched  in  front  of  the  abutting  prop- 
erty. The  form  shown  in  Fig.  12  is  free  from  this  objection.  A  nar- 
row berm  is  left  between  the  sidewalk  and  the  edge  of  the  slope  to 
prevent  crowding  the  gutter  too  close  to  the  shade  trees,  which  are 


ART.    1]  CONSTRUCTION  83 

usually  planted  just  outside  of  the  sidewalk.  The  gutter  shown  in 
Fig.  12  decreases  the  available  wheel  way,  and  consequently  in  some 
localities  would  be  undesirable.  This  cross  section  also  can  be  made 
and  maintained  with  the  ordinary  scraping  grader. 

j< aft >i< — Qff -** /? rft -»i 

Walk  \ ] ^^-— TI L-^. fl 

I*  4ft  <n  -^^-^^r-  /2fff,  j 

FIG.  13. — CROSS  SECTION  OF  VILLAGE  STREET. 

130.  The  crown  should  be  greater  on  steep  grades  than  on  the 
more  level  portions,  since  on  the  grade  the  line  of  steepest  descent  is 
not  perpendicular  to  the  length  of  the  road,  and  consequently  the 
water  in  getting  from  the  center  of  the  road  to  the  side  ditches  travels 
obliquely  down  the  road.  If  the  water  once  commences  to  run 
down  the  center  of  the  roadway  on  a  steep  grade,  the  wheel  tracks 
are  quickly  deepened,  and  the  road  becomes  rough  and  even  danger- 
ous. Under  these  circumstances,  it  is  necessary  to  construct  catch- 
waters  ("  water-breaks,"  "  hummocks,"  or  "  thank-you-marms  ") 
at  intervals  to  catch  the  water  which  runs  longitudinally  down  the 
road,  and  to  convey  it  to  the  side  ditches.  These  catch-waters  may 
be  either  broad  shallow  ditches  or  low  flat-ridges  constructed  across 
the  road;  and  they  may  slope  toward  one  or  both  side  ditches.  In 
the  former  case,  they  should  cross  the  road  diagonally  in  a  straight 
line;  and  in  the  latter  case,  in  plan  they  should  be  a  broad  angle 
with  the  apex  at  the  center  of  the  road  and  pointing  up  hill.  There 
is  little  or  no  difference  between  the  merits  of  the  ditch  and  the  ridge, 
unless  the  bottom  of  the  former  is  paved  with  gravel,  broken  stone, 
or  cobbles.  The  ridges  are  more  common,  but  usually  are  so  narrow 
and  so  high  as  to  form  a  serious  obstruction  to  travel,  a  fact  which 
is  especially  objectionable  since  the  introduction  of  the  automobile. 
However,  neither  the  ditches  nor  the  ridges  should  be  used  except 
on  steep  grades  where  really  necessary,  since  either  form  is  at  best 
an  obstruction  to  travel.  The  angle  that  the  catch-waters  shall 
have  with  the  axis  of  the  road  should  be  governed  by  the  steepness 
of  the  grade — the  steeper  the  grade  the  more  nearly  should  the 
catch- waters  run  down  the  road.  They  should  have  a  considerable 
breadth  so  that  wheels  may  easily  ascend  them  and  horses  will  not 
stumble  over  them. 

Catch-waters  should  be  constructed  also  in  a  depression  where 
an  ascending  and  a  descending  grade  meet,  in  order  that  they  may 
collect  the  water  that  runs  down  the  traveled  way  and  convey  it 


84  EARTH   ROADS  [CHAP.    Ill 

into  the  side  ditches.  These  catch-waters  should  run  square  across 
the  road,  and  should  be  quite  shallow  ditches,  the  bottom  of  which 
should  be  hardened  with  gravel,  broken  stone,  or  cobbles. 

131.  Some  writers  recommend  that  the  surface  of  a  road  on  the 
face  of  a  hillside  should  consist  of  a  single  slope  inclining  inwards 
(see  Fig.  14).  This  form  of  surface  is  advisable  on  sharp  curves,  but 


Fio.  14. — IMPROPER  CROSS  SECTION  OF  ROAD  ON  SIDE  HILL. 

is  of  doubtful  propriety  elsewhere.  The  only  advantage  of  this 
form  is  that  the  water  from  the  road  is  prevented  from  flowing  down 
the  outer  face  of  the  embankment;  but  the  amount  of  rain  water 
falling  upon  one  half  of  the  road  can  not  have  a  very  serious  effect 
upon  the  side  of  the  embankment.  With  a  roadway  raised  in  the 
center  and  the  water  draining  off  to  either  side,  the  drainage  will  be 
more  effectual  and  speedy  than  if  the  drainage  of  the  outer  half 
must  pass  over  the  inner  half.  If  the  surface  is  formed  of  one  plane, 
as  in  Fig.  14,  the  lower  half  of  it  will  receive  the  greater  share  of  the 
travel  as  the  tendency  is  to  keep  away  from  the  edge;  and  as  this 
part  of  the  surface  will  bo  more  poorly  drained,  it  is  nearly  certain 
to  wear  hollow.  This  will  interfere  with  the  surface  drainage;  and 
consequently  a  road  with  this  section  will  require  excessive  attention 
to  keep  it  in  good  condition.  Figs.  53  and  54,  page  198,  show  two 
forms  of  Swiss  hillside  roads  having  the  center  higher  than  either 
side. 

Whatever  the  form  of  the  road  surface,  if  the  hillside  is  steep 
there  should  be  a  catch-water  above  the  road  to  prevent  the  water 
from  the  hillside  above  flowing  down  on  the  road.  Fig.  14  shows 
such  a  catch-water.  It  should  be,  say,  6  feet  back  from  the 
excavation,  and  should  have  a  width  and  depth  according  to  the 
amount  of  water  to  be  intercepted. 

132.  EXCAVATION  AND  EMBANKMENT.  Side  Slopes.  The 
angle  of  the  slopes  of  the  cuts  and  fills  is  designated  by  the  ratio  of 
the  horizontal  to  the  vertical  distance.  Thus,  if  the  face  of  the  fill 


ART.    1] 


CONSTRUCTION 


85 


has  an  inclination  of  1  \  feet  horizontal  to  1  foot  vertical,  the  slope  is 
designated  as  If  to  1. 

The  slope  of  the  excavations  varies  with  the  nature  of  the  soil, 
being  for  economy  as  steep  as  the  tenacity  of  the  soil  will  permit. 
Solid  rock  may  be  cut  with  a  slope  of  \  to  1.  Common  earth  will 
stand  1  to  1,  or  1 1  to  1 — the  latter  being  safer  arid  more  usual.  Gravel 
requires  \\  to  1.  Some  clays  will  stand  1  to  1,  while  some  require 
a  much  flatter  slope — in  extrene  cases  6  to  1.  Fine  sand  requires 
a  slope  of  2  to  1,  or  3  to  1. 

The  slope  of  embankment  has  less  range  than  that  of  excava- 
tions, since  there  is  less  variety  in  the  nature  and  the  condition  of 
the  materials,  and  is  usually  \\  to  1. 

133.  In  both  railroad  and  wagon-road  work,  it  is  customary  to 
establish  all  earthwork  slopes  as  planes  intersecting  each  other  in 
right  lines.  The  original  form  is  never  maintained,  since  it  is  not  a 
form  of  equilibrium  and  stability.  Storm  water  soon  washes  away 
the  angle  formed  by  the  intersection  of  the  two  plane  surfaces  at  the 
top  of  the  embankment,  and  the  water  flowing  down  the  slope  soon 
rounds  out  the  angle  at  the  foot.  Such  construction  violates  one  of 
the  fundamental  principles  of  stability,  and  it  is  a.  needless  expense  to 
build  laboriously  a  form  of  construction  which  nature  will  inevitably 
destroy. 

The  transverse  contours  of  the  embankment  and  excavation 
shown  in  Figs.  15  and  16  are  designed  to  meet  the  above  objections 


FIG.  15. — CROSS  SECTION  FOB  EMBANKMENT. 


to   the   ordinary   forms   of   construction.     These   sections   are   de- 
signed in  accordance  with  the  forms  of  railroad  excavations  and 


'to       / 
IP     / 

I  /  OT/te  OT/'/e 

Fio.  16. — CROSS  SECTION  FOR  EXCAVATION. 


\» 


embankments  recommended  by  D.  J.  Whittemore,  the  distinguished 
chief  engineer  of  the  Chicago,  Milwaukee  and  St.  Paul  Railroad, 


EARTH   ROADS  [CHAP.    Ill 


whose  forms  have  met  with  the  unanimous   approval  of  leading 
engineers. 

It  is  customary  in  railroad  construction  to  make  the  top  of  the 
earth  embankment  wider  than  the  base  of  the  gravel  or  broken- 
stone  ballast,  which  gives  a  berm  between  the  base  of  the  ballast 
and  the  outer  edge  of  the  earth  embankment.  This  berm  has  been 
omitted  in  Figs.  15  and  16,  since  with  an  earth  surface  there  is 
nothing  corresponding  to  the  ballast. 

134.  If  the  natural  slope  above  the  cut  is  long  or  steep,  a  catch- 
water  drain  should  be  constructed  along  the  upper  edge  of  the  exca- 
vation slope  to  prevent  the  surface  water   from  above   washing 
down  over  the  face  of  the  cut;   but  the  catch-water  should  be  well 
back  from  the  edge  of  the  excavation,  to  prevent  the  water  in  the 
drain  from  softening  the  upper  angle  of  the  slope  (Fig.  14,  page  84). 

The  slopes  of  both  excavations  and  embankments  should  be 
sowed  with  grass  seed.  Sometimes  the  material  of  the  embank- 
ment is  such  that  grass  seed  will  not  grow,  in  which  case  it  may  be 
necessary  to  lay  sod;  but  of  course  this  is  very  expensive.  The 
loots  of  the  grass  will  hold  the  earth  from  slipping,  and  prevent 
the  face  of  the  slope  from  being  gullied  out  and  washed  down. 

135.  There  is  a  tendency  for  workmen  in  order  to  decrease  the 
amount  of  labor  required  to  leave  the  side  slopes  of  embankments 
hollow  and  those  of  excavations   rounding.     When   inspecting  the 
work,  this  tendency  should  be  borne  in  mind. 

136.  Setting  Siope  Stakes.    For  instructions  as  to  methods  of 
staking  out  the  ground  preparatory  to  beginning  the  work  of  exca- 
vating and  embanking,  see  any  of  the  standard  volumes  on  railroad 
engineering. 

137.  Computing    Earthwork.    For    the    methods    employed    in 
computing  the  contents  of  excavations  and  embankments,  see  any 
of  the  various  treatises  on  that  subject;  or  for  a  briefer  presentation 
of  the  subject,  see  books  on  surveying  or  railroad  engineering. 

138.  Balancing  Cuts  and  Fills.    Other  things  being  equal,  the 
most  economical  position  of  the  grade  line  is  that  which  makes  the 
amount  of  cuts  and  fills  equal  to  each  other.     If  the  cuts  are  the 
greater,  the  earth  therefrom  must  be  wasted,  i.  e.,  deposited  in  spoil 
banks;    and  if  the  fills  are  the  greater,  the  difference  must  be  ob- 
tained from  borrow  pits, — both  of  which  operations  involve  addi- 
tional expense  for  labor  and  land.     Sometimes  it  is  more  economical 
to  make  an  embankment  from  near-by  borrow  pits  than  to  bring 
the  necessary  material  from  a  far-distant  cut;   or,  vice  versa,  it  is 


ART.    1]  CONSTRUCTION  87 

sometimes  more  economical  to  waste  the  material  from  a  cut  than 
to  send  it  to  a  remote  fill.  The  most  economical  use  of  the  material 
depends  upon  the  machinery  used  in  moving  the  earth,  the  char- 
acter of  the  earth  in  both  cuts  and  fills,  the  road  over  which  the 
earth  must  be  transported,  the  cost  of  haul,  the  price  of  land,  the 
liability  of  cuts  being  filled  with  snow,  etc.;  and  the  matter  must 
be  decided  by  the  engineer  to  the  best  of  his  judgment  in  each 
particular  case. 

When  the  road  lies  along  the  side  of  a  hill,  one  side  of  the  road  is 
usually  in  cut  and  the  other  in  fill;  and  it  is  customary  so  to  place 
the  center  line  that  these  two  parts  are  at  least  nearly  equal.  How- 
ever, where  the  side  slopes  are  steep,  it  is  better  to  make  the  road 
mostly  in  cuts  on  account  of  the  difficulty  of  forming  stable  fills  on 
steep  slopes. 

139.  In  railroad  work  it  is  the  custom  to  balance  cuts  and  fills 
on  the  longitudinal  profile  of  the  road,  but  in  wagon-road  work  the 
fills  as  shown  by  the  profile  of  the  center  line  should  be  slightly  in 
excess,  to  provide  a  place  for  the  earth  taken  from  the  side  ditches. 
On  account  of  the  expense,  wagon  roads  follow  the  surface  more 
nearly  than  railroads;   and  consequently  the  earth  from  the  ditches 
is  proportionally  more  in  wagon-road  construction  than  in  railroad 
construction. 

140.  Shrinkage  of  Earthwork.     With  the  ordinary  soil,  the  act  of 
excavation  so  breaks  it  up  that  it  occupies  more  space  after  excava- 
tion than  before;    but  when  the  material  has  been  placed  in  an 
embankment  it  will  usually  occupy  less  space  than  in  its  original  posi- 
tion.    The  expansion  due  to  excavation  is  usually  8  to  12  per  cent 
of  the  volume,  and  in  extreme  cases  may  be  40  per  cent;    but  in 
placing  the  material  in  the  embankment,  it  is  compacted  by  the 
weight  of  the  embankment  itself,  by  the  pounding  of  the  hoofs  and 
by  the  action  of  the  wheels,  until  usually  the  final  volume  is  less 
than  the  original. 

At  first  thought  it  seems  strange  that  earth  should  occupy  less 
space  when  placed  in  an  embankment  than  when  in  its  original 
position,  seeing  that  it  is  not  so  hard  and  firm,  and  that  it  will 
usually  settle  still  farther  after  the  embankment  is  completed. 
The  following  facts  account  for  this  phenomenon:  1.  The  continued 
action  of  frost  has  made  the  soil  in  its  natural  position  more  or  less 
porous.  2.  Earths  which  have  been  lying  in  situ  for  centuries 
become  more  or  less  porous  through  the  slow  solution  of  their  soluble 
constituents  by  percolating  water.  3.  The  surface  soil  is  rendered 


£8  EARTH  ROADS  [CHAP.   Ill 

more  or  less  porous  by  the  penetration  of  vegetable  roots  which 
subsequently  decay.  4.  There  is  ordinarily  more  or  less  soil  lost 
or  wasted  in  transporting  it  from  the  excavation  to  the  embank- 
ment. 

The  amount  of  shrinkage  depends  chiefly  upon  the  character  of 
the  material  and  the  means  by  which  it  is  put  into  the  embankment, 
and  somewhat  upon  the  moisture  of  the  soil,  the  rainfall  conditions 
while  the  work  is  in  progress  and  soon  afterwards,  and  the  depth  to 
which  frost  usually  penetrates.  If  the  soil  is  moist  when  placed  in 
the  bank,  it  will  become  more  compact  than  if  it  is  dry.  Rain 
greatly  affects  the  shrinkage,  and  embankments  put  up  during  a 
rainy  season  will  be  more  compact  than  those  built  during  a  dry 
time.  Soil  from  above  the  usual  frost  line  is  more  porous  than  that 
not  subject  to  the  heaving  effect  of  alternating  freezing  and  thawing, 
and  consequently  shrinks  more  when  put  into  an  embankment. 

The  natural  shrinkage  of  the  ordinary  soils  is  in  the  following 
order:  (1)  sand  and  sandy  gravel  least,  (2)  clay  and  clayey  soil 
intermediate,  and  (3)  loams  most.  The  shrinkage  according  to 
the  method  of  handling  is  in  the  following  order,  beginning  with 
the  least:  (1)  drag  scrapers,  (2)  wheel  scrapers,  (3)  wagons,  (4)  cars, 
(5)  wheelbarrows.  The  usual  allowance  for  shrinkage  for  drag- 
scraper  work  is  as  follows:  gravel  8  per  cent,  gravel  and  sand  9 
per  cent,  clay  and  clayey  earth  10  per  cent,  loam  and  light  sandy 
earth  12  per  cent,  loose  vegetable  surface-soil  15  per  cent.  The 
above  results  are  for  ordinary  earth,  and  do  not  apply  to  such 
unusual  materials  as  "  buckshot,"  gumbo,  very  fibrous  soil,  etc., 
which  have  a  much  greater  shrinkage.  Solid  rock  will  expand  40 
to  50  per  cent. 

The  shrinkage  of  earth  should  be  considered  in  locating  the  grade 
lines  to  balance  the  cuts  and  fills. 

141.  Settlement  of  Embankments.  The  shrinkage  of  earth- 
work referred  to  above  takes  place  chiefly  during  construction,  but 
the  continued  action  of  the  weight  of  the  embankment  and  the 
effect  of  rain  and  traffic  will  usually  cause  a  comparatively  small 
settlement  after  completion.  Sand  or  gravel  embankments  built 
with  wheel  scrapers  will  usually  settle  1  to  2  per  cent  after  comple- 
tion, and  clay  or  loam  embankments  about  2  to  3  per  cent.  With 
drag  scrapers  the  settlement  will  usually  be  a  little  less  than  the 
above;  and  with  dump  carts  or  wagons,  a  little  more.  With  wheel- 
barrows the  settlement  is  usually  about  10  per  cent,  but  may  be  as 
much  as  25  per  cent,  depending  upon  the  moisture  in  the  soil,  the 


.    1]  CONSTRUCTION 


rain  during  construction,  and  the  length  of  time  under  construc- 
tion. 

The  settlement  of  the  embankment  after  completion  should  be 
taken  into  account  when  determining  whether  the  bank  has  been 
raised  to  the  proper  height.  The  .embankment  should  be  built  to 
such  a  height  that  after  it  has  ceased  to  settle  it  will  be  at  grade. 
The  length  of  time  required  for  this  settlement  depends  upon  the 
weather  conditions.  The  proper  adjustment  of  the  height  of  the 
embankments  to  compensate  for  future  settlement  is  an  important 
matter  with  broken-stone  roads  and  with  pavements. 

142.  The  above  remarks  about  settlement  do  not  apply  to  em- 
bankments built  with  the  elevating  grader   (§  161).     The  settle- 
ment of  earth  roads  put  up  by  these  machines  is  of  no  importance, 
and  depends  upon  the  amount  of  rolling  they  receive. 

143.  Rolling  the   Embankment.     Many  writers  on  roads  rec- 
ommend the  rolling  of  all  new  earth  embankments.     In  view  of 
the  usual  settlement  of  banks  built  with  drag  or  wheel  scrapers,  it 
does  not  appear  that  rolling  with  a  farm  roller  would  be  very  effect- 
ive, and  a  heavier  roller  is  seldom  available.     Simply  rolling  the 
top  of  the  finished  bank  is  not  worth  much,  since  the  effect  of  the 
roller  does  not  reach  very  deep ;  and,  besides,  no  roller  will  compact 
loose  earth  so  that  wheels  and  hoofs  will  not  make  depressions  in 
it.*    Further,  it  is  not  practicable  to  roll  the  bank  during  the 
progress  of  construction,  except  when  the  scraping  and  elevating 
graders  are  used.     Finally,  those  who  travel  the  road  most  are  gen- 
erally the  ones  who  pay  for  the  construction,  and  almost  univer- 
sally they  prefer  to  compact  the  earth  by  traffic. 

It  is  customary  to  roll  the  foundation  of  pavements,  but  the 
chief  object  of  so  doing  is  to  discover  soft  places  rather  than  to  con- 
solidate the  surface;  and,  besides,  the  foundation  of  a  pavement  is 
protected  from  rain  and  the  action  of  wheels,  and  therefore  the 
effect  of  the  rolling  is  permanent,  while  with  an  earth  road  it  is  not. 

144.  Over-haul.    When  earthwork  is  done  by  contract,  the  bid 
includes  the  cost  of  removing  excavated  material  and  depositing 
it  in  embankments,  provided  the  necessary  length  of  haul  does  not 
exceed  a  specified  limit.     When  the  material  must  be  carried  beyond 
this  limit  the  extra  distance  is  paid  for  at  stipulated  price  per  cubic 
yard  per  100  feet  of  haul.     This  extra  distance  is  known  by  the 
name   of   "  over-haul "   or   simply    "  haul."     For   an   explanation 

*  The  heaviest  steam  rollers  give  a  pressure  of  about  600  pounds  per  linear  inch,  while 
wagons  frequently  give  twice,  and  occasionally  three  times,  as  much, 


90  EARTH    ROADS  [CHAP.    Ill 

of  the  method  of  computing  "  haul,"  see  treatises  on  earthwork  or 
books  on  railroad  surveying. 

The  specified  limit,  i.  e.,  the  distance  of  free  haul,  depends  upon 
the  conditions.  It  is  sometimes  made  as  low  as  100  feet,  and  is 
sometimes  2,000  feet — the  latter  usually  only  in  street  work.  In 
railroad  work  500  feet  is  a  common  limit. 

145.  Frequently  all  allowances  for  over-haul  are   disregarded. 
The  profiles,  estimates  of  quantities,  and  the  required  disposal  of 
material  are  shown  to  bidding  contractors;    and  they  must  then 
make  their  own  allowances,  and  bid  accordingly.     This  method  has 
the  advantage  of  avoiding  possible  disputes  as  to  the  amount  of  the 
over-haul  allowance,  and  on  this  account  is  adopted  by  some  railroads. 

146.  Stability  of  Embankments.    The  principles  to  be  observed 
in  the  formation  of  an  embankment  depend  somewhat  upon  the 
machinery  employed  in  doing  the  work,  but  a  few  general  considera- 
tions are  not  out  of  place  here. 

Specifications  usually  require  that  "  all  matter  of  vegetable 
nature  must  be  carefully  excluded  from  the  embankment."  It  is 
impracticable  to  do  this  when  the  road  passes  through  grass  land — 
particularly  if  the  grade  is  built  with  a  scraping  grader  (§  155). 
It  is  desirable  to  remove  brush,  tall  grass,  and  high  weeds  from  the 
space  to  be  occupied  by  the  embankment  and  the  borrow  pit;  but 
small  twigs,  leaves,  and  sod  are  no  material  detriment,  and  their 
removal  is  a  needless  expense — except  at  the  point  where  the  road 
passes  from  cut  to  fill.  It  is  essential  that  all  vegetable  matter 
and  loose  porous  soil  should  be  removed  at  this  point,  otherwise 
there  will  be  a  soft  place  which  will  soak  up  water  and  make  a  mud 
hole  and  also  weaken  the  bank  just  below  it.  When  an  embank- 
ment is  to  be  made  across  a  swamp,  bog,  or  marsh,  the  site  should 
first  be  drained  as  thoroughly  as  possible. 

Perfect  solidity  should  be  the  aim,  and  all  necessary  precautions 
should  be  taken  to  prevent  or  lessen  the  tendency  of  the  bank  to 
slip.  To  secure  stability,  embankments  should  be  built  in  successive 
layers  not  more  than  3  or  4  feet  thick,  and  the  vehicles  conveying 
the  materials  should  be  required  to  pass  over  the  bank,  so  as  to  con- 
solidate the  earth.  Specifications  sometimes  state  that  the  layers 
shall  be  made  concave,  but  this  refinement  is  scarcely  ever  necessary, 
although  it  is  well  to  see  that  the  layers  are  never  very  much  convex. 
Embankments  are  sometimes  first  built  up  in  the  center,  and  after- 
wards widened  by  tipping  or  dumping  earth  over  the  side;  but  this 
never  should  be  allowed. 


ART.    1]  CONSTRUCTION  91 

When  embankments  are  to  be  formed  on  sloping  ground,  it  may 
be  necessary  to  plow  the  ground  or  to  cut  steps  in  a  rocky  surface 
to  keep  the  filling  from  sliding  down  the  natural  surface.  In  many 
cases  where  roads  are  to  be  constructed  along  steep  slopes,  it  is 
found  cheaper  to  use  retaining  walls  (§  192)  to  sustain  the  road 
upon  the  lower  side  and  the  earth-cut  on  the  upper  side  than  to 
cut  long  slopes  or  form  high  embankments. 

147.  IMPROVING  OLD  ROADS.     Country  roads  may  be  improved 
in  any  of  several  ways: 

1.  By  changing  the  location,  to  secure  better  alignment  or  lower 
gradients.     The  method  of  doing  this  has  been  discussed  in  Art.  2, 
Chapter  I. 

2.  By  cutting  down  the  hills  and  filling  up  the  hollows,  to  secure 
easier  gradients.     A  hill  may  be  cut  down  without  seriously  inter- 
fering with  traffic  by  cutting  one  side  of  the  roadway  down  a  foot 
or  two  with  drag  or  wheel  scrapers  (§  154),  and  then  turning  traffic 
on  this  portion  and  lowering  the  other  side,  continuing  to  cut  down 
each  side  alternately  until  the  desired  depth  is  reached.     If  the 
earth  is  deposited  upon  the  embankment  in  the  hollow,  the  traffic 
will  consolidate  the  road  as  it  is  built  up,  which  is  very  desirable. 

3.  By  laying  tile  and  cutting  open  ditches,  to  improve  the  drain- 
age, as  has  been  discussed  in  §  114-28. 

4.  By  re-forming  the  surface  by  the  use  of  the  scraping  grader 
to  improve  surface  drainage,  as  discussed  in  §  155-58. 

5.  By  adding  sand  or  gravel  to  a  clay  road,  or  clay  to  a  sand 
road,  to  improve  the  surface,  as  considered  in  Chapter  IV. 

148.  ROAD-BUILDING  MACHINERY.     In  recent  years  there  has 
been  a  great  advance  in  the  machinery  employed  in  building  earth- 
roads.     The  wheelbarrow  was  formerly  much  used  for  short  hauls, 
but  has  been  superseded  by  some  form  of  drag  scraper  (§  150)  drawn 
by  horses,  and  is  never  used  now  except  for  very  small  jobs,  or  in 
wet  and  swampy  places.     Formerly  an  embankment  was  constructed 
with  plows  and  drag-scoop  scrapers  (Fig.  17,  page  92),  while  now  it  is 
built  much  more  cheaply  and  better  with  either  the  scraping  grader 
(Fig.  23,  page  96),  or  with  the  elevating  grader  (Fig.  28,  page  101). 
Years  ago  earth  was  thrown  into  wagons  or  carts  by  hand  and  hauled 
to  its   destination,  while   now  it  is  moved  with   the   two-  or  four- 
wheel  scrapers   (Fig.   21    or    22,   page  95).     Earth   was  formerly 
moved  considerable  distances  with  the  drag  scraper,  while  now  the 
wheel  scraper  is  employed.     Formerly  the  surface  of  the  excavation 
was  finished  with  the  drag-scoop  scraper,  while  now  it  is  done  much 


92  EAKTH   ROADS  [CHAP.    Ill 

better  and  more  cheaply  with  the  tongue  scraper  (Fig.   18,  page 
93)  or  the  scraping  grader  (Fig.  23,  page  96). 

There  are  a  variety  of  plows,  dump  carts,  wagons,  etc.,  used  in 
moving  earth,  which  need  not  be  considered  here.  The  dump  cart 
is  much  in  favor  in  the  New  England  States,  but  is  never  used  in  the 
Mississippi  Valley.  The  steam  shovel  and  dump  cars  afford  the 
most  economical  method  of  handling  earth  when  the  amount  to  be 
moved  justifies  the  outlay  for  the  plant;  but  as  that  would  seldom 
be  the  case  in  highway  work,  this  method  will  not  be  considered. 

149.  Scrapers.     Scrapers  are  generally  used  to  move  material 
after  it  has  been  loosened  by  plowing.     There  are  two  principal 
kinds — the  drag  and  the  wheel  scraper. 

150.  Drag  Scrapers.     There  are  three  forms  of  the  drag  scraper 
— the  scoop  (Fig.  17),  the  pole-scraper  (Fig.  18,  page  93),  and  the 
Fresno  scraper  (Fig.  19,  page  93). 

151.  The  drag-scoop  scraper,  Fig.  17,  is  sometimes  referred  to 
as  the  drag  scraper  or  simply  the  drag,  and  also  as  the  slip  scraper  or 

the  slip.  It  is  made  in 
three  sizes.  The  smallest, 
for  one  horse,  has  a  capacity 
of  3  cubic  feet;  and  the  two 
larger  sizes,  for  two  horses, 
have  a  capacity  of  5  and 

FIG.  17.— DRAG-SCOOP  SCRAPER.  .  .      .  a 

7  feet  respectively.  Some 

have  metal  runners  on  the  bottom  and  others  have  practically  a 
double  bottom,  both  of  which  devices  decrease  draft  and  increase 
durability. 

The  drag-scoop  or  slip  scraper  is  much  used  for  moving  earth 
short  distances;  but  with  it  there  is  difficulty  in  building  a  bank 
of  uniform  solidity,  since  each  scraperful  is  deposited  in  a  compact 
mass  by  itself,  with  low  loose  places  between  them.  Nor  is  the 
slip  scraper  suitable  for  finishing  an  embankment,  since  the  surface 
made  with  it  is  a  succession  of  humps  and  hollows  which  is  very 
trying  to  drive  over  when  dry,  and  when  it  rains  the  low  places  fill 
with  water  which  speedily  softens  the  remainder  of  the  road,  and 
finally  produces  mud  holes.  The  pole  or  tongue  scraper  (§152)  is 
much  preferable  for  finishing  the  surface. 

The  drag-scoop  or  slip  scraper  is  sometimes  employed  in  loading 
wagons.  This  is  done  by  building  an  elevated  platform  under  which 
the  wagons  are  driven,  and  to  the  top  of  which  the  earth  is  drawn  in 
a  scoop  scraper  upon  an  inclined  runway.  In  the  middle  of  the  plat- 


ART.    1] 


CONSTRUCTION 


93 


form  is  a  hole  through  which  the  scraper  is  dumped.     This  arrange- 
ment of  platform  and  runways  is  called  a  trap. 

152.  The  pole  or  tongue  scraper,  Fig.  18,  is  ordinarily  used  for 
leveling   up   the   road   surface   in   excavations,    and   is   frequently 


FIG.  18. — POLE  OB  TONGUE  SCRAPER. 

employed  in  preparing  the  subgrade  for  pavements.  It  may  be 
used  to  transport  earth  short  distances,  but  is  not  so  good  for  this 
purpose  as  the  scoop  scraper.  It  is  made  in  two  sizes,  36  and  48 
inches  wide. 

153.  The  Fresno  scraper,  Figs.  19  and  20  is  the  outgrowth  of 


FIG.  19. — FRESNO  SCRAPER,  READY  FOR  LOADING. 

experience  in  irrigation,  and  has  some  advantages  over  the  com- 
mon scoop  scraper.  (1)  The  proportions  are  such  that  it  is  more 
readily  loaded  to  its  full  capacity.  (2)  It  distributes  the  earth  on 


94 


EARTH   ROADS 


[CHAP,  in 


the  bank  better,  as  it  can  be  adjusted  to  deliver  in  layers  from  1  to 
12  inches  thick.  (3)  The  runners  make  it  more  durable.  (4)  It 
is  more  easily  loaded.  (5)  It  will  follow  up  a  steep  bank  without 
dumping,  and  hence  runways  are  not  required. 

Fresno  scrapers  are  made  in  three  sizes,  the  cutting  edge  being 


FIG.  20. — FRESNO  SCRAPER,  DUMPED 

3J  feet,  4  feet,  and  5  feet;  and  their  respective  capacity  is  8,  10,  and 
12  cubic  feet. 

Under  favorable  conditions  this  form  of  scraper  will  push  con- 
siderable earth  along  in  front  of  it,  and  consequently  the  capacity 
is  frequently  stated  as  much  greater  than  that  given  above. 

154.  Wheel  Scrapers.  There  are  two  forms  of  wheel  scrapers, — 
those  with  two  wheels  and  those  with  four.  The  two-wheel  scraper 
consists  of  a  steel  box  mounted  on  wheels  and  furnished  with  levers 
for  raising,  lowering,  and  dumping,  Fig.  21,  page  95.  It  is  made  in 
three  sizes,  No.  1,  2,  and  3,  having  a  capacity  of  9,  12,  and  16  cubic 
feet,  respectively.  Some  manufacturers  make  an  automatic  front 
end-gate  which  adds  materially  to  the  load  the  scraper  will  carry,  par- 
ticularly on  a  rough  down-hill  road. 

The  four-wheel  scraper  is  a  steel  box  or  scoop  suspended  from 
a  frame  supported  upon  four  wheels,  see  Fig.  22,  page  95.  It  is  made 
in  two  sizes,  a  half-yard  and  a  yard  capacity.  It  may  be  loaded  by 
a  snatch  team  or  by  steam  power, — either  a  traction  or  a  hoisting 
engine.  The  larger  size  is  loaded  with  a  20  H.P.  steam  traction 


ART.    1] 


CONSTRUCTION 


95 


engine  or  a  30-60  H.P.  gasoline  tractor,  or  by  a  hoisting  engine; 
but  when   loaded,  it   is   drawn    by   a   two-horse   team.     The   pan 


FIG.  21.— TWO-WHEEL  SCRAPER,  FILLING. 

is  raised  and  lowered  by  the  tractive  power.  Work  can  be  done 
cheaper  with  the  four-wheel  scraper  than  with  the  two-wheel 
scraper,  because  the  former  carries  larger  loads  and  also  because  it 
is  self-loading  and  self-dumping. 
The  four-wheel  scraper  has  been 
called  a  self-loading  and  self- 
dumping  wagon. 

The  four-wheel  or  Maney 
scraper  was  first  made  in  1909, 
and  has  been  used  in  the  far 
west  in  railroad  work.  It  is  used 


extensively  in  preparing  the  sub-  FIG.  22.— FOUR-WHEEL  SCRAPER. 

grade  for  pavements. 

155.  Scraping  Grader.  There  are  several  forms  of  scraping 
graders  of  the  type  shown  in  Fig.  23,  which  differ  in  minor  details 
but  all  of  which  accomplish  substantially  the  same  work.  Each 
consists  of  a  frame  carried  on  four  wheels,  supporting  an  adjustable 
scraper-blade,  the  front  end  of  which  plows  a  furrow  while  the  rear 
end  pushes  the  earth  toward  the  center  of  the  road  or  distributes  it 
uniformly  to  form  a  smooth  surface.  The  blade  can  be  set  at  any 
angle  with  the  direction  of  draft,  or  at  any  height;  and  it  may  also 
be  tilted  forward  or  backward.  This  machine  will  work  in  almost 
any  soil — even  where  a  plow  will  not.  It  is  hauled  by  horses  or 
traction  engine,  usually  the  former,  and  makes  successive  rounds 


EARTH   ROADS 


[CHAP.    Ill 


or  cuts  until  the  desired  depth  of  ditch  and  crown  of  road  is  ob- 
tained. 

This  machine  is  often  called  a  road  grader  and  sometimes  a  blade 
grader;   but  it  is  here  designated  as  a  scraper  grader  to  distinguish 


FIG.  23. — SCRAPING  GRADER. 

it  from  the  elevating  grader  (§  161).  The  scraping  grader  is  an 
important  machine  in  both  the  construction  and  the  care  of  earth 
roads.  As  an  instrument  of  maintenance  it  has  been  called  a  road 
hone,  but  could  more  properly  be  called  a  road  plane. 

Fig.  24  to  27,  pages  98  to  99,  show  the  various  kinds  of  con- 
struction work  that  may  be  done  with  this  type  of  machine.  For 
a  discussion  of  the  work  of  this  machine  in  maintenance,  see  §  208-1 1 . 

Various  devices  are  employed  to  neutralize  the  lateral  resistance 
of  the  earth  to  being  pushed  sidewise  by  the  blade.  In  some  types 
the  whole  rear  end  of  the  machine  may  be  thrown  to  one  side  or  the 
other  by  operating  a  hand-wheel;  in  other  forms  the  rear  axle  is 
shifted  lengthwise  so  that  one  wheel  may  bear  against  the  unplowed 
bank  of  the  ditch;  in  other  cases  either  rear  wheel  can  be  moved  in 
or  out  independently;  in  still  other  types  either  the  front  or  rear 
wheels  or  both  may  be  set  at  any  inclination  by  operating  a  hand- 
wheel;  and  in  other  forms  the  wheels  have  a  flange  which  cuts  into 
the  earth  and  resists  the  lateral  thrust. 

The  scraping  grader  is  of  inestimable  value  in  constructing 
•earth  roads,  as  it  does  the  work  better  and  much  cheaper  than  it 
ran  be  done  either  by  hand  or  with  plows  and  scrapers.  The  work 
done  with  the  scraping  grader  is  also  superior  to  that  done  with 
plows  and  drag  scrapers,  since  the  plow  cuts  deeper  in  some  places 


ART.    1]  CONSTRUCTION  97 

than  others  and  these  places  are  left  full  of  loose  earth  and  soon 
form  holes  which  catch  and  hold  water. 

156.  There  are  scraping  or  blade  graders  on  the  market  having 
only  two  wheels,  and  also  those  having  no  wheels;  but  such  machines 
are  neither  common  nor  efficient.     They  have  been  superseded  by 
the  road  drag  (§  206). 

157.  Operating  the  Scraping  Grader.     To  build  a  road  with  the 
scraping  grader,  first  plow  a  light  furrow  with  the  point  of  the  blade, 
where  the  outside  of  the  ditch  is  to  be  (see  Fig.  24,  page  98).     To 
make  the  blade  penetrate  hard  or  stony  ground,  elevate  the  rear  end 
considerably  and  use  only  the  point.     On  the  second  round,  with 
the  front  and  rear  wheels  in  line  (see  Fig.  25),  drive  the  team  so 
that  the  point  of  the  blade  will  follow  the  furrow  made  the  first 
round,  plowing  a  full  furrow  with  the  advance  end  of  the  blade,  and 
dropping  the  rear  end  somewhat  lower  than  before.     The  third  time 
round,  move  the  earth  previously  plowed  over  toward  the  middle  of 
the  road.     In  moving  the  earth  toward  the  center  of  the  road, 
elevate  the  rear  end  of  the  blade  to  allow  the  earth  to  distribute 
under  it,  so  as  to  build  the  road  at  the  side  of  the  proper  crown 
before  filling  the  -center;   and  if  the  machine  slides  sidewise  instead 
of  pushing  the  ridge  of  earth  toward  the  center,  either  slue  the 
whole  rear  end  of  the  machine  toward  the  center,  or  move  one  hind 
wheel  or  the  whole  rear  axle  laterally  until  the  rear  wheel  bears 
against  the  bottom  of  the  unplowed  bank  at  the  ditch,  or  incline 
the   rear   wheels,   according   to  the   construction  of  the  machine. 
Finally,  return  to  the  ditch  and  plow  it  out  deeper,  moving  the  earth 
over  toward  the  middle  whenever  as  much  is  plowed  as  the  machine 
can  move  at  once.     Repeat  this  until  the  ditches  are  of  the  proper 
depth,  and  the  road  as  full  and  round  as  required. 

A  ridge  should  not  be  left  in  the  middle  of  the  road.  Usually  a 
skillful  handling  of  the  machine  will  prevent  the  formation  of  such 
a  ridge  by  elevating  the  rear  end  of  the  scraping  blade,  thus  allowing 
the  earth  to  lose  out  under  it  as  the  center  of  the  road  is  approached. 
If  the  road  is  very  rough,  it  may  not  be  possible  to  fill  all  the  ruts 
without  at  some  places  forming  a  ridge  in  the  center  of  the  road. 
If  the  ridge  is  formed,  it  can  be  flatted  down  by  setting  the  blade 
square  across  the  road  and  allowing  the  earth  to  flow  under  it;  or 
with  most  machines  the  center  ridge  can  be  leveled  down  by  revers- 
ing the  blade  and  using  the  back  of  it. 

If  the  ground  where  a  road  is  to  be  constructed  is  covered  with 
weeds  and  grass,  it  should  be  cleared  by  burning  or  by  mowing  and 


98 


EARTH   ROADS 


[CHAP.   Ill 


FIG.  24.— SCRAPING  GRADER  MAKING  FIRST  ROUND. 


FIG.  25. — SCRAPING  GRADER  MAKING  SECOND  ROUND. 


ART.    1] 


CONSTRUCTION 


99 


FIQ.  26. — SCBAPING  GRADER  PLOWING  BETWEEN  FRONT  WHEELS. 


FIG.  27. — SCKAPING  GRADER  CUTTING  AWAY  OLD  BANK. 


100  EARTH  ROADS  [CHAP.   Ill 

raking.  With  sod  ground  the  best  road  can  be  obtained  by  first 
cutting  the  sod  as  thin  as  possible  and  moving  it  to  the  center  of  the 
road,  and  then  going  back  to  the  ditch  and  continuing  the  grading 
as  described  above.  To  cut  a  thin  slice  of  sod,  the  scraper  blade 
should  be  as  sharp  as  possible.  When  the  ground  to  be  moved  is 
covered  with  sod  or  wee'ds,  some  operators  make  the  first  cut  on  the 
inside  of  the  ditch  and  at  each  successive  round  cut  a  little  farther 
out,  thus  distributing  the  sod  through  the  earth  forming  the  road- 
way. This  requires  too  much  cutting  with  the  unsharpened  end  of 
the  blade,  and  is  therefore  not  as  good  as  the  method  described  above. 

158.  It  is  best  not  to  put  more  than  4  to  6  inches  of  loose  earth 
into  the  road  at  one  working,  as  that  is  all  that  can  be  thoroughly 
packed  by  traffic.     If  a  greater  amount  is  thrown  up  at  one  time, 
the  bottom  of  the  grade  will  remain  soft  and  cause  the  road  to  cut 
into  deep  ruts  as  soon  as  the  top  has  become  thoroughly  soaked  by 
rain.    As  far  as  possible  the  grading  should  be  done  early  in  the 
summer,  giving  ample  time  for  the  loose  earth  to  settle  and  pack 
before  the  fall  rains.     If  worked   in  the  fall,  there  should  never 
be  more  than  4  inches  of  loose  earth  put  upon  the  road  at  one 
working. 

If  the  maximum  amount  of  earth  is  to  be  placed  upon  the  road 
at  once,  it  is  wise  to  roll  each  successive  layer  with  as  heavy  a  roller 
as  is  available  or  as  a  team  can  draw,  as  otherwise  traffic  will  con- 
solidate only  the  surface,  and  the  bottom  of  the  grade  will  long 
remain  soft  and  spongy. 

159.  The  scraping  grader  is  usually  drawn  by  four  or  six  horses, 
depending  upon  their  size,  and  the  character  and  condition  of  th& 
soil.     One  man  can  operate  the  machine,  and  one  or  two  men  are 
required  to  drive. 

A  traction  engine  is  sometimes  used;  and  it  is  a  better  power, 
since  it  gives  a  steady  draft  and  does  not  need  to  stop  to  rest.  At 
certain  seasons  of  the  year,  the  traction  engine  is  the  cheaper  power, 
and  at  other  times  horses  are  the  cheaper,  depending  upon  the  re- 
quirements of  horses  for  farm  work  and  the  demands  for  the  trac- 
tion engine  in  threshing  and  shelling. 

160.  The  cost  of  building  an  ordinary  prairie  road  with  horse- 
power with  this  machine  is  about  $30  to  $40  per  mile,  with  a  width 
of  30  or  35  feet  and  a  crown  of  6  inches  above  the  natural  surface. 
The  first  is  the  cost  when  there  is  no  sod,  and  the  second  when  there 
is  a  stiff  sod.    A  second  6  inches  may  be  added  for  about  $30  per 
inile, 


ART.  1]  CONSTRUCTION  101 

If  the  ground  is  very  dry  and  hard,  another  team  and  driver  will 
be  required,  and  the  above  prices  may  be  nearly  doubled. 

161.  Elevating  Grader.  The  best  known  form  of  the  elevating 
grader  is  shown  in  Fig.  28.  It  consists  of  a  frame  resting  upon  four 
wheels,  from  which  is  suspended  a  plow  and  a  frame  carrying  a 
wide  traveling  belt.  The  carrier  is  built  in  sections  and  its  height 
is  adjustable.  The  larger  carrier  will  deliver  earth  14,  17,  19,  or  22 
feet  horizontally  and  8  feet  vertically  from  the  plow;  while  the 
smaller  size  will  deliver  14  and  17  feet  horizontally  and  7  feet  verti- 
cally. The  smaller  machine  is  designed  for  highway  work.  The 


FIG.  28. — ELEVATING  GRADER. 

plow  loosens  the  soil  and  throws  it  upon  the  traveling  inclined  belt, 
which  delivers  it  upon  the  embankment  direct  or  into  wagons. 

This  is  an  exceedingly  effective  machine  for  building  open  ditches, 
earth  embankments,  or  filling  wagons.  By  changing  the  length  of 
the  carrier  and  by  properly  distributing  the  earth,  the  machine  will 
build  either  a  broad  low  embankment  from  a  narrow  deep  cutting, 
or  a  narrow  high  embankment  from  a  broad  shallow  cutting;  or 
the  machine  will  excavate  a  deep  narrow  ditch  with  flat  spoil  banks, 
or  a  shallow  ditch  with  narrow  spoil  banks.  This  machine  is  espe- 
cially adapted  to  building  earth  roads  in  a  prairie  country,  for 
which  purpose  it  has  been  very  largely  used. 

The  large  machine  is  usually  propelled  by  twelve  horses — eight 
in  front  and  four  behind, — and  the  smaller  by  eight  in  front.  Often 
a  traction  engine  is  cheaper  than  horses.  One  man  can  operate  the 
machine;  and  at  least  two  men,  and  usually  three,  are  required  to 
drive  the  larger  machine,  but  usually  two  drive  the  smaller  one. 


102 


EARTH  ROADS 


[CHAP,  in 


162.  Operating  the  Elevating  Grader.  To  build  a  new  road  of 
the  sections  shown  in  Fig.  11  and  12,  page  82,  first  mark  by  stakes 
a  line  10  feet  on  each  side  of  the  center  of  the  proposed  road.  With 
the  machine  arranged  to  throw  the  earth  17  feet  horizontally,  drive 
along  the  left-hand  row  of  stakes  and  back  on  the  other  side  of  the 
road  in  the  same  way.  The  streams  of  earth  as  delivered  will  overlap 
5  or  6  feet.  Start  the  machine  on  the  second  round  with  the  right- 
hand  forward  wheel  in  the  furrow  of  the  previous  round,  and  complete 
the  round.  A  harrow  should  follow  the  machine  to  break  up  the 
sod  and  level  the  bank.  Continue  to  make  rounds  until  the  ditches 
are  as  wide  as  desired. 

Commence  the  second  plowing  by  bringing  the  left-hand  wheel 
of  the  machine  to  the  left-hand  edge  of  the  first  furrow  cut,  which 
brings  the  plow  one  furrow  to  the  left  of  the  point  of  commencing 
the  first  plowing,  and  keep  this  relative  position  while  making  this 
round.  Make  the  second  round  with  the  left-hand  forward  wheel 
in  the  furrow  of  the  previous  round;  and  continue  to  make  rounds 
until  the  outside  of  the  ditch  is  reached  again.  For  the  best  results 
a  harrow  and  roller — the  heavier  the  better — should  follow  the 
grader  during  the  second  and  subsequent  rounds — see  Fig.  29. 

When  the  second  plowing  has  been  completed,  the  grade  will  be 


FIG.  29. — ELEVATING  GRADER  BUILDING  EARTH  ROAD. 

high  and  narrow;   and  therefore  the  carrier  should  be  shortened  to 
14  feet.     Then  start  the  machine  so  that  the  plow  will  take  a  furrow 


ART.    1]  CONSTRUCTION  103 

from  the  center  of  the  ditch,  and  continue  the  third  plowing,  as 
described  above  for  the  first  and  second,  to  the  outside  of  the  ditch. 
For  the  fourth  plowing  take  a  couple  of  furrows  from  the  outside  of 
the  excavation  to  deepen  the  ditch. 

The  final  result  should  be  about  as  in  Fig.  11  or  12,  page  82.  Most 
operators,  however,  leave  a  berm  at  the  inside  edge  of  the  ditch 
(Fig.  37,  page  122),  which  is  undesirable  since  it  interferes  with  the 
operation  of  the  scraping  grader  in  maintaining  the  road. 

163.  For  loading  wagons,  the  carrier  is  arranged  to  deliver  at  17 
or  19  feet  horizontally  from  the  machine,  the  wagons  are  driven  so 
that  the  earth  falls  from  the  carrier  into  the  wagon,  and  both  move 
at  the  same  speed  until  the  wagon  is  loaded;   and  then  the  grader 
slows  down  while  the  loaded  wagon  drives  out  and  an  empty  one 
drives  in.     Common  wagons  with  dump  boards  are  not  so  easily 
loaded  as  the  usual  dump  wagon,  since  they  are  narrower  and  longer. 
It  is  customary  to  estimate  three  dump  wagons  for  the  first  100  feet 
of  haul,  and  an  additional  wagon  for  each  100  feet  thereafter. 

164.  COST  OF  EARTHWORK.     Of  necessity,  general  estimates  of 
the  cost  of  earthwork  can  not  be  very  exact,  since  the  cost  will  vary 
with  the  condition  of  the  soil,  the  wages,  the  hours  constituting  a 
day's  work,  the  relative  amount  paid  for  supervision,  the  effective- 
ness of  the  supervision,  the  facilities  for  preventing  one  part  of  the 
crew  from  interfering  with  the  work  of  another,  the  proper  adjust- 
ment of  the  number  of  shovelers  per  wagon  or  cart,  or  of  scraper 
holders  to  scrapers,  etc.     The  following  data  have  been  checked  by 
engineers  and  contractors  of  wide  experience  and  are  believed  to  be 
reasonably  reliable.* 

165.  In  the  analysis  of  the  cost  of  earthwork  to  follow,  the  price 
for  a  man  will  be  assumed  to  be  $1.50  per  day  of  10  hours,  and  that 
for  a  team  and  driver  $3.50  per  day.     These  were  the  usual  wages 
formerly  paid  by  contractors,  which  are  the  prices  to  be  considered 
here;   for  if  the  work  is  done  under  the  labor-tax  system  ordinary 
estimates  will  not  apply  (§  51-52),  and  if  the  farmer  hires  out  to 
do  the  work  of  a  teamster  he  usually  demands  the  ordinary  pay  for 
that  class  of  work.     These  were  about  the  prices  current  for  a  number 
of  years  in  a  number  of  states,  before  the  rise  of  prices  incident  to 
the  Great  European  War.     Of  course,  if  wages  are  greater  than  as 
stated  above,  the  following  prices  can  be  changed  proportionally. 

166.  Cost  with  Scraping  Grader.     In  prairie  soil,  two  men  and 

*  For  an  instructive  discussion  of  methods  of  Handling  Earth  in  Road  Construction,  see  an 
article  by  Chas,  R,  Thomas  in  Engineering  and  Contracting,  Vol.  47  (1917),  pp.  406-08. 


104  EARTH    ROADS  [CHAP.    Ill 

four  horses  with  a  scraping  grader  can  build  a  mile  of  road  36  feet 
wide  from  inside  to  inside  of  ditch  with  a  crown  of  6  inches  at  the 
center  after  being  compacted,  for  $30  to  $40,  which  is  equivalent  to 
if  or  2J  cents  per  cubic  yard.  The  first  is  the  cost  when  there  is 
no  sod,  and  the  last  when  there  is  sod,  The  cost  for  a  crown  of  12 
inches  will  be  about  $70  per  mile,  or  If  cents  per  cubic  yard.  The 
above  prices  do  not  include  interest,  or  wear  and  tear  of  grader, 
which  would  be  about  £  cent  per  cubic  yard. 

In  hard  soil  requiring  an  extra  team  and  hence  another  driver, 
add  one  half  to  the  above  prices. 

167.  Cost  with  Elevating  Grader.    The  elevating  grader,  Fig.  28 
and  29,  pages  101  and  102,  will,  in  light  prairie  soil,  deposit  on  a  road 
1,000  cubic  yards  per  day  of  10  hours;    and  will  load  into  wagons 
500  yards  per  day.     The  outfit  required  is:   seven  two-horse  teams 
at  $2.50  each  plus  2  drivers  at  $2.00  each  plus  1  operator  at  $2.00  and 
1  at  $2.50  =  $29.50.     For  the  earth  deposited  on  the  road  this  is  2.9 
cents  per  cubic  yard,  and  for  that  loaded  into  wagons  5.8  cents, 
exclusive  of  interest,  depreciation,  and  administration. 

168.  Cost  with  Drag-Scoop  Scraper.   Drag  scrapers  are  admirably 
adapted  for  borrowing  at  the  sides  of  embankments  and  for  wasting 
from  cuts  or  ditches,  and  also  for  opening  the  mouth  of  large  cuts; 
but  are  not  economical  except  for  short  distances.     There  is  no 
danger  of  the  scraper  getting  out  of  order  until  it  is  worn  out  and 
unfit  for  use,  and  the  manner  of  using  it  is  quickly  learned  by  any 
one.     Drag  scrapers  are  made  in  three  sizes  having  a  capacity  of 
3,  5,  and  7  cubic  feet,  respectively;    but  it  must  not  be  assumed 
that  each  scraper  will  carry  to  the  embankment  an  amount  equal  to 
its  rated  capacity,  since  in  the  first  place  it  is  difficult  to  completely 
fill  the  scraper,  and  in  the  second  place  the  scraper  carries  loose 
earth  which  will  shrink  about  25  per  cent  when  compacted  in  the 
embankment.     Unless  the  soil  is  very  loose  and  easily  loaded,  it  is 
not  safe  to  assume  that  each  trip  of  the  scraper  will  make  of  com- 
pleted embankment  more  than  one  half  of  its  rated  capacity.     The 
larger  size  is  most  economical,  but  the  relative  advantage  is  not 
proportional  to  the  size,  since  the  larger  size  is  not  as  easy  to  handle 
nor  as  easy  to  fill.     Scrapers  should  be  used  in  gangs  of  not  less 
than  six  to  decrease  the  cost  of  loading,  superintendence,  spreading, 
etc. 

169.  Cost  of  Loosening.     Sand  or  sandy  loam  can  be  scraped 
without  plowing.     In  loam  a  two-horse  team  and  plow  will  loosen 
400  cubic  yards  per  day,  at  a  cost  of  $3.50  for  team,  plow,  and  driver, 


ART.    1]  CONSTRUCTION  /105 

and  $1.50  for  the  plow  holder,  making  a  total  of  $5.00,  or  1J  cents 
per  cubic  yard.  Sometimes  the  driver  can  also  hold  the  plow,  in 
which  case  loosening  will  cost  about  1  cent  per  cubic  yard,  since  the 
team  will  not  do  quite  as  much  work  as  when  there  is  a  plow  holder 
and  also  a  driver.  If  the  ground  is  hard  it  will  be  necessary  to  add 
another  team  and  also  a  man  to  "  ride  "  the  beam  of  the  plow.  If 
the  ground  is  not  very  hard,  this  force  will  loosen  400  cubic  yards 
per  day  at  a  cost  of  2.1  cents  per  yard. 

170.  Cost  for  25-foot  Haul.  The  cost  of  building  an  embank- 
ment from  a  borrow  pit  at  the  side  of  the  road  will  first  be  considered. 
For  a  60-foot  right-of-way  and  a  light  embankment,  the  length  of 
haul  or  "  lead  "  from  center  of  gravity  of  the  fill  to  the  center  of 
gravity  of  the  cut  will  be  about  25  feet.  This  distance  will  be  a 
little  more  or  a  little  less  according  to  the  height  and  width  of  the 
bank,  and  the  width  reserved  for  sidewalk;  but  slight  difference  in 
length  of  short  hauls  make  comparatively  little  difference  in  the 
cost  of  moving  the  earth,  because  in  the  first  place  a  considerable 
part  of  the  cost  of  hauling  is  due  to  time  consumed  in  turning  and 
loading,  and  in  the  second  place  the  cost  of  transportation  is  only 
about  half  the  total  cost  of  moving  the  earth. 

On  the  road,  an  ordinary  team  will  travel  220  feet  per  minute 
(2J  miles  per  hour),  but  in  scraping  considerable  time  is  consumed 
in  turning,  waiting  to  load,  etc.,  and  besides,  the  distance  traveled  is 
more  than  that  from  the  center  of  cut  to  the  center  of  fill;  therefore 
the  ordinary  speed  of  the  team  is  no  guide  in  this  connection. 
Experience  shows  that  a  team  will  use  from  a  minute  to  a  minute 
and  a  half  in  making  a  round  trip  at  the  above  distance,  or,  say,  li 
minutes  per  trip.  A  foot  vertically  is  equivalent  to  10  to  25  feet 
horizontally  (see  §  77),  and  in  estimating  the  length  of  haul  this 
fact  must  be  taken  into  account. 

Using  the  large  scraper,  a  scraperful  will  make  3J  cubic  feet 
of  compacted  embankment,  or  will  require  eight  trips  per  cubic 
yard.  Therefore  a  team  will  place  a  yard  of  earth  in  the  fill  every 
10  minutes,  or  6  yards  per  hour  and  60  yards  per  day.  In  light  loose 
earth,  where  it  is  easy  to  fill  the  scrapers  full,  a  team  may  make  70 
yards;  but  if  the  ground  is  hard,  or  obstructed  with  roots  and  grass, 
50  yards  may  be  the  maximum.  Assuming  a  day's  work  to  be  60 
yards,  the  cost  of  hauling  is  $3.50  -5-  60  =  5.83  cents  per  cubic 
yard. 

One  man  will  hold  and  fill  the  scraper  for  two  teams  at  a  cost  of 
$1.50  -T-  (2  X  60)  =  1.25  cents  per  yard.  One  man  on  the  dump 


106  EARTH    ROADS  [CHAP.    Ill 

will  distribute  and  level  the  earth  deposited  by  six  teams,  at  a  cost 
of  $1.50  -f-  (6  X  60)  =  0.4  cents  per  cubic  yard.  One  foreman 
will  be  required  at,  say,  $2.50  per  day;  or  $2.50  ^  (6  X  60)  =  0.69 
cents  per  cubic  yard.  For  wear  and  tear  of  scraper  we  may  allow 
10  cents  per  day  for  each,  or  60  cents  for  the  lot;  and  for  wear  of 
plow  and  cost  of  sharpening,  say,  30  cents,  making  a  total  of  90  cents 
or  0.25  cents  per  cubic  yard.  In  very  hard  ground  the  above  prices 
may  be  doubled. 

The  total  cost  of  moving  earth  25  feet  will  then  be  as  in  Table  18, 
page  108. 

171.  Cost  for  50-foot  Haul.     We  will  next  consider  the  cost  for  a 
50-foot  haul.     At  this  distance  a  scraper  holder  can  fill  for  three 
teams.     Each  team  can  put  in  about  50  cubic  yards  per  day.     The 
other  items  will  be  substantially  as  for  a  25-foot  haul,  and  the  total 
cost  will  be  as  in  Table  18. 

172.  Cost  for  100-foot  Haul.     Each  team  will  make  a  trip   in 
about  2|  minutes,  and  will  put  in  40  cubic  yards  per  day.     The 
total  cost  will  be  as  in  Table  18. 

173.  Cost  for  200-foot  Haul.     At  this  distance  a  scraper  holder 
can  fill  for  four  teams.     Each  team  will  make  the  trip  in  about  3J 
minutes,  and  put  in  about  35  cubic  yards  per  day.     The  total  cost 
will  then  be  as  in  Table  18.  . 

174.  Cost  for  Hard  Ground.     If  the  ground  is  so  difficult  to  plow 
as  to  require  a  second  team  and  a  man  to  ride  the  beam,  add  1  or  1| 
cents  to  the  values  in  Table  18  for  the  extra  cost  of  loosening;  and 
add,  say,  one  fifth  to  the  cost  of  hauling  to  allow  for  the  fact  that 
in  hard  ground  the  scrapers  are  not  as  well  filled  as  in  loose  light 
soil.     Also  add  one  half  to  the  above  estimated  cost  of  wear  and 
tear.     The  results  for  hard  ground  are  then  as  in  Table  18. 

175.  Cost  with  Two-Wheel  Scrapers.     Two-wheel  scrapers  are 
excellent  for  hauling  earth  distances  up  to  600  or  700  feet.     They 
are  made  in  three  sizes,  No.  1,  2,  and  3,  having  a  capacity  of  9,  12, 
and  15  cubic  feet,  respectively.     With  No.  1  the  team  fills  its  own 
scraper,  while  with  No.  3  an  extra  team  (a  snatch  team)  is  required 
to  fill  the  scrapers  reasonably  full;  and  unless  the  ground  is  very 
loose  and  light  an  extra  team  is  required  to  fill  No.  2.     Most  con- 
tractors use  either  No.  1  with  a  single  team  or  No.  3  with  a  snatch 
team.     It  usually  takes  about  five  loads  with  No.  1  to  make  a  cubic 
yard  in  place;  four,  with  No.  2;  and  three,  with  No.  3. 

176.  Cost  for  100-foot  Haul.     It  is  assumed  that   the   scrapers 
will  be  worked  in  a  gang  of  six,  which  will  require  one  foreman,  one 


ART.    1]  CONSTRUCTION  107 

plow,  three  scraper  holders,  and  one  man  on  the  dump.  The 
expense  for  these  items  will  be  the  same  as  for  the  drag  scrapers, 
and  are  so  entered  in  Table  19,  page  109.  At  this  distance  a  trip 
will  occupy  2J  minutes,  and  a  yard  will  be  deposited  every  10  min- 
utes, or  60  yards  per  day,  at  a  total  cost  $3.50,  or  5.83  cents  per 
cubic  yard  for  hauling. 

The  wear  and  tear  is  computed  on  the  assumption  that  a  scraper 
will,  last  for  200  days'  continuous  work,  making  a  cost  for  deprecia- 
tion and  repairs  of,  say,  20  cents  per  day  per  scraper.  The  wear 
and  tear  on  the  plow  will  be  estimated  at  30  cents  per  day.  The 
total  cost  will  then  be  as  in  Table  19. 

177.  Notice  that  the  cost  for  100  feet  with  the  two- wheel  scraper 
is  9.99  cents  per  cubic  yard,  while  with  the  drag  scraper  for  the  same 
distance  it  is  12.67  cents.    The  difference  is  in  the  cost  of  hauling, 
v,  hich  is  due  to  the  difference  in  the  capacity  of  the  scrapers. 

178.  Cost  for  200-foot  Haul.     A  trip  will  be  made  in  about   4 
minutes,  and  each  scraper  will  put  in  50  cubic  yards  per  day.     The 
three  scraper  holders  can  fill  an  additional  scraper,  making  nine 
in  all.     The  cost  will  thgn  be  as  in  Table  19. 

179.  Cost  for  300-foot  Haul.     In  this  case  another  scraper   can 
be  added,  making  four  scrapers  to  each  scraper  holder.     A  trip  can 
be  made  in  about  5j  minutes,  and  each  team  will  move  45  yards 
per  day.     The  cost  will  be  as  stated  in  Table  19. 

180.  Cost  for   400-foot   Haul.     It   is   difficult   to   determine  the 
most  economic  distance  for  each  size  of  scraper,  since  the  several 
sizes  are  seldom  available  for  making  the  test.     However,  at  300 
feet,  the  cost  with  a  No.  2  scraper  is  about  the  same  as  with  a 
No.  1  at  200  feet;  and  at  400  feet  the  cost  with  a  No.  2  is  about  the 
same  as  with  a  No.  1  at  300  feet.     But  at  400  feet  a  No.  3  is  more 
economical  than  a  No.  2. 

A  snatch  team  is  required  in  filling  No.  3  scrapers.  The  extra 
.  force  acquired  by  using  the  extra  team  completely  fills  the  scraper, 
and  also  packs  the  load  so  that  it  is  less  liable  to  spill  than  when 
loaded  by  a  single  team.  For  this  distance  it  is  most  economical 
to  work  the  scrapers  in  a  gang  of  eight;  and  two  men  will  be  re- 
quired to  hold  the  scrapers  while  being  filled.  Each  team  will  put 
into  place  45  cubic  yards,  or  360  for  the  gang.  The  total  cost  will 
be  as  shown  in  Table  19. 

181.  Cost  for  Other  Distances.    For  each  additional  100  feet  of 
lead,  add  1  cent  per  cubic  yard  to  the  cost  of  haul;   and  the  total 
cost  will  be  approximately  as  shown  in  Table  19. 


108 


EARTH   ROADS 


CHAP.    Ill 


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110  EARTH   ROADS  [CHAP.    Ill 

When  the  amount  of  earth  to  be  moved  is  considerable  and  the 
length  of  haul  is  great,  something  must  be  allowed  for  keeping  in 
repair  the  road  over  which  the  earth  is  transported.  A  wheel 
scraper  is  prone  to  wear  a  series  of  fyumps  and  hollows  along  the 
road  it  traverses,  and  these  must  be  kept  in  subjection,  if  the  work 
is  to  be  done  at  reasonable  cost.  The  proper  allowance  will  vary 
greatly  with  the  soil,  the  weather,  etc.  Trautwine  recommends 
0.1  cent  per  cubid  yard  per  100  feet  for  this  expense. 

182.  It  is  difficult  to  determine  at  what  distance  wagons  should 
supersede  two-wheel  scrapers;    but  usually  the  economic  limit  for 
two-wheel  scrapers  is  600  to  800  feet,  and  it  is  seldom  wise  to  use 
such  scrapers  beyond  800  feet — unless  they  are  at  hand  and  wagons 
are  not.* 

183.  Cost  with  Four-Wheel  Scrapers.     The  cost  of  moving  earth 
with  the  four-wheel  scraper  is  not  well  established.     With  enough 
scrapers  to  keep  the  loading  engine  reasonably  busy,  the  cost  of 
power  for   loading — operator,  fuel,  rent,  etc., — will  be  about   2  or 
3  cents  per  cubic  yard,  depending  upon  the  hardness  of  the  soil. 
The  saving  in  loading  is  5  to   6   cents  over  that  of  wagons — see 
§184. 

184.  Cost  with  Wagons.     It  will  be  assumed  that  the  wagons  are 
filled  by  hand,  that  they  are  used  in  gangs  of  nine,  and  that  the  haul 
is  700  to  800  feet.     If  the  roads  are  level  and  fairly  smooth,  a  load 
will  make  about  1|  cubic  yards  in  place;  and  with  ordinary  roads,  1 
yard  will  make  a  load ;  but  if  the  roads  are  soft  and  steep,  f  of  a  yard 
may  make  a  load.     The  amount  a  team  can  deliver  will  vary  greatly 
with  the  time  consumed  in  waiting  to  load  and  in  loading.     With 
short  wagon-hauls  and  well-organized  work,  half  of  the  time  is  thus 
consumed,  and  often  much  more  is  thus  consumed.     The  time  of 
the  wagon  while  loading  should  be  considered  as  a  part  of  the  cost 
of  loading,  and  this  will  be  discussed  more  fully  in  the  next  para- 
graph.     For  the  above  distance,  the  round   trip  will   consume  15 
minutes;  and  assuming  a  yard  as  a  load,  each  wagon  will  deliver 
50  cubic  yards  per  day,  at  a  cost  of  7.00  cents  per  cubic  yard  for 
hauling. 

There  is  very  great  variation  in  the  amount  of  earth  a  shoveler 
will  load  in  a  day.     In  well  managed  work,  the  shovelers  are  not 


*  For  the  results  of  an  elaborate  time  study  of  cost  of  moving  various  kinds  of  soils  dif- 
ferent distances  with  two-wheel  scrapers  and  Fresno  scrapers,  see  Engineering  and  Contracting, 
Vol.  41  (1914),  pp.  629-36. 


ART.    1]  CONSTRUCTION  111 

actually  engaged  in  loading  much  more  than  half  of  the  time;  while 
under  poor  management,  they  do  not  really  work  half  of  the  time. 
With  short  intervals  of  rest  equal  to  the  working  time,  a  man  should 
load,  in  a  day  of  10  hours,  20  cubic  yards  of  light  sandy  soil,  17 
yards  of  loam,  and  15  of  heavy  soil — provided  all  are  loosened  by 
plowing  or  picking.  Usually  five  or  seven  men  are  set  to  load  a 
wagon — two  or  three  on  each  side  and  one  at  the  rear.  Seven  men 
will  load  a  wagon  with  loam  in  5  minutes,  8  minutes  will  be  con- 
sumed in  traveling  to  and  -from  the  dump,  1  minute  in  dumping 
and  1  minute  in  getting  into  and  out  of  the  cut — making  in  all  15 
minutes  for  a  round  trip;  and  therefore  the  cost  for  wagon  and 
team  is  8.75  cents  per  cubic  yard,  as  above.  In  this  case  the  team 
works  only  about  half  the  time.  If  only  five  men  are  engaged  in 
loading  a  wagon,  7  minutes  will  be  consumed  in  loading,  and  the 
time  for  a  round  trip  will  be  17  minutes,  and  each  wagon  will  deliver 
only  35  cubic  yards,  making  the  cost  10  cents  per  cubic  yard.  In 
this  case,  the  team  really  works  less  than  half  the  time.  If  the 
men  shovel  only  12  to  15  cubic  yards  each,  as  is  very  common,  the 
loss  by  the  wagon's  waiting  for  a  load  is  considerably  more  than 
above.  The  proper  number  of  men  to  be  set  to  loading  a  wagon 
depends  upon  the  relative  wages  of  shoveler  and  wagon,  upon  the 
length  of  the  haul,  and  upon  the  quantity  loaded  per  day  per  man. 
Usually  seven  shovelers  are  employed  to  each  wagon,  but  this 
number  is  not  enough  to  secure  the  greatest  economy.  In  the  fol- 
lowing estimates  it  will  be  assumed  that  nine  shovelers  are  em- 
ployed to  each  wagon.  At  the  above  distance,  nine  shovelers,  each 
loading  17  yards  per  day,  will  be  required  to  keep  three  wagons 
going,  each  of  which  deposits  45  cubic  yards  per  day.  The  cost  of 
loading  will  then  be  8.8  cents  per  cubic  yard. 

The  cost  of  leveling  the  dump  is  small,  if  dump  wagons  are  used 
and  the  earth  is  dumped  over  the  end  of  the  embankment  or  wasted ; 
and  it  may  be  taken  the  same  as  for  scrapers,  i.  e.,  at  0.40  cents 
per  cubic  yard.  But  dump  wagons  are  so  heavy  and  expensive 
that  they  are  seldom  used;  and  if  ordinary  wagons  with  dump 
boards  are  employed,  the  expense  for  labor  on  the  dump  will  be 
about  three  times  as  great  as  above,  or,  say,  1.20  cents  per  cubic 
yard. 

The  driver  furnishes  his  own  wagon,  and  hence  no  account  is 
taken  of  the  wear  and  tear  of  it.  There  should  be  a  small  allowance 
made  for  the  wear  and  care  of  shovels,  say,  0.1  cent  per  cubic 
yard. 


112                                                             EARTH    ROADS  [CHAP.    Ill 

The  total  cost  of  moving  earth  700  to  800  feet  with  wagons, 
then,  is  as  follows: 

1.  Loosening 1 . 25  cts.  per  cu.  yd. 

2.  Shoveling 8.80 

3.  Depreciation  of  shovels 0. 10  "        " 

4.  Hauling  700  to  800  feet 7.00 

5.  Helper  on  dump 1 .20  "       " 

6.  Superintendence 0.69  "       " 

7.  Water  boy 0.16  "       " 


Total  cost 18.20 

185.  For  longer  distances  add  1  cent  per  cubic  yard   for   each 
100  feet  of  distance — the  usual  charge  for  over-haul. 

186.  Other  Methods.     When  the  haul  is  more  than  600  or  800 
fe^et,  and  when  the  amount  of  work  to  be  done  is  sufficient  to  justify 
the  initial  expense,  it  is  more  economical  to  use  portable  track  and 
small  dump  cars  than  to  use  wagons.     However,  such  conditions 
seldom  occur  in  wagon-road  construction. 

187.  Finishing  the   Slopes.     In   addition    to   the    elements    of 
cost  discussed  above,  there  is  always  some  expense  in  leveling  off 
the  bottom  of  the  cut,  in  digging  ditches,  in  trimming  up  the  slopes 
of  embankments  and  excavations,  and  in  cutting  a  catch-water  at 
the  top  of  the  slope  in  excavation.     The  cost  of  these  items  will 
very  greatly  with  the  degree  of  finish  required  and  also  with  the 
depth  of  the  cut  or  the  fill;   and  it  may  amount  to  0.25  or  0.50  of 
a  cent  per  cubic  yard.     If  the  bottom  of  the  cut  can  be  leveled  off 
with  the  scraping  grader,  and  if  the  ditch  also  can  be  made  with 
this  machine,  the  cost  of  this  item  will  be  considerably  reduced. 

188.  Profits  and  Administration.     The  proper  allowance  under 
this  head  will  vary  according  to  the  magnitude  of  the  work,  the 
risks  involved,  etc.;   but  will  usually  be  5  to  15  per  cent.     Out  of 
this  the  contractor  must  pay  the  expense  of  assembling  the  plant, 
the  cost  of  tool  house,  the  wear  and  tear  on  small  tools,  interest 
on  investment,  profits,  etc. 

189.  BRIDGES.    This  subject  will  not  be  considered  here,  since 
the  space  available  is  not  sufficient,  and  since  there  are  a  number 
of  elaborate  treatises  on  bridges.     None  of  these,  however,  gives 
an  adequate  treatment  of  the  very  small  highway  bridge,  or  fairly 
represents  current  practice  for  moderate  spans. 


ART.    1]  CONSTRUCTION  113 

190.  WATERWAYS.     The     determination     of    the    amount    of 
waterway  required  for  any  particular  bridge  or  culvert  is  a  matter 
of  importance.     Although  the  problem  does  not  admit  of  an  exact 
mathematical   solution,    it   requires   intelligent   treatment.     For   a 
discussion  of  this  subject,  see  pages  564-69  of  the  tenth  edition  of 
author's  Masonry  Construction.* 

191.  CULVERTS.     For  a  discussion  of  the  cost  and  method  of 
construction  of  culverts — wood  box,  vitrified  pipe,  cast  iron  pipe, 
stone  box,  and  masonry-arch  culverts, — see  the  author's  Masonry 
Construction,  pages  569-605.* 

One  common  defect  of  earth  roads  is  that  culverts  are  made  too 
short,  which  concentrates  the  traffic  upon  the  portion  of  the  road 
usually  least  able  to  bear  it.  A  short  culvert  may  be  permissible 
when  the  cost  per  unit  of  length  is  great,  but  the  defect  is  common 
where  this  cost  is  quite  small. 

192.  RETAINING  WALLS.     Retaining  walls  are  masonry  struc- 
tures employed  to  support  the  sides  of  roads  on  hillsides  or  in  places 
where  land  for  the  ordinary  earth  slopes  is  not  readily  obtainable. 
For  a  discussion  of  retaining  walls,  see  the  author's  Masonry  Con- 
struction, pages  489-534.* 

193.  GUARD  RAILS.     Roads  on  steep  hillsides  or  on  high  em- 
bankments, and  particularly  on  sharp  curves  in  mountain  roads, 
should  be  protected  to  insure  vehicles  against  the  possibility  of 
falling    down    the    slope.     In    Europe    such    protection   is   usually 
afforded  by  a  stone  wall,  or  by  stone  posts  set  at  frequent  intervals. 
In  this  country  the  usual  protection  is  by  means  of  wood  posts  and 
wood  guard  rails.     The  description  of  the  guard  rails  used  on  the 
state-aid  roads  in  Massachusetts  is  as  follows:     "  Posts  of  cedar,  or 
other  wood  which  endures  well  in  the  soil,  are  set  at  intervals  of  8 
feet,  and  1  foot  in  from  the  edge  of  the  embankment.     These  posts 
are  planted  to  the  depth  of  3  feet,  and  project  for  3  feet  6  inches 
above  the  ground.     The  top  of  this  post  is  transversely  notched,  so 
as  to  receive  one  half  of  a  rail  4  inches  square.     Half-way  down, 
the  post  is  notched  to  receive  another  rail  2  by  6  inches  in  size. 
These  rails,  preferably  of  planed  spruce  wood,  are  spiked  to  the 
posts.      To   insure   the  better   preservation   of  the   wood   and  it 
visibility  in  the  night-time,  it  is  painted  with  two  coats  of  oil  paint 
of  some  light  color."     For  a  diagram  of  this  guard  rail,  see  Fig.  52, 
page  198. 

*  A  Treatise  on  Masonry  Construction,  by  Ira  O.  Baker,  745 +xv  pp.,  6X9  inches,  cloth, 
10th  edition.     John  Wiley  &  Sons,  New  York.     Price  $5.00. 


114  EARTH   ROADS  [CHAP.    Ill 

The  Massachusetts  Highway  Commission  wherever  practicable 
widens  the  base  of  the  embankment  until  a  slope  of  1  to  4  is  ob- 
tained, and  then  dispenses  with  the  guard  rail.  This  plan  is  believed 
to  be  more  economical,  and  to  give  a  better  appearance. 

194.  GUIDE   POSTS.     Some    states,    by    statute,  require   guide 
posts  at  all  intersections;  and  their  value  to  the  occasional  traveler 
is  sufficient  to  justify  the  expense.     The  guide  post  may  be  a  plain 
post,  supporting  near  its  top  a  board  upon  which  is  the  name  of 
the  place  reached  by  the  road,  with  figures  showing  the  distance, 
and  a  OP^  to  show  the  direction. 

195.  ARTISTIC    TREATMENT.      Engineers    are    accustomed    to 
study  chiefly  or  only  the  economic  side   of  construction,  and  are 
therefore  likely  to  neglect  the  artistic  treatment  of  the  highway. 
In  the  attempt  to  beautify  the  roadside,  it  may  be  necessary  to  sac- 
rifice a  little  of  utility  to  secure  a  pleasing  effect.     Masses  of  foliage 
and  shade  add  beauty  to  the  roadside,  but  tend  to  keep  the  traveled 
way  damp — usually  the  bane  of  good  earth  roads.     Trees  are  a 
necessary  adjunct  to  a  beautiful  highway,  but  are  anything  but  a 
benefit  to  the  traveled  way.     If  beauty  is  desired  at  the  expense  of 
utility,  highways  can  scarcely  be  too  much  shaded  by  over-arching 
boughs.     However,  a  happy  medium  will  suffice  in  most  places. 

The  varieties  of  trees,  suitable  for  the  ornamentation  of  highways 
are  almost  infinite.  The  elm,  with  its  graceful  arching  branches 
and  delicate  lace-like  foliage,  is  not  surpassed;  and  the  hard  maple 
and  the  oaks  are  very  handsome  for  this  purpose.  The  walnut,  the 
butternut,  the  hickory,  the  beech,  the  poplar,  and  the  pine,  ranging 
from  the  most  delicate  to  the  most  somber  and  rugged,  are  all  more 
or  less  adapted  to  particular'  requirements  and  circumstances. 
Trees  such  as  willow,  the  roots  of  which  spread  extensively  or  seek 
water  vigorously,  should  not  be  permitted  to  grow  near  tile  drains, 
as  the  small  roots  frequently  entirely  obstruct  the  tile.  Trees 
should  not  be  planted  close  to  the  traveled  way,  but  near  the  edge  of 
the  right-of-way,  or  if  possible  on  the  private  property  bordering 
the  right-of-way. 

196.  The  roadside  fences  are  usually  the  property  of  the  adjoin- 
ing land  owner,   and  may  mar  or  beautify  the  landscape.     The 
hedge  rows  of  England  and  the  stone  fences  of  New  England  are  all 
that  can  be  desired  for  appearance;   but  in  localities  where  there  is 
much  snow  they  catch  the  drifting  snow  and  so  obstruct  the  high- 
way.    The  only  thing  favorable  to  the  appearance  of  the  common 
wire  fence  is  that  it  is  inconspicuous. 


ART.    2]  MAINTENANCE  115 


ART.  2.     MAINTENANCE 

197.  Proper  maintenance  is  as  important  as  good  construction. 
A  distinction  should  be  made  between  maintenance  and  repairing. 
The  former  keeps  the  road  always  in  good  condition;    the  latter 
makes  it  so  only  occasionally.     If  the  road  is  not  properly  main- 
tained, it  deteriorates  in  a  geometrical  ratio.     A  small  depression 
fills  with  water  and  soon  becomes  a  mud  hole  which  travel  makes 
deeper  and  deeper;   or  an  obstructed  side  ditch  forces  the  water  to 
run  down  the  center  of  the  road,  and  gullies  out  the  surface.     A 
defect  which  could  be  remedied  in  the  beginning  with  a  shovelful  of 
earth  and  a  minute's  time,  if  neglected  may  require  a  wagon  load  of 
earth  or  an  hour's  time,  besides  being  in  the  meantime  an  annoyance 
or  a  damage  to  travel.     The  better  the  state  in  which  a  road  is  kept, 
the  less  are  the  injuries  to  it  by  ordinary  travel  and  the  weather. 

198.  DESTRUCTIVE  AGENTS.     The   agents   tending  to  destroy 
the  road  are:  water,  narrow  tires,  the  tracking  of  the  front  and  rear 
wheels,  the  horse  not  being  hitched  before  the  wheel. 

199.  Water.     Water  is  the  natural  enemy  of  good  earth  roads. 
The  chief  object  of  maintenance  should  be  to  keep  the  surface  smooth 
and  properly  crowned  so  that  rain  will  be  shed  into  the  side  ditches. 
These  should  be  kept  open  so  that  the  water  may  be  carried  entirely 
away  from  the  road.     This  subject  is  fully  considered  in  §  205-15. 

200.  Narrow  Tires.     It  is  desirable  that  a  wagon  in  passing 
over  the  road  should  help  to  make  or  preserve  it,  and  not  to  destroy 
it;    and  therefore  as  far  as  the  road  is  concerned,  within  reason- 
able limits,  the  broader  the  tire  the  better. 

Tables  3  and  4,  pages  15  and  16,  show  the  relative  tractive  resist- 
ance of  broad  and  narrow  tires.  Although  there  is  not  much  differ- 
ence between  the  tractive  power  of  broad  and  narrow  tires,  the  latter 
are  much  more  destructive  on  any  road,  particularly  on  an  earth 
one.  But  in  deciding  upon  the  proper  width  of  tire,  there  are  other 
factors  besides  the  tractive  resistance  and  the  preservation  of  the 
road,  that  must  be  considered. 

If  wagons  were  employed  only  upon  the  public  highway,  it 
might  be  wise  to  use  wide  tires  and  sacrifice  some  tractive  power 
for  the  benefit  of  the  roads.  Other  things  being  equal,  a  wagon 
with  broad  tires  is  not  so  easily  managed  as  one  with  narrow  tires. 
To  be  equally  easy  to  turn,  the  broad-tired  wagon  should  have  a 
narrower  bed,  or  a  longer  front  axle,  or  a  smaller  front  wheel.  In 


116  EARTH   ROADS  [CHAP.    Ill 

Europe  it  is  customary  to  adopt  the  smaller  front  wheel,  which  is 
very  destructive  of  the  broken-stone  roads  so  common  in  that 
country.  Increasing  the  length  of  axle  interferes  with  getting  the 
wagon  up  to  cribs,  warehouses,  etc.,  and  increases  the  difficulty  in 
going  through  gates,  passing  buildings,  etc.  It  is  not  clear  that  laws 
should  be  passed  regulating  the  width  of  tires,  many  claims  to  the 
contrary  notwithstanding. 

"  The  best  argument  against  the  enactment  of  laws  concerning 
broad  tires  is  found  in  the  fact  that  the  numerous  and  long-enforced 
English  statutes  on  this  matter  have  of  late  years  been  abrogated, 
a  century  of  experience  having  shown  that  they  are  difficult  to 
administer  and  generally  disadvantageous."  The  Massachusetts 
Highway  Commission,  after  an  elaborate  discussion  of  the  subject,* 
says:  "  It  is  a  matter  of  doubtful  expediency  to  endeavor,  in  the 
present  state  of  our  highways,  by  general  legislation  to  control  the 
width  of  tires  or  the  diameter  of  wheels." 

201.  Many  European  countries  have  laws  regulating  the  width 
of  tires.     In  England  for  100  years  the  law  required  1  inch  of  tire 
for  each  500  pounds  of  load,  but  all  general  laws  in  that  country 
regulating  the  width  of  tires  have  been  repealed.     In  France  the  tires 
of  market  carts  vary  from  3  to  10  inches  in  width,  being  generally 
from  4  to  6  inches,  with  the  rear  axle  about  14  inches  longer  than  the 
forward  one. 

In  this  country  a  number  of  the  states  have  statutes  concerning 
the  width  of  tires,  many  of  which  take  the  form  of  a  rebate,  either 
cash  or  part  of  the  road  tax,  to  those  using  tires  of  a  prescribed 
width.  The  following  is  the  legal  width  in  Ohio  for  vehicles  using 
gravel  or  broken-stone  roads: 

Minimum  width  of  tire  for  load  of  2,500  to  3,500  Ib 3    inches 

"  "  3,500  "  4,000  " 3£     " 

4,000  "  6,000  " 4       " 

6,000  "  8,000" 5       " 

8,000  or  more  " 6       " 

According  to  wagon  manufacturers  about  60  per  cent  of  the 
wagons  used  on  country  roads  have  tires  1^  to  If  inches  wide,  those 
of  the  remaining  40  per  cent  being  2  to  4  inches.  The  broad  tire  is 
of  comparatively  recent  introduction  on  rural  roads  in  this  country. 

202.  There  is  greater  justification  for  limiting  the  load  per  unit 
of  width  of  tire  on  pavements  than  on  earth  roads,  since  with  the 

*  Report  of  the  Massachusetts  Highway  Commission  for  1893,  p.  56-62. 


ART.    2]  MAINTENANCE  117 

latter  the  damage  is  not  so  great  and  can  be  more  easily  repaired. 
For  a  discussion  of  limiting  loads  on  pavements,  see  §  39.  Notice, 
however,  that  these  regulations  apply  only  to  loads  heavier  than  any 
likely  to  traverse  earth  roads. 

203.  Equal  Axles.     Since  with  equal  axles  the  hind  wheel  follows 
in  the  track  of  the  fore  wheel,  it  increases  the  depth  of  the  rut,  and 
consequently  increases  the  destructive  effect  of  the  wagon  upon  the 
road.     The  remedy  would  be  to  make  the  lengths  of  the  two  axles 
unequal,  but  this  would  make  the  wagon  more  difficult  to  manage 
and  would  also  increase  the  tractive  resistance.     The  advantage  of 
not  permitting  one  wheel  to  exactly  follow  another,  is  shown  by  the 
fact  that  there  are  no  ruts  at  a  corner  or  a  sharp  turn  in  the  road;  but 
it  is  not  practicable  to  secure  this  advantage  generally,  either  by 
making  the  two  axles  of  unequal  length  or  by  preventing  a  wagon 
from  traveling  in  the  ruts  already  made. 

204.  Horse  not  Hitched  before  Wheel.     On  broken-stone  roads, 
the  horses'  feet  loosen  fragments  of  stone,  which  tends  to  destroy 
the  surface;   and  if  the  horses  were  hitched  directly  in  front  of  the 
wheels,  the  stones  loosened  by  the  horses'  feet  would  be  rolled  down 
by  the  wheels  of  the  wagon.     This  is  a  matter  of  some  moment  with 
broken-stone  roads,  but  is  not  important  with  earth  roads.     How- 
ever, a  few  teamsters  using  earth  roads  hitch  their  horses  in  front  of 
the  wheel,  to  enable  their  horses  and  wheels  to  run  in  the  beaten 
track  made  by  the  feet  of  preceding  horses  not  hitched  in  front  of 
the  wheel. 

205.  CARE   OF  THE   SURFACE.     The  most  important  work  in 
maintaining  an  earth  road  is  to  keep  the  surface  smooth  so  that  the 
rain  water  will  flow  quickly  into  the  side  ditches.     If  the  surface  of 
an  earth  road  is  properly  formed  and  kept  smooth,  the  water  will  be 
shed  into  the  side  ditches  and  do  comparatively  little  harm;  but  if  it 
remains  upon  the  surface,  it  will  be  absorbed  and  convert  the  road 
into  mud. 

There  are  three  classes  of  machines  in  use  for  filling  ruts  and 
depressions  and  in  keeping  the  surface  smooth.  They  are  the  road 
drag,  the  scraping  grader,  and  the  V  road-leveler. 

206.  Road  Drag.     The  road  drag  is  the  simplest,  the  cheapest, 
and  the  most  effective  implement  for  smoothing  the  surface  of  a 
road.     Four  type  forms  of  the  road  drag  are  shown  in  Fig.  30,  31, 
32,  and  33,  pages  118  and  119.     Each  consists  essentially  of  one  or 
more  cutting  edges  which  stand  obliquely  across  the  road  as  the 
implement  is  drawn  along  the  road.     The  road  drag  is  designed  to  do 


118 


EARTH    ROADS 


[CHAP.    Ill 


three  things,  viz.,  (1)  smooth  the  surface  by  paring  off  the  high 
places  and  filling  up  the  low  spots;  (2)  move  a  little  earth  toward  the 
center  of  the  road  to  compensate  for  the  wash  of  the  water  in  reducing 
the  crown;  and  (3)  puddle  or  harden  the  surface  by  dragging  it 
when  wet.  To  accomplish  the  first  purpose,  the  cutting  edge  must 
make  a  considerable  angle  with  the  line  of  draft,  so  that  the  earth 
that  is  pared  from  the  high  places  will  drift  along  in  front  of  the  cut- 
ting edge  and  fill  up  the  low  spots.  To  secure  the  second  object, 
the  front  end  of  the  cutting  edge  should  be  toward  the  outside  of 
the  traveled  way,  so  the  drifted  earth  will  be  moved  toward  the  center 
of  the  trackway.  To  accomplish  the  third  purpose,  the  road  should 
be  dragged  when  it  is  wet.  Working  the  soil  when  it  is  wet,  puddles 
it;  when  it  dries,  it  will  be  hard  and  more  nearly  waterproof.  Under 
favorable  conditions  each  successive  dragging  adds  a  thin  layer  of 
tough  and  impervious  material;  and  consequently  the  frequent 
dragging  of  an  earth  road  builds  up  a  crust  that  does  not  easily  become 
muddy. 

The  slicker,  or  lapped-plank  drag,  Fig.  30,  is  easily  and  cheaply 
made;  and  is  to  be  preferred  when  the  road  is  quite  soft,  and  also  in 
soil  that  is  too  sticky  to  flow  along  the  blade  of  the  other  forms  of 
drags.  In  these  cases  the  slicker  smooths  the  road  partly  by  com- 


FIG.  31. — SPLIT-LOG  ROAD  DRAG. 


FIG.  32. — PLANK  ROAD  DRAG. 


pressing  the  high  places  and  partly  by  cutting  them  off,  and  thus 
fills  up  the  low  places  and  gives  the  water  an  opportunity  to  run  off. 


ART.   2] 


MAINTENANCE 


119 


The  split-log  drag,  Fig.  31,  is  commendable  chiefly  because  of  the 
ease  with  which  it  can  be  made  when  a  suitable  log  is  at  hand.  The 
log  should  be  7  or  8  feet  long,  and  10  or  12  inches  in  diameter.  The 
braces  between  the  two  halves  of  the  log  may  be  roughly  hewn  and 
the  ends  be  made  to  fit  into  2-inch  auger  holes  in  the  slabs.  The 
braces  should  be  long  enough  to  hold  the  two  slabs  about  30  inches 
apart.  The  braces  should  be  fastened  in  place  by  driving  wedges 


FIG.  33. — ADJUSTABLE  THREE-BLADE  STEEL  FiG.  34. — ROAD  SLICKER  AT  WOBK. 

ROAD  DRAG. 


Fio.  35. — WORK  OF  ROAD  SLICKER. 


FIG.  36. — SPLIT-LOG  DRAG  AT  WORK. 


into  their  outer  ends.  Fig.  36  shows  a  crude  split-log  with  removable 
platform  doing  excellent  work. 

The  dimensions  and  construction  of  the  plank  drag  are  shown 
in  Fig.  32.  If  the  main  planks  are  hard  wood  and  2  J  or  3  inches  thick, 
the  reinforcing  planks  may  be  omitted. 

Fig.  33  shows  an  adjustable  steel  road  drag.  The  cutting  blades 
can  be  tilted  forward  or  backward  by  the  hand  lever.  Similar  steel 
drags  are  made  with  two  blades.  Non-adjustable  2-blade  and 
3-blade  steel  road  drags  also  are  upon  the  market. 


120  EARTH   ROADS  [CHAP.    Ill 


207.  Rules  for  Using  Road   Drag.     The  following  rules  should 
be  observed  in  dragging  earth  roads: 

1.  Remove  all  loose  stones  from  the  road  before  dragging  it. 

2.  The  depth  of  cutting  may  be  regulated  by  the  length  of  the 
hitch  chain.     A  short  chain  causes  an  uplift,  and  hence  a  lighter  cut; 
and  vice  versa,  a  long  hitch  causes  less  uplift,  and  hence  a,  deeper 
cut.     The  depth  of  cut  can  also  be  varied  by  the  driver  moving 
forward  or  backward  on  the  drag,  or  e\en  by  inclining  his  body. 

3.  The  driver  should  stand  upon  the  drag,  and  be  ready  to  shift 
his  position  as  the  circumstances  demand.     However,  if  an  unusually 
soft  portion  of  the  road  is  encountered,  it  may  be  best  for  the  driver 
to  walk. 

4.  Drive  the  team  in  a  walk. 

5.  Drive  a  horse  on  each  side  of  the  right-hand  wheel  track,  with 
the  front  end  of  the  cutting  edge  on  the  outside  of  the  traveled  way, 
and  proceed  to  the  end  of  the  portion  of  the  road  to  be  dragged;  and 
then  on  the  return  do  similarly  for  the  other  wheel  track.     Of  course, 
if  the  traveled  way  is  wide,  it  may  be  necessary  to  make  more  than 
one  round  trip. 

6.  The  best  time  to  use  the  drag  is  when  the  road  is  drying  out, 
while  the  soil  cuts  easily,  and  is  damp  or  wet  enough  to  puddle ;  but  the 
soil  should  not  be  so  sticky  as  to  cling  to  the  cutting  edges  of  the  drag. 

7.  Do  not  move  too  much  earth  toward  the  center  of  the  road. 
There  are  two  reasons  for  this  rule.     First,  it  is  important  to  keep 
the  road  surface  as  hard  as  possible,  and  hence  no  more  earth  should 
be  loosened  than  just  enough  to  make  the  surface  smooth.     An  excess 
of  loose  earth  will  make  the  road  dusty  in  dry  weather  and  muddy 
in  wet  weather.     Second,  only  enough  earth  should  be  moved  toward 
the  center  to  counteract  the  effect  of  the  rain  in  reducing  the  crown. 
An  excess  crown  will  concentrate  travel  at  the  center  and  cause  deep 
ruts  to  form,  which  will  hold  water  and  make  mud.     If  the  crown 
becomes  too  high,  drag  it  once  or  twice  in  such  a  way  as  to  work 
the  soil  away  from  the  center. 

8.  By  hitching  the  team  close  to  the  front  end  of  the  drag,  the 
blade  will  cut  like  a  plow ;  and  the  machine  can  then  be  used  to  remove 
weeds  or  deepen  the  side  ditch.     With  this  hitch,  the  driver  should 
stand  near  the  forward  end  of  the  blade,  and  should  throw  his 
weight  forward  or  backward  as  the  work  may  require.     Care  must 
be  taken  that  the  entire  drag  is  not  tipped  over  forward.     When 
weeds  clog  the  cutting  edge,  they  can  be  removed  by  the  driver's 
shifting  his  weight  to  the  rear  end  of  the  cutting  edge. 


ART.    2]  MAINTENANCE  121 

9.  The  road  should  be  dragged  after  it  has  been  roughened  by 
being  used  when  muddy;  but  the  dragging  should  not  be  postponed 
until  the  soil  has  ceased  to  be  mellow  and  easily  moved.     It  is  a 
waste  of  time  to  postpone  the  dragging  until  the  road  has  become 
dry  and  hard. 

10.  The  most  effective  time  to  drag  a  road  is  in  the  spring  imme- 
diately after  the  frost  is  out. 

11.  If  the  roadway  is  very  rough  and  contains  many  deep  ruts  and 
holes,  it  may  be  best  to  drag  it  when  it  is  quite  slushy.     In  this  case 
a  lapped-plank  smoother,  or  slicker,  is  better  than  the  split-log, 
plank,  or  steel  drag.     The  hitch  to  the  slicker  should  be  such  that  the 
outer  end  of  the  machine  is  a  little  ahead,  so  as  to  fill  up  the  holes 
and  ruts. 

12.  If  a  slushy  road  is  dragged  or  smoothed  just  before  freezing 
weather,  the  surface  will  freeze  and  make  a  fine,  smooth,  hard  road. 

208.  Cost  of  Dragging.     The  cost  of  dragging  the  road  depends 
somewhat  upon  the  condition  of  the  road  and  also  upon  the  demand 
for  men  and  teams  for  agricultural  work;   but  usually  the  cost  per 
mile  per  round  trip  of  an  8-foot  drag  will  be  about  50  to  60  cents. 
See  §  229  for  further  details. 

209.  Scraping  Grader.    This  machine  is  described  in  §  155-56, 
and  its  use  in  road  construction  is  explained  in  §  157-60.     It  is  pro- 
posed here  to  consider  the  use  of  this  machine  in  the  maintenance 
of  a  road. 

If  the  road  drag  (§  206)  were  frequently  and  efficiently  used,  the 
road  would  be  kept  in  a  fairly  good  condition;  but  for  one  reason 
or  another,  the  roads  are  not  usually  dragged  when  the  work  can  be 
done  most  efficiently,  and  consequently  are  allowed  to  dry  out 
rough  and  full  of  ruts.  After  they  have  reached  this  stage,  it  is 
nearly  or  quite  impossible  to  secure  a  smooth  surface  with  any  form 
of  road  drag.  Under  these  conditions  the  surface  can  be  restored  by 
running  a  scraping  grader  over  the  road  so  as  to  plane  off  the  ridges 
and  fill  up  the  ruts. 

210.  Operating  the  Scraping  Grader.     Commence  at  the  ditch  and 
work  toward  the  center,  scraping  with  the  entire  length  of  the  blade. 
The  blade  should  stand  nearly  square  across  the  road,  and  consider- 
able earth  should  be  shoved  along  in  front, — enough  to  fill  the  depres- 
sions;— but  only  enough  earth  should  be  moved  toward  the  center  of 
the  roadway  to  replace  that  washed  down  by  the  rains.     The  sur- 
plus earth  should  be  uniformly  distributed  over  the  surface,  by 
carrying  the  rear  end  of  the  blade  a  little  higher  than  the  point.     A 


122  EARTH    ROADS  [CHAP.    Ill 

ridge  of  earth  should  not  be  left  in  the  center  of  the  road,  since  it 
will  but  slowly  consolidate  and  is  likely  to  be  washed  into  the  side 
ditches  to  make  trouble  there. 

This  work  should  be  done  early — before  the  ground  becomes 
hard  and  difficult  to  work,  before  traffic  has  been  compelled  par- 
tially to  do  the  work  of  the  road  leveler,  and  while  the  surface  is  in 
condition  to  unite  with  the  loose  earth  left  by  the  machine,  and 
when  the  roots  of  grass  and  weeds  do  not  interfere  with  the  work  of  the 
blade.  Unfortunately  this  work  is  often  postponed  until  the  ground 
is  so  hard  that  it  is  impossible  to  do  a  thoroughly  good  job.  If 
the  ground  is  a  little  too  wet  for  tillage,  it  is  all  the  better  for  road 
making,  since  it  will  pack  and  harden  better  than  though  it  were 
drier.  After  the  ground  becomes  dry  and  hard,  it  is  not  only  more 
laborious  and  expensive  to  secure  a  smooth  surface;  but  the  newly 
repaired  road  may  for  weeks  be  in  a  worse  condition  than  before 
it  was  worked,  since  the  loose  earth  is  too  dry  to  pack  under  traffic. 

211.  A  common  error  in  scraping  roads  is  not  to  begin  far  enough 
down  in  the  ditch,  thus  leaving  a  shoulder  which  prevents  the  water 
from  flowing  from  the  roadway  into  the  side  ditch.  Fig.  37  shows  a 


FIG.  37. — OBJECTIONABLE  SHOULDERS  LEFT  BY  SCRAPING  GRADER. 


road  finished  in  this  way.  The  shoulders  not  only  dam  back  the 
water,  but  also  narrow  the  roadway;  and  after  weeds  and  grass  have 
got  a  good  start,  it  is  improbable  that  the  shoulder  will  be  cut  off 
next  time  the  road  is  scraped,  and  in  all  probability  each  successive 
scraping  will  make  a  bad  matter  worse.  However,  with  a  skilful 
use  of  the  scraping  grader  these  shoulders  can  be  cut  off. 

Not  infrequently  writers  claim  that  material  from  the  side 
ditches  should  not  be  placed  upon  the  roadway.  Unquestionably 
silt  from  the  bottom  of  the  ditches  is  undesirable  material  with 
which  to  built  or  repair  a  road;  but  in  ditches  properly  constructed 
and  cared  for,  there  is  not  much,  if  any,  of  such  material,  and  if  any 
of  it  is  removed  with  the  scraping  grader  it  is  so  thoroughly  mixed 
with  good  material  before  it  reaches  the  roadway  as  to  be  practi- 
cally harmless.  The  advice  against  fine  material  from  the  side 
ditches  originated  when  the  drag  scraper  was  the  chief  tool  used  in 
repairing  roads,  and  the  advice  has  unfortunately  outlasted  its 
usefulness. 


ART.    2]  MAINTENANCE  123 

212.  Cost  with  Scraping  Grader.     The  scraping  grader  may  be 
drawn  by  three  2-horse  teams  or  by  a  traction  engine;    but  unless 
the  roads  are  very  hard  and  tough,  horses  are  more  economical  than 
the  tractor,  since  the  grader  is  too  small  to  be  used  economically  with 
the  ordinary  tractor. 

To  shape  up  the  road  in  the  spring,  six  horses  and  three  men  are 
required  to  operate  the  scraper.  The  wages  of  a  team  and  driver 
will  usually  be  $3.00  or  $3.50  per  day,  since  generally  the  scraping 
should  be  done  when  farmers  are  busy  with  farm  work,  and  since 
the  work  is  hard  on  teams.  The  cost  of  operating  the  grader  is  then 
$9.00  to  $10.50  per  day.  A  scraper  will  on  the  average  smooth  up 
3  or  4  miles  per  day,  at  an  expense  of  $3.00  to  $3.50  per  mile,  or,  in 
round  numbers,  including  repairs  and  loss  by  bad  weather,  say, 
$4.00  per  mile.  If  the  road  is  not  very  rough,  two  rounds  are 
enough;  and  if  it  is  very  bad  four  may  be  required,  but  usually  three 
rounds  are  sufficient.  If  the  work  is  postponed  too  long,  the  cost  may 
be  nearly  double  the  above. 

The  cost  of  smoothing  up  city  streets  would  be  considerably 
more  than  the  above,  because  of  the  time  consumed  in  passing  side- 
walk crossings  or  in  turning  to  avoid  them.  Particularly  under  such 
conditions,  the  amount  of  work  accomplished  in  a  day  depends  greatly 
upon  the  training  of  men  and  horses. 

213.  The  V  Road-Leveler.     One  form  of  this  machine  is  shov/n 
in  Fig.  38.     It  consists  of  two  cutting  blades  18  or  25  feet  long, 
suspended  from  a  platform  which 

carries  the  operating  machinery. 

The  spread  of  the  cutting  blades 

and  also  the  relative  height  of 

the    two    ends    are    adjustable. 

The  machine  is  drawn  behind  a 

15  to  25  H.P.  tractor.     With  the 

longer    blades    the    leveler   will 

smooth  up  a  maximum  width  of 

30  feet  at  one  time;   and  with 

the     shorter     blades     22     feet. 

Either  size  machine   can   shape  FIG.  ss.— THE  v  ROAD-LEVELER. 

up   a    roadway    10   or    12    feet 

wide.     There  are  at  least  two  other  somewhat  similar  forms  of  this 

machine. 

The  scraping  grader  is  too  large  for  a  convenient  number  of  horses, 
and  too  small  for  a  tractor;  and  hence  the  V  road-leveler  was 


124  EARTH   ROADS  [CHAP.    Ill 

invented  to  economically  utilize  the  full  power  of  an  ordinary  'trac- 
tion engine. 

Sometimes  three  large  adjustable  steel  road  drags  are  hitched 
behind  a  traction  engine,  and  do  substantially  the  same  work  as  a 
V  road-leveler. 

Sometimes  a  drag  or  a  roller  is  hitched  behind  the  V  road-leveler, 
to  level  down  or  consolidate  the  loose  earth  left  between  the  cutting 
blades. 

214.  Filling  Holes.    After  the  road  has  been  smoothed  by  the 
scraping  grader  or  the  V  road-leveler,  it  is  a  good  plan,  particularly 
if  the  road  is  very  rough,  to  send  a  man  with  a  shovel  to  fill  up  all 
ruts  and  depressions  that  were  too  deep  to  be  filled  by  the  scraper. 
If  a  deep  hole  has  been  filled  by  the  scraper,  it  is  well  to  add  a  little 
more  earth  to  provide  for  settlement  in  order  to  prevent  the  re- 
appearance of  the  hole.     The  new  material  should  be  trodden  or 
tamped  solidly  into  place. 

Holes  and  ruts  in  an  earth  road  should  never  be  filled  with  stone, 
brick,  or  coarse  gravel.  The  hard  material  does  not  wear  uniformly 
with  the  rest  of  the  road,  but  produces  bumps  and  ridges,  and 
usually  results  in  making  two  holes,  each  larger  than  the  original 
one.  It  is  a  bad  practice  to  cut  a  gutter  from  a  hole  to  drain  it  to 
the  side  of  the  road.  Filling  the  hole  is  the  proper  course,  whether 
it  is  dry  or  contains  mud. 

215.  Removing  Stones.    All  loose  stones  larger  than  2  inches 
in  diameter  should  be  removed;    and  stones  projecting  above  the 
surface  should  be  dug  out.     They  should  be  taken  entirely  away, 
or  be  piled  beyond  the  side  ditches;   and  should  never  be  left  just 
outside  of  the  trackway,  as  is  sometimes  done,  where  they  restrict 
traffic  and  obstruct  the  flow  of  water  from  the  center  of  the  side 
ditches. 

216.  CARE  OF  SIDE  DITCHES.     The  side  ditches  should  be  ex- 
amined in  the  fall  to  see  that  they  are  free  from  dead  weeds  and 
grass;   and  late  in  the  winter  they  should  be  examined  again  to  see 
that  they  are  not  clogged  with  corn  stalks,  brush,  etc.,  washed  in 
from  the  fields.     The  mouth  of  culverts  should  also  be  cleared  of 
rubbish,  and  the  outlet  of  tile  drains  should  be  opened.     Attention 
to  side  ditches  will  prevent  overflow  and  washing  of  the  road-bed, 
and  will  also  prevent  the  formation  of  ponds  at  the  roadside  and 
the  consequent  saturation  of  the  road-bed.     The  road  care-taker 
should  frequently  go  over  his  portion  of  the  road  just  as  a  heavy 
fall  of  snow  is  going  off,  for  it  is  then  that  water  does  most  damage. 


ART.    2]  MAINTENANCE  125 

217.  CARE   OF   ROADSIDE.     It   is   desirable  that  the  roadside 
should  be  so  cared  for  as  to  secure  a  coating  of  grass  instead  of  un- 
sightly and  noxious  weeds.     This  can  usually  be  accomplished  at  a 
slight  expense  by  an  occasional  mowing. 

218.  Care   of  Trees   and   Hedges.     Earth   roads   should   have 
plenty  of  light  and  air.     Trees  along  the  road  may  add  beauty  to 
the  landscape  (§195),  but  shade  is  nearly  sure  to  breed  mud  holes. 
In  some  localities  and  under  some  conditions,  shade  upon  the  road 
surface  should  be  eliminated  by  cutting  down  the  trees  or  by  trim- 
ming them  so  as  not  to  keep  the  breeze  and  sunlight  from  the  road; 
but  in  other  localities  and  under  other  conditions,  a  little  of  the  utility 
of  the  road  may  be  sacrificed  to  secure  attractiveness  in  the  general 
surroundings. 

A  tall  hedge  cuts  off  the  view  of  the  adjacent  country,  shuts  out 
the  breeze,  in  a  dry  time  keeps  in  the  dust,  and  in  a  wet  time  retards 
the  drying  of  the  road.  The  hedges  usually  belong  to  the  adjacent 
private  property,  but  in  most  states  the  height  is  limited  by  statute; 
and  in  such  cases  the  road  officials  should  enforce  the  law.  If  there 
is  no  law  governing  hedges  and  trees  near  the  road  on  private 
property,  the  road  officials  should  use  all  possible  diplomacy  to  have 
trees  and  hedges  trimmed  with  reference  to  the  benefits  of  the  road. 
In  this  connection,  see  §  219. 

219.  OBSTRUCTION  BY  SNOW.     In  localities  subject  to  heavy 
falls  of  snow,  it  is  an  important  matter  to  keep  the  roads  from  be- 
coming obstructed  by  it  during  the  winter.     In  some  countries  where 
there  is  only  an  occasional  fall  of  snow,  as  in  France,  it  is  customary 
to  remove  it  from  the  surface  of  the  road;  but  where  there  is  much 
snow,  it  is  only  necessary  to  compact  it  so  as  to  make  the  road  pass- 
able.    This  is  done  by  driving  horses  or  cattle  back  and  forth  along 
the  road,  or  by  rolling  the  road  with  a  heavy  farm-roller.  The  use 
of  the  roller  should  commence  with  the  first  storm  of  the  season  and 
be  continued  as  often  as  necessary  through  the  winter.     In  the  case 
of  a  very  heavy  storm,  the  roller  should  be  sent  over  the  roads  at 
intervals  during  its  continuance.     Obviously  this  work  must  be 
done  by  the  residents  along  the  road. 

Snow  and  ice  frequently  accumulate  in  the  side  ditches  to  such  a 
height  as  to  make  the  surface  of  the  road  the  principal  line  of  drain- 
age. In  the  spring,  when  this  occurs  on  earth  roads,  a  large  volume 
of  snow-water  flows  down  the  road,  and  often  seriously  damages  it 
by  washing  gullies  in  the  surface.  The  best  water-bound  macadam 
roads  may  be  seriously  injured  in  this  way;  and  in  some  localities 


HAT. 


to  remove  the  snow 
of  the  character.    The  cfiffieofty  and 

free  from 
jlrrp  and  narrow,  particularly 

to  maintain  a  eulitat  or  cohered 
and  private  drives  with  the 

%'V3"%  h&bie  tiO  Mt^ff^F^*  ^M^fBBPCi  ^ili> 

to 


: 


This  difficulty  could  be  obviated.  01 
detreascd,  by  oaoBtmftmg  dbaflow  ade  ditches 
a  large  tile  drain  tinder  the  ditch  to  cam 


K  orMMnnm  ny  snow  can  ue  oecreasea  uy  proper 
Dees,  UDdaUudi,  efcCL,  along  the  side  of  the  road. 
•ed  by  the  obstruction  of  the  currents  of  air  near 
tint  cany  the  drifting  snow.    In  forests  the  winds 
tent  wfiuul5  to  cany  the  enow,  and  consequently 
of  a  uniform  depth,    but  in  the  open  country 
mind.    Fences  and  shrubbery  which  retard  the 
the  mam  to  blow  through,  cause  the  snow  to  pile 
1  fide  and  possibly  to  block  the  road  and  ditches. 
he  either  quite  open  or  very  dose.     A  High  tight 
wind,  and  causes  the  snow  to  pile  up  on  the  wind- 
inadaiiie  is  partially  obstructed,  the  wind  moves 
o  earth  cots  and  also  into  the  beaten  snow  path, 
Fifihig  the  snow  trackway  gradually  raises  the 
f  the  road  until  turning  out  into  the  loose  snow 

nt,  •"  in  many  townships  the  cost  of  keeping  the 
ti*fr  winter  is  one  third,  and  in  some  one  half,  of 
rinrmifd  on  the  highways,  and  the  average  for 
boV  or  $UO  per  mile  per  annum. 
bfc  coat  of  maintenance  on  account  of  snow  should 
(Bating  a  road  (§  100-02). 

OT  MADrTEHAJfCE.     The  administration  of  the 
•Hi  toads  k  a  matter  of  great  difficulty.    The 
Bae  justifiable  expense  is  comparatively  small; 
wo  does  the  work  must  have  charge  of  a  consider- 
•BqpBntrjr  can  get  over  the  road  only  at  infre- 
•Mny  states  the  maintenance  is  done  in 
W»r  fax,  which  at  best  is  inefficient.    3.  The 


.    2]  MAINTENANCE 


imount  of  work  required  may  vary  suddenly  and  greatly  with  local 
storms.  4.  The  administration  is  usually  in  the  hands  of  an  inexpert 
man  or  board  to  whom  the  care  of  the  roads  is  only  an  incident  in 
orivate  or  official  duties.  In  this  connection,  see  §  41. 

As  a  rule  inadequate  attention  has  been  given  in  this  country 
to  the  maintenance  of  roads,  and  this  is  particularly  true  of  earth 
roads.  For  the  latter  there  have  been  only  a  few  attempts  to  develop 
an  efficient  system,  and  even  they  are  still  in  the  experimental  stage. 
Further,  the  conditions  vary  so  greatly  in  different  parts  of  the 
country  that  a  system  that  is  reasonably  successful  in  one  locality 
may  be  wholly  inapplicable  in  another. 

The  various  systems  that  have  been  attempted  may  be  classified 
somewhat  roughly  as  follows:  (1)  intermittent  repairs;  (2)  continu- 
ous repairs;  (3)  continuous  maintenance;  and  (4)  contract  system. 
These  systems  differ  greatly  and  over-lap  as  applied  in  different 
localities. 

223.  Intermittent  Repairs.     This  system  is  with  propriety  often 
called  the  pathmaster  system.     In  this  system  the  care  of  the  roads 
is  left  to  the  official  pathmaster,  who  has  charge  of  8  or  10  miles  of 
roads,  and  who  superintends  the  working  out  of  the  labor  road-tax. 
This  is  the  most  common  but  least  efficient  system.     This  system 
has  all  the  objections  enumerated  in  §  222. 

In  practice  this  system  is  a  method  of  intermittent  repairs  rather 
than  maintenance,  that  is,  under  this  system  the  roads  are  allowed 
to  get  into  a  comparatively  bad  condition  before  they  are  repaired 
or  restored. 

224.  Continuous   Repairs.     This   system     consists    in    putting 
the  care  of  the  roads  of  a  township  or  its  corresponding  administrative 
road-unit  into  the  hands  of  a  smaU  squad  of  men  who  give  all  or 
substantially  all  of  their  time  to  the  care  of  the  roads.     These  men 
ire  provided  with  a  scraping  grader  (§  155)  or  a  V  road  leveler 
(§213),  and  teams  or  a  traction  engine,  shovels,  picks,  etc. 

The  theory  of  this  system  is  that  the  roads,  or  at  least  the  main 
)nes,  will  not  get  very  bad,  or  be  bad  very  long,  before  the  repair 
2;ang  will  be  along.  The  advantages  of  this  system  are:  (1)  the  squad 
is  employed  continuously,  and  hence  becomes  more  expert;  and  (2) 
:he  amount  of  road  cared  for  is  so  great  as  to  warrant  providing  the 
squad  with  a  good  outfit.  The  disadvantages  are:  (1)  the  work  is 
lot  done  at  the  most  advantageous  time,  i.  e.,  when  the  soil  is  most 
3asily  worked;  and  (2)  the  roads  are  not  in  good  condition  all  the 
ime. 


128  EARTH   ROADS  [CHAP.    Ill 

225.  Continuous   Maintenance.     The   essential   feature   of  this 
system  is  that  the  care  of  a  definite  road  is  allotted  to  one  man,  who 
makes  this  his  first  business.     The  system  is  sometimes  called  the 
patrol  system,  and  takes  this  name  from  the  method  long  employed, 
chiefly  in  Europe,  in  caring  for  water-bound  macadam  roads  in  which 
a  man  devoted  all  of  his  time  to  patrolling  and  caring  for  a  compara- 
tively short  piece  of  road. 

It  is  not  usual  to  attempt  formally  to  maintain  any  but  the  most 
traveled  earth  roads.  Even  on  these  roads  the  amount  of  work 
required  varies  so  much  with  the  season  and  with  th3  frequency 
of  storms,  that  the  section  must  be  comparatively  short;  and  there- 
fore at  times  there  is  not  work  enough  to  require  the  full  time  of  the 
patrol.  To  meet  this  condition  it  is  customary  to  employ  a  man  who 
lives  near  the  road,  to  labor  upon  the  road  when  directed.  The 
direction  of  the  patrols  is  in  the  hands  of  a  township  official  or 
foreman;  and  a  general  supervision  of  all  the  foremen  is  in  the  hands 
of  the  county  engineer. 

The  only  work  ordinarily  attempted  is  to  drag  the  road  as  needed. 
The  overseer  or  superintendent  communicates  with  the  patrols  by 
telephone,  both  to  inquire  as  to  the  condition  of  the  roads  and 
to  order  work  done.  The  patrol  is  usually  required  to  report  by 
postal  card  as  to  the  work  done  and  the  time  required.  In  some  cases 
a  report  is  required  for  each  day's  work,  but  sometimes  only  a  weekly 
report  is  demanded.  In  some  states  a  portion  of  the  road  tax  is  set 
apart  by  statute  to  pay  for  dragging,  and  can  not  be  used  for  any  other 
purpose. 

The  length  of  the  sections  vary  with  the  possibility  of  securing 
competent  patrols;  but  are  preferably  not  more  than  a  mile  or  two 
each,  so  as  not  to  interfere  too  much  with  the  other  duties  of  the 
patrol. 

Prizes  are  sometimes  given  for  the  best  work,  the  money  for  the 
same  in  some  cases  being  taken  from  the  road  taxes  and  in  other  cases 
is  contributed  by  Chambers  of  Commerce,  etc. 

By  this  system  of  maintenance  the  roads  are  kept  all  the  time 
in  a  fairly  good  condition.  In  a  number  of  cases  where  tried  the  con- 
clusion reached  was  that  the  condition  of  the  roads  was  much  better 
under  this  than  under  other  systems,  and  at  the  same  time  the  total 
cost  was  less.* 

226.  In  those  states  in  which  an  attempt  is  made  to  continuously 

*For  one  example,  see  Engineering  Record,  Vol.  73   (1916),  p.  643-44;    and  for  another 
see  Engineering  and  Contracting,  Vol.  38  (1912),  p.  714. 


ART.  2]  MAINTENANCE  129 

maintain  the  earth  roads,  there  is  usually  a  law  prohibiting  the  use 
of  the  dragged  surface  until  it  has  dried  out  so  that  a  wheel  will  not 
make  a  rut. 

227.  Maintenance    by    Contract.     In    view    of    the   ordinarily 
inefficient  system  of  caring  for  roads,  it  has  frequently  been  pro- 
posed to  maintain  them  by  contract.     As  a  rule,  work  done  under 
the  supervision  of  a  contractor  who  has  pecuniary  interest   in   the 
result  is  more  economical  than  that  performed  under  the  direction 
of  a  public  official;  but  it  is  not  wise  to  do  work  by  contract  unless 
the  amount  required  can  be  approximately  known  beforehand,  and 
also  unless  the  character  of  the  performance  can  be  easily  deter- 
mined  after   completion.     Neither   of  these   important   conditions 
would  be  present  in  a  contract  for  the  maintenance  of  a  public  high- 
way.    Owing  to  the  indefiniteness  as  to  the  amount  and  character 
of  the  work  to  be  done,  it  is  not  at  all  certain  that  the  maintenance 
could  be  provided  for  by  contract  for  a  sum  less  than  the  public 
officials  could  do  the  work  under  the  present  system.     The  inspec- 
tion would  finally  depend  upon  the  road  official,  and  the  letting  of  a 
contract  would  increase  the  difficulties  and  expense  of  supervision. 

It  is  claimed  that  the  contractor  could  maintain  a  trained  corps, 
and  therefore  do  better  work  than  can  be  done  by  the  present  system; 
but  it  is  doubtful  if  contract  work  would  be  any  cheaper  or  better 
than  the  method  described  in  §  225. 

228.  EXPENDITURES  FOR  MAINTENANCE.    There  are  but  few 
data  concerning  cost  of  maintaining  earth  roads,  and  much  of  that 
is  very  indefinite  since  the  conditions  of  soil,  weather,  etc.,  are  not 
stated  and  also  since  no  definite  information  can  be  stated  as  to  the 
quality  of  the  maintenance. 

229.  Dragging.     The  cost  of  systematically  dragging  a  road  in 
Arkansas  was  $11  per  mile  per  annum,  or  50  cents  for  each  dragging.* 

In  Tennessee  30  miles  of  roads  in  sections  of  3  miles  each  were 
dragged  during  the  months  of  December,  January,  February,  and 
March,  under  the  continuous  maintenance  system.  The  price  for  a 
man  and  a  2-horse  team  was  30  cents  per  hour.  The  county  furnished 
the  drags.  Prizes  were  offered  for  the  best  kept  road,  and  the  prize 
was  awarded  to  a  road  for  which  the  cost  was  $5.00  per  mile -per 
annum. 

In  Hale  Township,  Carroll  County,  Missouri,  an  overseer  is  in 
charge  of  every  8  miles  of  road,  and  has  ten  patrols,  each  in  charge 
of  a  section.  After  each  rain  the  overseer  by  telephone  calls  upon  the 

*  Bui.  No.  48  of  the  U.  S.  Office  of  Public  Roads,  p.  46-7. 


130  EARTH   ROADS  [CHAP.    Ill 

patrols  to  drag  the  roads.  The  cost  of  maintaining  the  roads  dur- 
ing April,  May  and  June  is  from  $10  to  $15  per  mile,  including  $15 
for  each  overseer. 

Clayton  County,  Iowa,  from  1913  to  1916  maintained  the  county 
roads,  i.  e.,  226  of  the  1350  miles  in  the  county,  by  the  continuous 
maintenance  system.  The  patrol  section  was  from  7  to  10  miles. 
Some  of  the  patrolmen  put  in  all  their  time,  but  some  only  part 
time.  The  patrolmen  hired  help  as  was  necessary.  Each  patrolman 
furnished  his  own  team  and  wagon.  Usually  three  horses  were  used 
on  a  drag.  The  average  pay  was  27f  cents  per  hour  for  patrolmen, 
47  J  cents  per  hour  for  man  and  team,  and  10  cents  per  hour  for  each 
extra  horse.  Patrolmen's  assistants  were  paid  25  cents  per  hour,  and 
45  cents  per  hour  for  man  and  team.  The  average  cost  of  dragging 
was  56J  cents  per  mile  for  one  round  trip.  The  total  cost  for  main- 
tenance and  repairs  averaged  about  $56  per  mile  per  year.  The 
average  cost  of  dragging  in  Clayton  County  was  56J  cents  per  mile 
per  round  trip;  while  that  in  other  counties  of  the  state  was  71.3 
cents  per  mile  per  round  trip.* 

230.  Total  Cost  of  Maintenance.     The  following  data  are  for 
the  maintenance  of  70  miles  of  road  during  the  years  1909  and  1910 
in  northern  Michigan,  f    The  roads  were  maintained  by  the  patrol 
system  under  the  direction  of  the  County  Engineer.     The  roads 
were   "  floated,"  i.   e.,   dragged  with    the  slicker   or   lapped-plank 
drag,  once  when  the  frost  was  partly  out  and  once  after  it  was  com- 
pletely out.     The  roads  were  dragged  after  every  heavy  or  protracted 
rain  during  the  season.     After  the  roads  had  settled  in  the  spring, 
every  hole  was  filled,  and  the  roads  otherwise  put  into  good  condition. 
The  patrols  were  required  to  be  on  the  road  and  work  upon  either 
the  road-bed  or  the  drainage  system,  two  specified  days  in  each  month. 
They  were  expected  to  mow  weeds  and  brush  on  the  roadsides,  break 
through  the  snow  in  winter,  and  keep  the  road  to  the  standard  of  a 
good  earth  road.     The  cost  was  as  shown  in  the  tabular  statement 
on  the  opposite  page. 

"  The  greater  cost  of  dragging  in  1909  was  because  the  roads 
had  not  previously  been  dragged;  and  the  greater  cost  of  general 
repairs  in  1910  was  because  of  the  higher  standard  of  maintenance."  J 

231.  Table  20,  page  132,  gives  the  results  of  a  test  in  maintenance 


*  Engineering  Record,  Vol.  73  (1916),  p.  643. 

t  K.  I.  Sawyer,  in  Proc.  1915  Short  Course  in  Highway  Engineering,  University  of  Michigan, 
p.  62-64. 

J  Private  letter  from  Mr.  Sawyer. 


ART.    2]  MAINTENANCE  131 

conducted  by  the  U.  S.  Office  of  Public  Roads.*  "  Before  the  main- 
tenance was  undertaken  the  county  repaired  the  road  and  put  it  in 
good  shape.  The  repairs  consisted  in  shaping  parts  of  the  road  with 
a  scraping  grader,  clearing  and  widening  the  ditches  and  clearing 
the  culverts,  and  applying  gravel  to  a  section  of  the  road.  The  cost 

1909  1910 

Length  maintained,  miles 70 . 5  72 . 5 

Length  of  patrol  section,  miles 6  4-6 

Average  cost,  per  mile : 

Dragging $26 . 17 

Patching  surface,    culvert  and   ditch   work,    cutting 

weeds  and  brush .  .  8 . 56 


Total  cost  per  mile .  .  $34.73         $28.42 

Cost  of  dragging  one  mile  one  time 0 . 925 

of  the  repairs  was  $700.  On  the  8  miles  of  road  there  are  4  bridges, 
19  culverts,  54  drain  pipes  under  driveways,  59  intersecting  roads 
with  drain  pipes,  42  intersecting  roads  without  drain  pipes,  and  10 
small  wooden  bridges  across  the  gutter.  The  entire  8  miles  of  road 
is  well  traveled,  and  there  is  considerable  heavy  teaming  over  parts 
of  it  A  portion  of  the  road  is  also  used  by  United  States  cavalry. 
There  is  also  considerable  automobile  traffic  on  some  portions. 
A  travel  census  for  3  days  in  March  on  one  section  of  the  road  shows 
the  following :  Loaded  1 -horse  wagons,  15;  unloaded  1 -horse  wagons, 
58;  loaded  2-horse  wagons,  38;  unloaded  2-horse  wagons,  49;  loaded 
4-horse  wagons,  9;  unloaded  4-horse  wagons,  4;  saddle  horses,  96; 
and  motor  runabouts,  1.  The  patrolman  furnished  a  horse,  cart, 
and  small  tools.  He  was  supplied  with  a  plank  road-drag,  and  re- 
quired to  furnish  two  horses  to  drag  the  road  whenever  it  was  in 
suitable  condition  for  dragging,  usually  following  each  rain.  He 
was  paid  $60  per  month  and  $1.00  per  day  extra  whenever  he 
used  two  horses  to  drag  the  road.  His  presence  was  required  on 
the  road  from  8  a.m.  to  4:30  p.m.,  with  thirty  minutes  allowed  for 
lunch." 

"  The  cost  of  dragging  was  approximately  $1.25  per  mile  for  each 
dragging  of  three  round  trips.  The  item  of  $169.88  for  repairing, 
clearing  and  improving  ditches  and  underdrains  was  large,  because 
it  was  found  necessary  as  the  year  progressed  to  rebuild  entirely 
portions  of  the  gutters  and  ditches. 

*  Engineering  and  Contracting.  Vol.  38  (1912),  p.  714. 


132 


EARTH   ROADS 


[CHAP,  in 


TABLE  20 
COST  OF  MAINTENANCE  OF  8  MILES  OF  ROAD  IN  ALEXANDRIA  COUNTY,  VIRGINIA, 

From  December   17,   1911,   to  June  30,   1912 


Ref. 
No. 

Kind  of  Work. 

DAYS. 

COST. 

Total. 

Per  Cent. 

Total. 

Per  Mile. 

1 

2 

3 
4 
5 
6 

7 

Dragging.  . 

38.5 

73. 
26.5 
10.5 
10. 
5.5 

6. 

22.7 

42.9 
15.6 
6.2 
5.9 
3.2 

3.5 

$128.89 

169.88 
61.78 
24.55 
23.36 
12.67 

13.86 

$16.11 

21.22 

7.72 
3.06 
2.92 
1.58 

1.73 

Repairing,  cleaning  and  improving 
ditches  and  underdrains 

Cutting  brush,  etc  

Picking  off  stones  

Taking  census  

Inspection  during  storms  
Clearing    fallen    trees,     building 
guard  rails,  etc 

Total  for  6.5  months 

170.00 

100.0 

$434.99 

$54.34 

"  The  following  conclusions  are  clearly  demonstrated  by  the 
experiment:  (1)  The  use  of  the  drag  has  greatly  improved  the 
daily  condition  of  the  road  and  rendered  it  smooth  and  comfortable 
for  travel  for  a  greatly  increased  number  of  days  in  bad  weather. 
(2)  A  width  of  earth  road  in  excess  of  24  feet  is  unnecessarily  expensive 
to  maintain.  (3)  The  presence  of  the  patrolman  during  storms  and 
immediately  after,  saves  considerable  expense  for  repairs  due  to  the 
wash  of  surface  water.  (4)  The  existence  of  poorly  drained  private 
driveways  and  intersecting  roads  is  a  constant  expense  for  main- 
tenance. (5)  The  use  of  small  tiles  for  side  drains  and  the  building 
of  wooden  bridges  over  gutters  at  driveways  is  a  serious  obstacle 
to  proper  drainage.  The  pipe  is  usually  laid  at  insufficient  depth, 
and  becomes  broken  and  clogged.  It  would  appear  that  paved  gut- 
ters at  driveways  would  not  be  unduly  expensive  in  the  long  run,  and 
would  certainly  provide  better  surface  drainage.  (6)  It  is  not 
economical  to  employ  a  patrolman  during  the  winter  months,  unless 
his  time  can  be  used  to  advantage  in  clearing  brush  and  rubbish  from 
the  right-of-way;  but  a  man  should  be  constantly  in  charge  of  every 
mile  of  road  to  inspect  it  during  storms,  and  to  free  the  ditches. 
(7)  The  presence  of  old  cobble  stones  and  poorly  consolidated 
coarse  gravel  is  a  serious  impediment  to  the  use  of  the  drag.  The 
stones  must  be  removed  from  the  road  before  dragging  can  be  suc- 
cessful. (8)  There  is  ample  work  for  one  man  continuously  during 
8  or  9  months  of  the  year;  and  there  is  difficulty  in  combining  road- 
patrol  work  with  the  dragging  of  earth  roads." 


ART.   3]  SURFACE   OILING  133 

232.  Table  21  gives  details  of  the  average  annual  expenses  for 
roads  in  Champaign  County,  Illinois.  Notice  that  part  of  the 
expenditures  are  for  maintenance  proper,  while  part  are  for  im- 
provements in  the  original  construction. 

TABLE  21 
AVERAGE  EXPENDITURES  PER  MILE  OF  EARTH  ROADS  IN  CHAMPAIGN  Co.,  ILL. 

1.  New  steel  bridges — exclusive  of  county  aid  * $16.20 

2.  Drainage 6.32 

3.  Tile  culverts 1 .32 

4.  Repairs  of  bridges  and  culverts 2 . 93 

5.  Grading  (not  simply  smoothing  and  leveling) , 1 .43 

6.  Smoothing  and  leveling  (not  grading) 2 . 83 

7.  Mowing  the  roadsides 1.14 

8.  Administration 2 . 69 


Total  average  annual  expenditure $34 . 86 

It  is  not  known  that  any  data  similar  to  those  in  Table  21  were 
ever  before  collected,  and  hence  there  is  no  means  of  knowing 
whether  these  data  are  representative.  These  expenditures  were 
in  1900  before  the  use  of  oil  in  maintaining  earth  roads  (see  Art. 
3  of  this  chapter).  It  is  probable  that  the  expenditure  for  bridges 
is  considerably  larger  than  the  average.  Champaign  County  is 
rolling  prairie  situated  in  the  corn  belt.  There  are  no  large  streams, 
and  practically  all  the  land  is  under  cultivation.  Farm  lands  with- 
out buildings  then  sold  at  $80  to  $100  per  acre.  There  are  1.97 
miles  of  road  per  square  mile  of  area  outside  of  cities  and  villages. 
All  the  roads  have  a  black  loam  surface. 

ART.  3.    SURFACE  OILING 

233.  The  surface  of  an  earth  road  is  sometimes  treated  with  oil 
to  prevent  dust  and  also  to  aid  in  keeping  the  surface  smooth.     In 
small  towns  and  villages  the  former  is  the  chief  purpose;   while  on 
rural  roads  the  latter  is  the  main,  or  sole,  object. 

234.  PREVENTING   DUST.    The   annoyance  from  dust  usually 
reaches  its  maximum  in  small  towns  and  villages,  owing  to  the 
concentrated  travel  and  the  presence  of  more  people  to  be  incon- 
venienced.    The  dust  can  be  greatly  reduced  by  properly  dragging 
the  road.     The  surface  should  never  be  dragged  when  dry,  since  the 

*  In  Illinois  the  county  pays  half  the  expenses  of  bridges  costing  more  than  a  specified  per 
cent  of  the  assessed  value  of  the  township.  The  expenditures  by.  the  county  for  new  steel 
bridges  is  nearly  as  much  as  by  the  township. 


134  EARTH  ROADS  [CHAP.    Ill 

resulting  loose  earth  will  speedily  be  ground  into  dust;  and  for  the 
same  reason,  an  excess  of  loose  earth  should  not  be  left  in  the  center 
of  the  road. 

Within  the  last  decade  many  dust  palliatives  and  preventatives 
have  been  used;  but  oil  is  the  agent  most  frequently  employed  on 
earth  roads.  Crude  tar  has  been  employed  as  a  dust  palliative; 
but  on  account  of  its  injury  to  rubber  tires  and  also  on  account  of 
its  tracking  into  houses,  it  is  not  satisfactory. 

235.  EFFECT  OF  OIL  ON  MAINTENANCE.  Loam  and  clay  roads 
are  improved  by  a  little  moisture — just  enough  to  keep  them  damp 
and  dark  without  making  them  soft  or  spongy.  In  dry  climates  the 
roads  not  only  become  excessively  dusty,  which  is  a  great  discom- 
fort, but  also  wear  into  pot-holes,  which  are  dangerous,  since  being 
filled  level-full  of  dust  their  presence  is  not  revealed  until  a  wheel  or 
a  horse's  foot  plunges  into  them.  In  some  localities  the  dust  at 
times  is  practically  hub  deep,  and  is  not  only  an  annoyance  but 
greatly  increases  the  tractive  resistance.  In  arid  climates  and  even 
in  dry  times  in  humid  climates,  sprinkling  with  water  is  an  effective 
means  of  maintenance.  A  layer  of  straw  is  sometimes  put  upon  the 
road  to  subdue  or  prevent  the  dust;  but  of  course  the  effect  is  only 
temporary. 

Recently  crude  petroleum  has  been  employed  on  highways, 
instead  of  water,  to  prevent  dust.  Oil  has  been  used  for  this  purpose 
in  Southern  California  more  than  elsewhere,  primarily  on  account 
of  the  high  grade  of  oil  that  is  available  at  low  cost,  but  also  on 
account  of  the  sandy  soil,  the  semi-arid  climate,  and  the  absence 
of  freezing  weather. 

Oil  when  applied  to, loam  and  clay  roads,  reduces  the  dust,  makes 
the  road-bed  at  least  partially  non-absorbent,  and  gives  a  dark- 
colored  surface  which  is  more  pleasing  to  the  eye  than  the  ordinary 
light,  dusty  soil.  Since  the  road-bed  is  less  absorbent,  it  is  not  so 
easily  worked  into  mud;  and  besides  the  oily  surface  more  readily 
sheds  the  rain  water  into  the  side  ditches.  In  localities  where  there 
is  frequent  thawing  and  freezing  and  also  much  rain,  the  effect  of 
oil  on  loam  and  clay  roads  does  not  last  through  the  succeeding 
winter,  except  in  case  of  an  unusually  dry  winter  and  spring.  In 
comparatively  dry  climates  and  upon  a  sandy  soil,  the  continued 
application  of  suitable  oil  to  the  surface  tends  to  gradually  improve 
the  condition  of  the  road-bed.  However,  whatever  the  character 
of  the  soil,  roads  having  a  considerable  hauling  are  not  materially 
improved  either  temporarily  or  permanently. 


ART.    3]  SURFACE    OILING  135 

236.  PREPARING  THE  SURFACE.     The  surface  should  be  smooth 
and  properly  crowned,  so  as  to  shed  water  into  the  side  ditches. 
The  surface  can  be  properly  prepared  with  a  road  drag  (§  206)  or 
a  scraping  grader  (§  155),  according  to  the  degree  of  roughness  and 
the  dryness  of  the  soil.     If  much  earth  is  moved  in  shaping  the  sur- 
face, the  road  should  be  subjected  to  travel  to  consolidate  the  loose 
earth;   and  if  depressions  appear  these  should  be  filled,  and  be  con- 
solidated by  travel  or  other  means  before  the  oil  is  applied.     The 
expense  incurred  in  securing  a  hard,  smooth,  properly  crowned  sur- 
face will  more  than  be  made  up  by  the  increased  effectiveness  of  the 
oil  treatment.     For  the  best  results  the  upper  2  inches  of  the  road 
should  be  fairly  dry;  but  the  surface  should  be  free  from  dust. 

237.  THE  OIL.     For  a  discussion  of  the  origin  and  character 
of  road  oils,  the  method  of  shipping  them,  and  specifications  for  oils 
suitable  for  the  different  kinds  of  soil,  see  Art.  2  of  Chapter  VIII. 

238.  APPLYING  THE  OIL.     The  oil  should  be  applied  .at  the  rate 
of  one  fourth  to  one  half  gallon  per  square  yard  of  surface.     If  the 
road  has  never  been  oiled,  or  if  more  than  one  season  has  elapsed 
since  a  previous  oiling,  about  a  half  gallon  per  square  yard  will  be 
required.     If  the  road  has  been  oiled  earlier  in    the  season,    one 
fourth  to   one  third   of  a  gallon  per  square   yard  will  usually  be 
satisfactory.     It    is    much    better    to    apply    a    small  amount  of 
oil  twice  each  season  rather  than  to   put   on    the  full    quantity 
in  one  application.     When  too  much  oil  is  applied,  it  is  not  only 
wasted,  but  is  often  disagreeable  to  the  users  of  the  road.     The 
first  time  the  road  is  oiled,  the  best  results  may  be  secured  by  using 
a  thin  product  that  will  penetrate  the  road-bed  to  a  considerable 
distance  and  at  the  same  time  contain  as  much  binding  material  as 
possible.     The  oil  should  be  thin   at  ordinary  atmospheric  tem- 
peratures, and  applied  cold.     If  a  thick  oil  is  used  for  the  first 
application,  it  will  not  penetrate  to  any  considerable  distance,  but 
will  form  a  mat  upon  the  surface;   and  consequently,  if  the  road  is 
not  well  underdrained,  the  accumulation  of  moisture  under  the  mat 
may  cause  the  road  to  dry  out  more  slowly,  and  may  also  cause  the 
mat  to  break  up.     After  the  top  layer  of  the  road  has  been  satu- 
rated, a  heavier  oil  may  then  be  used;  and  it  is  best  applied  when 
hot. 

239.  The  uniform  distribution  of  the  oil  is  one  of  the  essential 
requirements  for  success.     An  ordinary  street  sprinkler  or  a  home- 
made device  attached  to  a  thresher  tank-wagon  may  be  utilized 
for  distributing  the  oil,  although  considerable  care  is  required  to 


136  EARTH    ROADS  [CHAP.    Ill 

secure  the  right  amount  and  a  uniform  distribution.  Much  better 
results  can  be  secured  by  the  use  of  specially  designed  pressure  dis- 
tributor tank  wagons  or  trucks.  There  are  a  number  of  such  wagons 
and  trucks  on  the  market.  Some  of  them  are  equipped  with  a  heat- 
ing device  so  that  hot  oil  may  be  applied  when  required;  and  all 
have  pumps  for  distributing  the  oil  uniformly  and  under  pressure, 
and  some  have  a  device  for  spraying  the  oil  upon  the  road. 

On  earth  roads  the  use  of  the  pressure  distributor  is  useful  mainly 
to  secure  the  proper  quantity;  but  on  a  gravel  or  broken-stone  road 
the  pressure  distributor  is  important,  since  applying  the  oil  with 
force  blows  the  dust  off  the  pebbles  or  stones  and  permits  a  better 
adhesion  of  the  oil.  The  chief  advantage  of  the  spray  is  that  it 
secures  a  more  uniform  distribution. 

The  distributor  should  be  so  regulated  that  the  width  to  be  oiled 
will  be  covered  by  one  or  more  uniform  strips  without  any  over- 
lapping. Any  spots  between  the  strips  that  are  not  covered  by  the 
machine  distributor,  should  be  covered  with  a  hand-pouring  can 
following  immediately  after  the  distributor.  For  the  best  results 
the  oil  should  be  applied  in  two  equal  coats  with  an  interval  of  at 
least  5  or  6  hours  between  them  to  allow  the  first  to  be  absorbed 
before  the  second  is  applied.  Travel  should  be  barred  from  the  road 
for  3  days  after  the  first  coat  is  applied,  to  allow  the  oil  to  be  absorbed. 

Fig.  39  shows  a  pressure  tank  wagon  for  distributing  road  oil. 
This  road  oiling-machine  is  provided  with  an  oil-burning  heater  and 
a  jacket  around  the  tank;  and  also  has  two  pumps  for  applying 
the  oil  under  pressure  and  regulating  the  amount.  Fig.  40  shows 
another  form  of  road  oiler. 

240.  COST  OF  OILING.  The  cost  of  preparing  an  earth  road 
for  oiling  will  vary  greatly,  depending  upon  the  condition  of  the 
surface.  If  the  surface  is  not  already  well  crowned,  the  road 
should  be  treated  with  either  the  road  drag  or  the  scraping  grader. 
However,  such  work  should  not  be  considered  as  part  of  the  cost 
of  oiling;  but  should  be  considered  as  part  of  the  cost  of  construc- 
tion or  of  maintenance,  since  the  road  should  be  properly  crowned 
and  be  kept  so  whether  or  not  it  is  oiled.  Even  though  the  sur- 
face is  properly  maintained,  it  will  probably  be  necessary  to  drag 
the  road  and  otherwise  shape  it  up;  and  this  cost  may  be  $10  to 
$25  per  mile. 

The  price  of  oil  for  earth  roads  varies  from  4  to  8  cents  per 
gallon  (§  560-61).  The  oil  is  usually  applied  at  the  rate  of  J  to  \ 
gallon  per  square  yard,  and  the  oiled  width  is  generally  15  feet;  and 


ART.    3] 


SURFACE   OILING 


137 


FIG.  39. — ROAD  OILING- MACHINE. 


FIG.    40. — MACHINE  APPLYING  ASPHALT  OIL. 


138  EABTH   ROADS  [CHAP.  Ill 

therefore  with  the  smaller  values  above,  the  oil  may  cost  $88  per 
mile,  and  with  the  larger  values  $352. 

The  cost  of  applying  the  oil  will  depend  upon  the  length  of  the 
haul,  the  size  of  the  tank,  and  the  method  of  applying;  and  may 
vary  from  $50  to  $150  per  mile  for  machine  application.  There- 
fore, the  total  cost  of  oiling,  including  only  slight  preparation  of 
the  road  surface,  and  excluding  rent  and  depreciation  of  equipment, 
and  also  excluding  general  administrative  expense,  will  vary  from 
$148  to  $527  per  mile. 


CHAPTER  IV 
SAND  AND  SAND-CLAY  ROADS 

ART.  1.     SAND  ROADS 

243.  As  a  rule  roads  on  pure  sand  are  the  worst  in  existence, 
since  they  are  good  only  when  wet,  and  therefore  are  at  their  worst 
most  of  the  year;   while  in  most  localities  clay  or  loam  roads  are  at 
their  best  most  of  the  time.     If  the  sand  is  fine,  a  dry  sand  road  is 
worse  than  any  muddy  road. 

244.  DRAINAGE.     Roads  on  pure  or  nearly  pure  sand  require 
very  different  treatment  from  those  on  clay  and  loam.     Dampness 
improves  a  sand  road,  while  it  damages  a  clay  or  -loam  road;    and 
therefore  the  preceding  rules  for  the  drainage  of  loam  or  clay  roads 
must  be  reversed  for  sand  roads.     Wet  sand  makes  a  better  road 
than  dry  sand,  and  therefore  draining  a  sand  road  is  useless  and 
possibly  a  damage.     Of  course,  this  is  not  true  of  quicksand,  since 
that  is  improved  by  drainage;  but  there  is  very  little,  if  any,  of  this 
material  in  roads. 

245.  GRADING.     Sand  roads  are  usually  nearly  level  longitudi- 
nally;   and  hence  need  little,  if  any,  grading.     They  should  not  be 
crowned,  since  they  do  not  need  surface  drainage.     The  traveled 
portion  should  be  simply  leveled  off. 

246.  SHADE.     While  shade  harms  a  loam  or  clay  road,  it  im- 
proves a  road  of  sand  or  broken  stone,  since  it  prevents  the  evapo- 
ration of  the  moisture  from  the  road-bed.     Therefore  a  sand  road 
can  be  permanently  improved  by  planting  trees  so  as  to  shade  the 
traveled  way.     They  will  prevent,  in  part,  the  drying  effect  of  the 
winds,  as  well  as  intercept  the  rays  of  the  sun. 

247.  HARDENING   THE   SURFACE.    The  great  disadvantage  of 
pure  sand  as  a  road  material  is  the  freedom  with  which  the  grains 
move  one  on  the  other;  and  therefore  to  improve  a  sand  road  grass 
should  be  encouraged  to  occupy  all  the  space  possible,  since  its  roots 
will  decrease  the  movement  of  the  grains  under  the  tread  of  the 

139 


140  SAND    AND    SAND-CLAY    KOADS  [CHAP.    IV 

hoofs  and  wheels.  It  is  an  advantage  if  low  growing  bushy  vege- 
tation occupies  the  surface  clear  up  to  the  traveled  way — both  for 
the  shade  and  for  the  binding  effect  of  the  roots  and  the  leaves.  The 
leaves  fall  into  the  ruts  and  also  aid  in  binding  the  sand. 

Where  no  other  recourse  is  possible,  it  is  advantageous  to  have 
two  roadways  adjacent  to  each  other,  one  of  which  is  planted  with 
grass  while  the  other  is  in  use.  If  the  traffic  is  not  very  great,  the 
effect  of  the  grass  will  last  for  a  year  or  two  after  the  road  is  again 
used  by  the  wheels.  A  fertilizer  is  sometimes  applied  to  stimulate 
a  growth  of  grass  upon  the  wheelway.  In  some  localities  the  sand 
is  so  fine  that  it  drifts  like  snow,  and  fills  the  partially  hardened 
way,  in  which  case  the  road  is  improved  by  planting  the  roadsides 
with  grass  to  prevent  the  sand  from  being  blown  into  the  road. 

A  road  on  pure  sand  is  improved  temporarily  by  covering  it 
with  a  thin  layer  of  any  vegetable  fiber,  as  decaying  leaves,  straw, 
marsh  hay,  waste  from  sorghum  mills  (bagasse),  fibrous  or  string- 
like  shavings,  etc.  This  fibrous  material  soon  becomes  incorporated 
with  the  sand  and  decreases  its  mobility;  but  the  vegetable  matter 
wears  out  and  decays,  and  consequently  the  effect  is  only  temporary. 
The  length  of  time  such  expedients  will  last  depends  upon  the 
climate  and  the  amount  of  travel. 

248.  In  this  connection  it  is  a  significant  fact  that  the  sand 
shoulders  of  a  broken-stone  road  soon  become  firm  and  hard,  owing 
to  the  infiltration  of  the  fine  dirt  and  stone  dust  washed  from  the  sur- 
face of  the  roadway.  The  fine  particles  of  dust  between  the  grains 
of  sand  act  mechanically  to  decrease  the  mobility  of  the  sand,  and 
to  increase  capillary  attraction  and  diminish  percolation,  which 
in  turn  keeps  the  sand  damp  and  still  further  decreases  its  mobility. 
Apparently,  then,  the  incorporation  of  fine  dust  in  a  sand  road  will 
improve  it;  but  it  will  be  difficult  to  procure  sufficient  dust  for  this 
purpose. 

ART.  2.     SAND-CLAY  ROADS 

250.  A  sand  road  is  best  when  wet,  and  a  clay  road  is  worst  when 
wet;  but  a  road  surface  constructed  of  sand  and  clay  mixed  in  proper 
proportions  possesses  the  good  qualities  of  both  the  sand  and  the  clay, 
and  frequently  is  better  then  either.  Such  roads  are  called  sand-clay 
roads,  and  give  the  best  results  where  the  ground  is  not  subject  to 
deep  freezing.  Materials  suitable  for  the  construction  of  sand-clay 
roads  are  found  in  greater  abundance  in  the  southern  states  than  in 


ART.    2]  SAND-CLAY   ROADS  141 

any  other  portion  of  this  country;  and  consequently  sand-clay 
roads  are  much  more  common  in  that  portion  of  the  country,  although 
sand-clay  roads  have  been  built  to  a  considerable  extent  in  several 
northern  states.  There  are  many  localities,  particularly  in  the 
South,  where  sand-clay  roads  are  the  only  improved  roads  which 
are  economically  possible.  In  many  cases  a  sand-clay  road  gives 
good  service  at  comparatively  low  cost  of  both  construction  and 
maintenance. 

Three  distinct  methods  are  employed  in  constructing  sand-clay 
roads,  viz.:  (1)  a  natural  mixture  of  sand  and  clay  is  placed  on 
top  of  either  a  sand  or  a  clay  road ;  (2)  a  layer  of  sand  is  incorporated 
in  the  road-bed  of  a  clay  road;  or  (3)  a  layer  of  clay  is  added  to  a  sand 
road. 

251.  THE  DESIGN.     The  width  and  thickness  adbpted  will  of 
course  depend  upon  the  travel  and  upon  the  money  available. 
For  a  single  track  the  improved  width  is  usually  10  or  12  feet,  and 
for  a  double  track  14  or  16  feet.     For  a  discussion  of  the  super-eleva- 
tion and  width  on  curves,  see  §  90  and  97,  respectively. 

The  thickness  at  the  center  varies  from  6  to  10  inches,  usually 
6  to  8;  and  at  the  sides  4  to  8,  usually  4  to  6,  but  feathers  out  at  the 
very  edge.  The  crown  should  be  f  to  f  inch  per  foot  of  total  width. 

252.  NATURAL  MIXTURES  OF  SAND  AND  CLAY.    Sometimes  a 
natural  mixture  of  sand  and  clay  is  found  in  such  proportions  as  to 
make  an  excellent  road  surface  for  moderate  travel  under  most  or 
all  weather  conditions. 

The  next  chapter  treats  of  gravel  roads,  and  a  special  case  of  a 
gravel  road  is  one  in  which  a  layer  of  natural  cementing  gravel  is 
added  to  a  clay  or  loam  road;  but  such  a  form  of  construction  will 
not  be  considered  here. 

253.  To  Test  the  Sand-Clay  Mixture.    To  determine  the  probable 
wearing  power  of  natural  mixtures  of  sand  and  clay  proceed  as 
follows: 

1.  Examine  the  existing  road  to  see  if  there  are  any  portions 
that  are  reasonably  good  under  all  weather  conditions,  to  identify 
the  character  of  soil  desired  for  other  portions  of  the  road. 

2.  Observe  out-crops  of  sandy  clay  or  clayey  sand.     The  best 
mixtures  of  sand  and  clay  for  road  building  purposes  will  stand  at 
relatively  steep  slopes,  will  develop  few  surface  cracks  in  drying,  and 
will  appear  dense  and  firm  in  dry  weather. 

3.  Determine  the  percentage  of  clay  and  sand  in  the  mixture. 
To  do  this,  thoroughly  dry  a  sample  of  the  soil,  weigh  it,  place  it  in 


142  SAND   AND   SAND-CLAY   ROADS  (CHAP.    IV 

a  vessel  several  times  larger  than  the  sample,  fill  the  vessel  with  water, 
agitate  the  water,  and  pour  off  the  muddy  water.  Before  pouring 
off  the  water,  allow  it  to  stand  a  minute  or  two,  so  the  sand  may 
settle  and  thus  prevent  its  being  carried  away  with  the  clay.  Repeat 
the  washing  until  the  water  remains  clear;  and  then  dry  the  sand 
and  weigh  it.  Fair  results  may  be  expected  if  the  sand  content  is 
from  50  to  70  per  cent;  but  the  best  results  are  obtained  when 
the  sand  varies  from  60  to  70  per  cent;  or  in  other  words,  for 
the  best  results  about  two  thirds  of  the  mixture  should  be  sand. 
Further,  the  greater  the  proportion  of  coarse  to  fine  sand  th:3 
better.* 

4.  Sometimes  a  natural  mixture  may  be  improved  by  combining 
it  with  another  natural  mixture  or  with  nearly  pure  clay  or  pure 
sand.  A  determination  of  the  sand  and  clay  contents  of  the  natural 
mixture  will  give  some  indication  of  the  element  needed  to  improve 
it;  but  the  only  way  to  determine  it  definitely  is  to  make  several 
trial  proportions  and  test  them.  To  determine  which  of  several 
natural  or  artificial  mixtures  will  probably  give  the  best  results  in 
the  road,  proceed  as  follows:  Mix  the  sample  to  a  stiff  mortar; 
spread  a  small  quantity  of  each  mixture  upon  a  board  or  plate  of 
glass  to  the  thickness  of  about  1  inch;  and  with  some  improvised 
equivalent  of  a  biscuit  cutter,  cut  from  each  mixture  two  samples 
containing  1  to  2  cubic  inches  each.  It  is  essential  to  cut  only  equal 
quantities  from  each  mixture.  Roll  these  samples  between  the  palms 
of  the  hands  into  approximately  true  spheres;  scratch  a  number  on 
each;  and  then  place  them  in  the  sun  to  dry.  When  thoroughly 
dried,  one  sphere  of  each  sample  should  be  tested  dry  and  the  other 
wet.  To  test  a  sphere  dry,  rub  it  lightly  with  the  thumb,  and  if  it 
has  too  much  sand  ii,  will  disintegrate  rapidly;  while  if  it  contains  an 
excess  of  clay,  it  will  speedily  rub  into  dust.  If  it  has  a  suitable 
proportion  of  sand  and  clay,  it  will  simply  become  slightly  glazed, 
and  will  offer  considerable  resistance  to  abrasion.  To  test  the 
spheres  wet,  place  them  in  a  circle  in  a  flat  pan  or  dish,  and  gently 
pour  enough  water  into  the  pan  to  cover  them,  being  careful  not  to 
pour  water  directly  upon  any  sphere.  The  specimens  containing 
too  much  sand  will  break  down  first;  those  having  an  excess  of  clay 
will  usually  disintegrate  next;  and  those  having  the  best  proportions 
will  endure  longest.  These  experiments  will  indicate  the  least 

*  For  a  series  of  complete  sieve  analyses  of  fourteen  samples  of  material  from  sand-clay 
roads  that  had  given  good  service,  and  a  discussion  of  the  relative  merits  of  the  same  sec 
Trans.  Amer.  Soc.  of  Civil  Eng'rs,  Vol.  77  (1914),  p.  1465-75. 


ART.    2] 


SAND-CLAY   ROADS 


143 


desirable  mixtures,  and  will  also  show  what  other  proportions 
should  be  tested.  A  second  test  of  the  most  promising  mixtures 
will  probably  indicate  whether  or  not  any  of  the  samples  are  worthy 
of  a  trial  in  an  experimental  section  of  road. 

254.  By    proceeding   as    described    above    one   may    determine 
within  comparatively  narrow  limits  the  possibility  of  successfully 
using  any  available  mixtures  of  sand  and  clay  for  a  road  surface. 

255.  Construction.     When  a  suitable  mixture  of  sand  and  clay 
is  available,  it  is  only  necessary  to  add  a  layer  of  this  material  to  a 
sand   road  or  to   a  clay  road.     The  road-bed   on  clay  should  be 
underdrained  and  crowned  as  described  in  Art.  1  of  Chapter  III, 
and  the  surface  of  the  sand  road  should  be  prepared  as  stated  in 
Art.  1  of  this  chapter.     The  amount  of  the  initial  crown  will  depend 
upon  the  thickness  of  sand  and  clay  to  be  added. 

The  sand-clay  mixture  is  spread  upon  the  road  to  the  desired 
width  and  thickness  (§251),  and  is  smoothed  with  a  scraping  grader 
(§  155)  or  drag  (§  206).  Travel  may  then  be  admitted;  but  the 
road  should  not  be  considered  finished  until  it  has  been  thoroughly 
soaked  by  rains,  harrowed  to  break  up  lumps,  and  again  shaped 
with  the  scraping  grader  or  drag. 

After  the  road  has  been  in  service  for  a  considerable  time,  if  it 
develops  that  either  clay  or  sand  is  lacking  in  the  surface,  a  thin  layer 
of  that  element  may  be  spread  and  be  incorporated  into  the  road-bed 
with  a  disk  or  toothed  harrow. 

Finally  the  sand-clay  road,  if  properly  built,  will  become  smooth, 
nearly  dustless,  and  resilient;  and  under  moderate  travel  should 
continue  so  without  much,  if  any,  expense  for  maintenance. 


Single  Track  -tO  f 
Double  Truck- 14  ft. 


FIG.  41. — MIXTURE  OF  SAND  AND  CLAY  ON  NATIVE  SUBGRADE. 


256.  SAND  ON  CLAY  SUBGRADE.  The  object  in  this  form  of 
construction  is  to  incorporate  sufficient  sand  with  the  clay  subgrade 
to  obtain  a  mixture  of  sand  and  clay  that  will  fulfill  the  conditions 
stated  in  §  253.  For  the  best  results  the  amount  of  clay  in  the  fin- 
ished road  surface  should  not  much  exceed  the  amount  required  to 


144 


SAND   AND    SAND-CLAY   ROADS 


[CHAP.    IV 


fill  the  voids  in  the  sand.     Ordinarily  about  two  parts  of  sand  to  one 
part  of  clay  gives  satisfactory  results. 


Fio.  42. — CLAY  ON  SAND  STJBQRADE. 

257.  The   Sand.     The  sand  to  be  added  to  a   clay  subgrade 
should  preferably  be  a  coarse-grained  pure  silica  sand.     Any  sand 
containing  any  considerable  percentage  of  mica  is  not  desirable. 

For  the  best  results,  not  less  than  45  per  cent  nor  more  then 
60  per  cent  of  the  sand  should  be  finer  than  that  caught  on  a  stand- 
ard No.  10  sieve,  and  coarser  than  that  caught  on  a  No.  60  sieve; 
and  that  caught  on  No.  20,  40  and  60  sieves  should  be  about  equal 
to  each  other. 

258.  The  Proportions.    The  proportions  of  sand  and  clay  can 
be  determined  approximately  by  finding  the  amount  of  water  that 
can  be  poured  into  a  vessel  full  of  sand.     To  do  this  determine 
the  weight  of  water  required  to  fill  any  suitable  vessel;    and  then 
fill  the  vessel  with  sand.     Next  determine  the  weight  of  water  that 
can  be  poured  into  the  vesselful  of  sand.     For  example,  if  the  pail 
holds  12.4  Ib.  of  water,  and  3.5  Ib.  of  water  can  be  poured  into  the 
pailful  of  sand,  then  the  voids  in  the  sand  are:    3.5  -r-  12.4  =  28 
per  cent.     Therefore  in  the  finished  road  the  proportion  of  clay  should 
be  about  28  per  cent,  and  that  of  sand  about  72  per  cent.     This 
proportion  and  others  differing  slightly  therefrom  should  be  tested 
by  the  method  described  in  paragraph  4  of  §  253. 

However,  since  exact  mathematical  proportions  are  not  possible 
in  incorporating  the  sand  with  the  clay  in  the  road  surface,  mathe- 
matical refinement  in  these  experiments  is  inappropriate. 

259.  The  Thickness.     For  average  rural-road  travel,  the  depth 
of  the  sand-clay  surface  should  be  about  6  to  8  inches  at  the  center 
and  4  to  6  inches  at  the  edges  of  the  traveled  way  feathering  out 
on  the  shoulders;    and  hence  the  thickness  of  the  layer  of  sand 
to  be  added  to  the  road  should  be  roughly  two  thirds  of  the  above 
depth,  the  exact  thickness  depending  upon  the  best  proportion  of 
sand  and  clay  as  determined  by  the  method  described  in  §  253. 


ART.    2]  SAND-CLAY   ROADS  145 

260.  Construction.     The  surface  of  the  clay  road  is  shaped  up  as 
described  for  an  earth  road  (§  129-31),  and  is  then  thoroughly  plowed 
to  a  depth  of  6  or  8  inches  according  to  the  thickness  of  sand  to  be 
added.     Next  the  thickness  of  sand  determined  as  above  is  spread 
upon  the  surface  and  leveled  down  with  the  scraping  grader  or  the 
road  drag.     The  road  is  then  plowed  again,  as  deep  as  possible,  to 
mix  the  clay  and  the  sand;  and  after  this  plowing  the  sand  and  clay 
are  further  mixed  by  harrowing  with  a  disk  harrow,  which  can  be 
done  best  when  the  road  is  wet.     Attempting  to  mix  the  materials 
dry  is  usually  unsuccessful.     After  the  sand  and  clay  are  thoroughly 
mixed,  the  surface  is  smoothed  and  crowned  with  the  blade  grader 
or  the  road  drag,  and  travel  is  admitted  to  complete  the  mixing  and 
to  compact  the  road.     While  the  road  is  new  it  should  be  watched 
carefully,  and  the  surface  should  be  kept  free  from  ruts  and  saucer- 
like  depressions  by  going  over  it  when  necessary  with  the  scraping 
grader  or  the  V  road  leveler.     When  the  road  has  begun  to  solidify, 
the  road  drag  is  not  very  effective — at  least  neither  the  split-log  nor 
the  plank  drag.     It  will  likely  be  necessary  to  add  clay  in  spots  where 
the  road  surface  is  loose,  and  sand  where  it  is  sticky.     Probably 
the  road  will  not  arrive  at  its  best  condition  for  a  year  or  two  or  at 
least  not  until  after  several  long-continued  wet  spells  during  which 
the  travel  will  thoroughly  consolidate  the  road. 

261.  CLAY  ON  SAND  SUBGRADE.      The  clay  is  added  to  serve 
as  a  binder  to  hold  the  grains  of  sand  together.     For  the  form  of 
construction  under  consideration  here,   clay  is  the  only  material 
available  for  permanent  effect.     For  the  proportions  of  clay  and  sand 
to  be  attained,  see  §  253. 

262.  The  Clay.     Clay  varies  more  in  its  suitability  for  road  build- 
ing purposes  than  sand;   and  it  is  difficult  to  determine  in  advance 
the  result  of  a  service  test  in  the  road. 

All  clays  contain  more  or  less  sand,  and  as  the  clay  is  to  serve 
as  a  binder  for  the  sand,  the  less  sand  the  clay  contains,  that  is  the 
more  nearly  it  is  a  pure  clay,  the  better;  and  hence  loam,  which 
is  a  mixture  of  clay,  sand,  and  vegetable  matter,  is  not  the  most  suit- 
able for  this  purpose. 

The  less  the  clay  is  affected  by  the  presence  of  water  the  better. 
Clays  are  known  as  slaking  and  non-slaking.  The  former  absorb 
water  freely  and  slake  or  fall  to  pieces  when  put  into  water,  some- 
what like  a  piece  of  quick-lime;  and  are  deficient  in  binding  power, 
and  hence  are  undesirable  as  a  binder  for  a  sand-clay  road. 

To  compare  the  slaking  qualities  of  several  clays,  make  small 


146  SAND    AND    SAND-CLAY    ROADS  [CHAP.    IV 

bails  of  each  of  approximately  the  same  size  by  rolling  between 
the  hands,  leave  the  balls  in  the  sun  or  put  them  into  an  oven  until 
they  are  well  dried  out,  and  then  place  them  under  water.  The 
ball  which  holds  its  shape  longest  has  the  highest  resistance  to 
slaking,  and  contains  the  clay  most  suitable  for  use  as  binder  in  a 
sand-clay  road.  To  make  a  fairly  trustworthy  comparison  the 
samples  should  contain  substantially  the  same  proportion  of  sand; 
and  therefore  the  percentage  of  cand  in  each  sample  of  clay  should 
be  determined  (see  §  253)  before  beginning  the  above  test,  and  sand 
should  be  added  so  as  to  give  all  samples  the  same  proportion  of 
sand. 

The  above  method  of  testing  may  be  employed  also  to  determine 
the  slaking  qualities  of  the  clay  when  mixed  with  different  propor- 
tions of  sand,  and  therefore  may  afford  valuable  information  in  fix- 
ing the  proportions  of  clay  and  sand  to  be  used  in  the  road  surface. 

Other  things  being  equal,  the  clay  which  shrinks  least  in  drying 
out  is  best  suited  for  use  in  a  road  surface.  The  relative  shrinkage 
may  be  determined  by  observing  the  balls  used  in  the  test  above, 
while  they  are  being  dried  out  before  being  immersed. 

263,  The  Construction.  The  road-bed  should  be  provided  with 
side  ditches  and  be  graded  as  described  for  earth  roads  (§  129-31), 
except  that  the  surface  should  have  but  little,  if  any,  crown.  Then 
spread  a  layer  of  clay  of  such  thickness  that  when  thoroughly  mixed 
with  the  sand  of  the  read-bed,  the  mixture  will  have  the  required 
depth,— 6  or  8  inches  as  the  case  may  be.  The  thickness  of  the  loose 
clay  required  will  usually  be  a  little  greater  than  one  third  of  the 
ultimate  thickness  of  the  combined  sand  and  clay.  The  exact  pro- 
portion of  clay  to  secure  this  condition  is  to  be  determined  as  de- 
scribed in  §  253. 

After  the  clay  has  been  spread  and  leveled  off  with  a  road  drag, 
the  clay  and  the  sand  are  to  be  thoroughly  mixed  by  successively 
plowing  and  then  harrowing  with  a  disk  harrow.  Roughly  the 
plowing  should  extend  into  the  sand  to  twice  the  depth  of  the  layer 
of  clay  added.  It  is  better  at  first  to  have  too  little  sand  rather  than 
too  much,  for  it  is  easier  to  correct  the  proportions  by  adding  sand 
from  the  subgrade  then  by  hauling  clay  from  a  distance.  When 
the  sand  and  clay  have  been  thoroughly  mixed  in  the  correct  propor- 
tion, the  road  should  be  properly  shaped  by  the  use  of  a  scraping 
grader  or  road  drag,  and  then  travel  may  be  permitted.  After  the 
first  soaking  rain,  plow  and  harrow  the  surface  again  until  the  sur- 
facing material  practically  becomes  mud,  after  which  shape  up 


ART.    2] 


SAND-CLAY   ROADS 


147 


the  surface  and  keep  it  in  shape  by  repeated  dragging  until  it  has 
dried  out  and  is  thoroughly  compacted.  At  this  stage  the  road 
roller  should  not  be  used,  since  it  will  harden  only  the  surface  and 
prevent  the  travel  from  consolidating  the  mixture  from  top  to  bottom. 
The  crust  formed  by  the  roller  will  carry  the  travel  until  the  first 
wet  spell,  and  then  it  will  cut  through  and  the  road  will  break  up; 
and  nothing  permanent  will  have  been  gained  by  the  use  of  the 
roller. 

The  road  should  be  watched  carefully  for  several  months,  and  ruts 
and  other  depressions  should  be  filled  by  the  use  of  the  steel  road 
drag  or  the  scraping  grader.  Any  deficiencies  in  sand  or  clay  that 
are  revealed  should  be  corrected  by  adding  the  lacking  ingredient. 

264.  COST.     In  the  Southern  states  the  cost  of  constructing  a 


TABLE  22 

COST  OF  SAND  CLAY  ROADS* 
Exclusive  of  grading  and  materials 


Items. 

Mixture  of 
Clay  and 
Sand. 

Sand  on  Clay 
Subgrade. 

Clay  on  Sand  Subgrade. 

Brook- 
ville, 
Fla. 

Gray 
Head, 
Miss. 

Mos- 
cow, 
Miss. 

Jack- 
son, 
N.  C. 

Pear- 
shall, 
Tex. 

San 
An- 
tonio, 
Tex. 

Tar- 
boro, 
N.  C. 

Sayre, 
Okla. 

Length  surfaced,  miles  

Width  graded,  feet: 
Cuts  
Fills 

0.45 

28 
20 

16     . 

660 
0.75 
3 

830 
0.50 

7 

$0.175 
.50 

.031 

'.'425 
.374 

.019 
.0035 

.0035 
.0157 

2.19 

22 
22 

16 

1  700 
0.125 
6 

2  300 
0.10 
6 

$0.20 
.50 

.010 
.223 
.344 

.003 
.010 

.004 
.006 

0.78 

30 
30 

17 

1524 
1.5 

7 

0.45 

30 
30 

14 

890 
1.00 

7 

0.79 

26 
26 

15 

1.00 

40 
26 

16 

2.50 

22 
22 

18 

0.75 

28 
28 

14 

Width  surfaced,  feet  

Materials  : 
Sand,  cu.  yd  
Distance  hauled,  miles.  .  . 
Depth  applied,  inches.  .  .  . 
Clay,  cu.  yd  

1556 
0.19 
8 

$0.12 
.30 

.0064 
.091 
'.9ii 

.032 
.0017 

.0005 
.003 

1815 
0.057 

7 

$0.15 
.345 

.0088 
.072 

1  .286 
.0069 
.0015 

2815 

1  383 
1.42 
8 

$0.16 
.30 

.0096 
.134 
";567* 

.072 
.007 

Distance  hauled,  miles.  .  .  . 
Depth  applied,  inches.  .  .  . 
Wages  per  hour: 

$0.10 
.25 

.003 
"!49 

.007 
.006 

$0.125 
.35 

.002 

0.29 

.84 

.077 

.002 
.003 

6 

$0.08 
.24 

.0018 
.005 

"  !  255 

.0178 
.0024 

.0022 
.0007 

Teams  

Cost: 
Subgrade,  per  sq.  yd  
Stripping    surfacing    ma- 
terial, per  sq.  yd  
Hauling  sand,  per  sq.  yd  . 
Hauling  clay,  per  sq.  yd.  . 
Spreading     material,    per 
sq.  yd  
Mixing  sand  jand  clay,  per 
sq.  yd  
Final  shaping,  per  sq.  yd  . 
General  expense,  per  sq.yd  . 

Total  cost,  per  sq.yd. 

0.198 

0.082 

0.105 

0.233 

.121 

.089 

.036 

.187 

*  Constructed    by   U.  S.  Office    of    Public  Roads   and   Rural   Engineering,  Bui.   No.  463 
(1917),  p.  45. 


148  SAND   AND    SAND-CLAY   ROADS  [CHAP.    IV 

16-foot  sand-clay  surface,  exclusive  of  grading,  usually  ranges  between 
$500  and  $1500  per  mile  and  nearly  proportionally  for  other  widths. 
For  details  of  the  cost  of  some  such  roads,  see  Table  22. 

In  Michigan,  which  state  seems  to  have  more  sand-clay  roads 
than  any  other  state  except  Georgia,  the  cost  of  a  well-built 
9-foot  sand-clay  road  consisting  of  6  inches  of  clay  on  a  sand  sub- 
grade  ranges  from  $1,000  to  $1,800  per  mile. 

265.  MAINTENANCE.    The  sand-clay  road  is  really  a  superior 
type  of  earth  road;  and  therefore  all  that  has  been  said  in  Chapter 
III  concerning  the  maintenance  of  earth  roads  applies  also  the  sand- 
clay  surfaces.     As  in  the  case  of  ordinary  earth  roads,  economical 
maintenance  depends  largely  upon  proper  original  construction.     The 
use  of  too  fine  sand  and  the  insufficient  mixing  of  the  sand  and  clay 
are  the  common  defects  of  construction  that  should  be  remedied  by 
proper  maintenance. 

The  surface  should  be  kept  smooth  and  properly  crowned  by  the 
use  of  the  road  drag,  particularly  after  severe  or  long-continued 
wet  spells.  If  a  hole  forms  because  of  an  excess  of  clay,  it  should 
be  filled  mainly  or  entirely  with  coarse  sand,  after  first  loosening  the 
material  in  the  bottom  of  the  hole  with  a  pick  so  that  the  new  mate- 
rial will  bond  with  the  old.  Sometimes  the  fine  sand  washes  to  the 
side  of  the  road  and  leaves  an  excess  of  clay,  in  which  case  a  thin 
layer  of  new  coarse  sand  should  be  spread  upon  the  surface  when  it 
is  soft  or  at  least  wet.  The  sand  that  has  washed  out  should  never 
be  used  again. 

If  the  whole  surface  has  a  tendency  to  rut  and  form  holes  in 
wet  weather,  it  usually  means  that  too  much  clay  has  been  used  in 
the  construction  of  the  road;  and  it  may  then  be  necessary  to  cover 
the  entire  surface  with  a  layer  of  sand  and  harrow  it  in.  If,  on  the 
other  hand,  the  surface  is  too  sandy,  clay  must  be  added  in  the  same 
way. 

If  lack  of  proper  dragging  has  allowed  the  road  to  become  badly 
worn,  then  it  must  be  plowed  up  with  a  rooter  plow,  after  which  it 
should  be  harrowed,  preferably  with  a  disk  harrow,  and  be  re-shaped 
with  a  grader. 

266.  The  ability  of  a  sand-clay  road  to  carry  travel,  particularly 
motor-driven  vehicles,  is  indicated  roughly  by  Table  23,  page  149. 

267.  Apparently,  in  the  Southern  States,  representative  sand-clay 
roads  are  maintained  in  reasonably  good  condition  at  an  expense  of 
$5  to  $10  per  year  per  mile,  the  chief  or  sole  expense  being  for 
dragging. 


ART.    2] 


SAND-CLAY    ROADS 


149 


TABLE  23 
APPROXIMATE  TRAVEL  ON  SAND-CLAY  ROADS  IN  GEORGIA 


Average  Numbers  of  Vehicles  both  Ways, 
per  Day. 


lype  01  vemcie. 

e 

c8 

1-5 

ja 

£ 

si 

B 

<: 

& 

§ 

•-) 

>> 
3 
>-> 

M 

3 
•< 

"ft  . 
£ 

"S 

o 

>' 

0 

fc 

1 

Horse-drawn: 
Buggies            .                     .                 

20 
60 
60 
4 

30 
90 
90 
6 

35 
100 
120 
6 

25 
75 
75 
4 

20 
60 
60 
4 

20 
60 
60 
4 

20 
60 
60 
4 

25 
70 
70 
5 

40 
120 
120 
8 

40 
120 
120 
8 

25 
75 
75 
4 

20 
40 
40 
4 

1-horse  wagon  (net  load  750  Ib.)  

2-horse  wagon  (net  load  1,500  Ib.)  
4-horse  wagon  (net  load  3  000  Ib  ) 

Total 

144 

.10 
6 
2 

2 

216 

10 
6 
2 
2 

261 

15 
8 
4 
3 

179 

20 
10 
6 

4 

144 

20 
10 
6 
4 

144 

30 
15 

8 
2 

144 

30 
15 
8 
2 

170 

20 
10 
8 
2 

288 

30 
15 
8 
4 

288 

30 
15 
8 
4 

179 

20 
10 
6 
2 

104 

10 

6 
2 
2 

Motor-driven  : 
2-passenger  automobile  . 

6-passenger  automobile  

Trucks  (net  load  1,500  Ib  ) 

Total    

20 
12 

20 

8 

30 
10 

40 
18 

40 
22 

55 
28 

55 

28 

40 
19 

57 
14 

57 
14 

38 
26 

20 
16 

Per  cent  motor-driven  

t  Trans.  Amer.  Soc.  of  Civil  Eng'rs,  Vol.  77  (1914),  p.  1492. 


CHAPTER  V 
GRAVEL  ROADS 

270.  Gravel  may  be  defined  as  a  mass  of  small,  more  or  less 
rounded  fragments  of  stone  which  have  been  broken  out  and  shaped 
by  the  action  of  water  or  ice.     When  properly  used  gravel  makes 
an  excellent  road  surface;   and  is  much  used  on  account  of  its  wide 
distribution,  the  ease  with  which  it  can  be  applied,  the  good  results 
obtainable  under  widely  varying  conditions  of  soil,  climate  and  travel, 
and  the  low  cost  of  construction  and  maintenance  under  a  moderate 
amount  of  travel.    Many  gravel  roads  have  been  poorly  constructed 
or  inadequately  maintained,  or  over-burdened  with  travel;    and  as 
a  consequence  many  people  believe  the  building  of  gravel  roads  a 
waste  of  money  under  any  condition.     Nevertheless  good  gravel 
roads  have  a  large  field  of  usefulness.     Gravel  roads   constitute 
about  one  third  of  the  total  mileage  of  improved  roads  in  the  United 
States;  and  in  1914  constituted  41  per  cent  of  the  state-aid  roads. 

A  gravel  surface  is  most  suitable  for.  country  highways  not  having 
exceedingly  heavy  traffic,  for  unfrequented  streets  in  villages  and 
small  cities,  and  for  park  roads. 

ART.  1.    THE  GRAVEL 

271.  REQUISITES  FOR  ROAD   GRAVEL.    To  be  suitable  for 

road-building  purposes,  gravel  should  fulfill  the  following  conditions: 
1.  The  fragments  should  be  so  hard  and  tough  that  they  will  not  be 
easily  ground  into  dust  by  the  impact  of  wheels  and  hoofs.  2.  The 
pebbles  should  be  of  different  sizes,  each  in  the  proper  propor- 
tion. 3.  There  should  be  intermixed  with  the  coarser  particles 
some  material  which  will  cement  and  bind  the  whole  into  a  solid 
mass. 

272.  Durability.     From  the  nature  of  its  origin,  it  is  apparent 
that  gravel  may  differ  widely  in  the  nature  of  the  stones  composing 
it.     Not  only  do  different  gravels  differ  from  each  other,  but  any 

150 


ART.    1]  THE   GRAVEL  151 

particular  gravel  may  be  composed  of  fragments  of  a  variety  of 
rocks.  Having  been  transported  a  considerable  distance  by  water 
and  ice,  gravel  is  usually  fairly  durable,  since  the  softer  and  more 
friable  fragments  have  been  worn  away.  In  many  parts  of  the 
country  the  rocky  fragments  transported  by  water  and  ice  are  more 
durable  than  any  of  the  native  rocks.* 

273.  Sizes.     If  the  pebbles  are  too  large,  the  road  will  not  be 
homogeneous,  and  the  large  stones  will  work  to  the  surface  under  the 
action  of  traffic  and  frost;    but,  on  the  other  hand,  if  the  pebbles 
are  too  small,  the  gravel  will  partake  too  much  of  the  character  of 
sand,  and  will  be  difficult  to  bind  properly.     The  best  results  are 
obtained  when  the  largest  pebbles  are  not  more  than  f  to  1  inch,  or 
at  most  If  inches,  in  greatest  dimension.     With  stones  larger  than 
1  inch,  it  is  difficult  to  keep  the  surface  from  breaking  up  when  dry. 
Small  gravel  makes  a  pleasanter  road  to  ride  upon  and  one  that  is 
easier  to  keep  in  order.     If  stones  larger  than  Ij  or  2  inches  are 
present  in  the  gravel,  they  may  be  screened  out  and  used  in  the 
foundation  (§303). 

It  is  desirable  that  the  several  sizes  should  be  so  proportioned 
that  the  smaller  ones  are  just  sufficient  to  fill  the  interstices  between 
the  larger  ones,  since  then  less  binder  is  required.  The  binder  is 
usually  the  least  durable  ingredient,  and  hence  the  less  there  is  of  it 
the  better.  Gravel  can  often  be  improved  by  screening — either  co 
remove  an  undesirable  size  or  to  separate  it  into  several  sizes 
afterward  to  be  combined  in  new  proportions.  The  proper  pro- 
portion depends  upon  the  nature  of  the  gravel — whether  the  binding 
material  is  already  present  in  the  form  of  dust,  or  whether  some  of 
the  pebbles  must  be  crushed  to  produce  the  binder. 

274.  Binder.    The   most   important   requisite   for   good    road- 
building  gravel  is  that  it  shall  bind  or  pack  well.     If  it  does  not  pack 
well,  the  wheels  will  sink  into  the  gravel  and  increase  the  tractive 
resistance,  and  the  rain  water  will  penetrate  the  road-bed  and  soften 
it.     To  bind  well,  the  several  fragments  should  be  in  contact  with 
one  another  at  as  many  points  as  possible,  in  order  that  they  may 
be  firmly  supported,  and  that  friction  may  act  to  the  best  advantage 
to  resist  displacement.     To  secure  contact  at  every  point,  all  the 
interstices  between  the  fragments  should  be  filled — those  between 
the  large  pebbles,   with  small  pebbles;    those  between  the  small 
pebbles,  with  sand  grains;    and,  finally,  those  between  the  sand 

*  For  a  discussion  of  the  merits  of  the  principal  stones  for  road -building  purposes,  see  Art.  1. 
Chapter  VI. 


152  GRAVEL   ROADS  [CHAP.    V 

grains,  with  some  finer  material,  called  a  binder.  The  binding 
material  must  be  very  finely  divided,  so  that  it  can  be  worked  into 
the  smallest  interstices;  and  for  this  reason,  it  is  the  least  durable 
part  of  the  gravel,  being  easily  washed  out  or  blown  away.  For 
the  best  results,  then,  the  sizes  of  the  coarser  particles  should  be  so 
adjusted  as  to  require  a  minimum  amount  of  binder. 

The  binding  material  may  consist  of  clay,  loam,  silica,  stone  dust, 
iron  oxide,  etc.,  or  some  other  ingredient  which  will  crush  under  traffic 
and  furnish  a  fine  dust. 

Clay  is  by  far  the  most  common  binding  material;  but  the  only 
recommendations  for  it  are  (1)  that  it  is  easily  reduced  to  an  im- 
palpable powder  by  the  action  of  wheels  or  by  water,  and  (2)  that 
it  is  often  found  already  mixed  with  the  gravel,  and  (3)  that  if  it 
must  be  artificially  mixed,  it  is  plentiful  and  cheap.  Clay  is  an  un- 
desirable binder,  since  its  binding  action  depends  in  a  large  measure 
upon  the  state  of  the  weather.  During  a  rainy  period  it  absorbs  water 
and  loses  its  binding  power,  and  the  road  becomes  soft  and  muddy; 
while  in  dry  weather  it  contracts  and  cracks,  thus  releasing  the 
pebbles  and  giving  a  loose  surface.  Clay  is  also  very  susceptible 
to  the  action  of  frost;  and  consequently  when  the  frost  is  going  out, 
a  gravel  road  with  a  clay  binder  ruts  up  badly  and  frequently  breaks 
entirely  through.  When  the  weather  is  neither  too  damp  nor  too 
dry,  a  gravel  road  with  clay  binder  is  very  satisfactory.  The  clay 
should  be  no  more  than  enough  to  fill  the  voids  in  the  pebbles  and 
sand,  and  for  a  good  road-gravel  should  not  exceed  15  to  20  per  cent 
of  the  mass.  Not  infrequently  much  greater  quantities  of  clay  are 
present.  This  surplus  may  sometimes  be  removed  by  screening; 
but  often  it  can  be  removed  only  by  washing — a  process  which  is 
usually  so  expensive  as  to  be  prohibitive. 

Loam  is  chiefly  clay  mixed  with  sand  and  a  little  vegetable 
matter,  lime,  etc.;  and  as  a  binding  material  has  all  the  charac- 
teristics of  clay. 

A  very  finely  divided  silica,  easily  mistaken  for  clay,  is  occa- 
sionally present  in  gravel,  and  makes  an  excellent  binding  material. 

Iron  oxide  is  frequently  found  as  a  coating  on  the  pebbles  in 
such  quantities  as  to  cement  them  firmly  together.  These  ferru- 
ginous gravels  when  broken  up  and  put  upon  a  road,  will  again 
unite — often  more  firmly  than  originally,  because  of  the  greater 
pressure — and  form  a  smooth  hard  surface,  impervious  to  water. 
They  are  much  used  in  road  building,  gravel  from  Shark  River, 
N.  J.,— much  used  around  New  York  City— and  that  from  the  Ohio 


ART.    1]  THE   GRAVEL  153 

river  near  Paducah,  Ky., — largely  used  in  the  neighboring  states — 
being  examples. 

275.  Comparatively   coarse    gravel    frequently    contains   some 
other  ingredients,  as,  for  example,  fragments  of  limestone  or  shale, 
which  under  the  action  of  traffic  and  the  weather  reduce  to  a  powder 
and  form  a  good  binding  material.     Sometimes  gravel  contains  bits  of 
iron-stone  (clay  cemented  with  iron  oxide)  in  the  form  of  thin  flat 
chips  which  break  and  crush  easily  under  the  wheels,  and  if  present 
in  any  quantity  make  a  most  excellent  binding  material. 

276.  The  binding  action  referred  to  in  the  preceding  discussion 
is  mechanical;   and  we  come  now  to  the  consideration  of  an  action 
not  yet  well  understood,  but  which  for  the  present  at  least  will  be 
called  chemical  action.     Experiments  seem  to  prove  that  if  fine 
powder  of  certain  stones  is  wetted  with  water  and  subjected  to 
compression,    a   true    chemical   cementation   takes   place.    Conse- 
quently some  stones  when  broken  into  small  fragments,  wetted  and 
traversed  by  heavy  wheels  or  by  a  road-roller  will  be  cemented 
together  to  a  considerable  degree.     This  cementation  is  due  to  the 
fact!  that  the  friction  of  one  small  piece  of  stone  upon  another  pro- 
duces a  very  fine  powder  at  the  point  of  contact,  which  when  wetted 
and  compressed,   forms  a  weak  cement.     Owing  to  the  rounded 
surfaces  of  water-worn  pebbles,  this  cementing  action  is  much  less 
with  gravel  than  with  rough  angular  fragments  of  broken  stone; 
but  with  gravel  composed  of  undecayed  rocky  fragments  this  action 
takes  place  to  a  considerable  degree.    As  a  rule,  pebbles  of  bluish 
color  will  thus  cement  together,  while  reddish  or  brown  ones  will 
not,  which  accounts  in  part  at  least  for  the  well  known  superiority 
of  blue  gravel  for  road  purposes.     Trap  rock  possesses  the  property 
of  cementation  in  a  high  degree,  and  hence  trap  gravel  is  a  very 
excellent  road-building  material.     Limestone  possesses  a  fair  de- 
gree of  cementation,  but  is  too  soft  to  wear  well.     Quartz  wears 
well  but  produces  little  or  no  dust  for  cementation,  and  besides  its 
surfaces  are  so  smooth  and  hard  that  the  binder  has  but  little  effect; 
and  therefore  it  rarely  happens  that  a  gravel  of  which  more  than 
one  half  of  its  bulk  is  white  quartz  pebbles  proves  to  be  a  good  road 
gravel. 

The  cementation  of  rocky  fragments  is  much  more  important  in 
a  water-bound  macadam  road  than  in  a  gravel  one,  and  therefore 
the  subject  will  be  more  fully  considered  in  Chapter  VI. 

277.  The  binding  elements  heretofore  discussed  exist  naturally 
in  the  gravel;   but  gravels  are  often  found  that  do  not  contain  any 


154  GRAVEL   ROADS  [CHAP.    V 

binding  material,  and  in  such  cases  it  is  necessary  to  add  some 
cementing  material. 

Clay,  shale,  hard-pan,  marl,  loam,  etc.,  are  often  used  for  this 
purpose,  chiefly  because  they  are  so  plentiful  and  easily  applied; 
but  none  of  them  are  suitable  for  the  purpose,  as  they  all  have  the 
characteristics  of  a  clay  binder  (see  §  274).  With  any  of  them,  it 
is  difficult  to  keep  the  gravel  from  breaking  up — particularly  under 
heavy  traffic. 

In  some  localities  a  poor  iron  ore  is  found,  which,  when  mixed 
with  gravel,  makes  an  excellent  binder  and  gives  a  smooth  hard 
road  surface.  Bog  iron-ore,  which  occurs  in  marshes,  is  usually 
very  good  for  this  purpose. 

The  fine  dust  from  a  stone  crusher,  when  mixed  with  gravel,  will 
bind  it  together;  but  it  is  seldom  feasible  to  use  stone  dust  on 
account  of  the  expense.  When  this  method  of  binding  gravel  is 
resorted  to,  the  construction  partakes  of  the  character  of  a  water- 
bound  macadam  road — a  subject  foreign  to  this  chapter  (see  Chapter 
VI).  The  chief  difference  between  a  gravel  and  a  crushed-stone 
road  is  in  the  thoroughness  of  the  binding.  The  binding  of  a  gravel 
road  is  due  chiefly,  and  usually  solely,  to  the  mechanical  action  of 
the  binder;  while  the  binding  of  the  broken  stone  is  due  to  both  the 
mechanical  and  the  chemical  action  of  the  binder,  and  both  are 
stronger  with  rough  angular  fragments  of  broken  stone  than  with 
water-worn  pebbles. 

278.  DISTRIBUTION  OF  GRAVEL.  The  gravel  beds  of  the 
glacial  drift  furnish  excellent  road-making  materials.  The  glacial 
ice  sheet,  often  a  mile  or  more  thick,  covered  New  England  and 
Canada  and  all  of  the  United  States  north  of  an  irregular  line  start- 
ing on  the  Atlantic  Coast  a  little  south  of  New  York  City  and  run- 
ning thence  successively  to  the  southwest  corner  of  the  State  of  New 
York,  to  Cincinnati,  to  a  point  a  little  north  of  the  mouth  of  the 
Ohio  river,  to  the  mouth  of  the  Missouri  river,  to  Topeka,  Kansas, 
thence  north  and  west  a  little  west  and  south  of  the  Missouri  river 
to  the  head  waters  of  that  stream,  and  thence  west  to  the  Pacific 
ocean.  All  of  the  area  north  of  the  above  described  line  was  cov- 
ered with  the  ice  sheet  except  small  portions  of  southeastern  Minne- 
sota, northeastern  Iowa,  northwestern  Illinois,  and  a  considerable 
portion  of  southwestern  Wisconsin.  As  this  ice  sheet  crept  to  the 
southward,  it  rent  great  quantities  of  stone  from  the  bed  rocks;  and 
these  materials  were  borne  southward,  either  in  the  slow-moving 
ice  or  hurried  along  by  the  violent  currents  of  water  which  swept 


ART.    1]  THE    GRAVEL  155 

forward  to  the  margin  of  the  ice  field.  Thus  impelled  the  under- 
ice  streams  were  able  to  bear  toward  the  margin  of  the  glacier  great 
quantities  of  stone.  The  original  range  of  the  glacial  gravels  has 
been  greatly  extended  here  and  there  by  the  streams,  which,  flowing 
southward  beyond  the  drift  belt,  have  often  carried  quantities  of 
the  hard  detritus  for  many  miles  beyond  the  limits  of  the  ice- 
field. 

Unfortunately  the  glacial  gravel  deposits  have  not  been  studied 
from  the  point  of  view  of  the  road-maker.  However,  it  is  known 
that  east  of  the  Hudson  river  the  glacial  supply  of  road  gravels  is 
only  here  and  there  of  economic  importance,  for  in  most  of  that  field 
the  glacial  waste  lies  on  native  rocks  which  are  suitable  for  road- 
making;  and  that  from  the  Hudson  to  the  Mississippi,  the  glacial 
deposits  of  bowlders  and  gravel  afford  better  road-building  mate- 
rials than  any  of  the  native  rocks.  Glacial  gravels  exist  in  consid- 
erable quantities  in  western  Pennsylvania,  in  the  greater  part  of 
Ohio,  in  northern  Indiana,  and  in  northern  Illinois,  and  to  some 
extent  in  several  of  the  states  of  the  Northwest. 

279.  South  of  the  glacial  district,  the  rocks  exposed  to  the 
weather  have  decayed  by  a  process  of  leaching,  which  in  many  cases 
has  removed  strata  hundreds  of  feet  thick.  The  rocky  portion  is 
removed  in  proportion  to  its  solubility;  and,  as  a  result,  there  are 
often  left  concretions  of  cherty  matter  which  were  originally  con- 
tained in  beds  of  limestone.  This  cherty  residuum  of  flinty  mate- 
rial generally  lies  in  a  comparatively  thin  sheet  of  fragments  min- 
gled with  sand  and  clay;  but  occasionally  it  is  found  in  deposits 
from  which  the  clay  and  sand  have  been  removed  by  recent  or 
ancient  streams,  leaving  the  material  well  suited  for  spreading  upon 
a  road.  Sometimes  this  cherty  residuum  is  found  in  layers  of 
fragments  many  feet  thick,  and  is  valuable  for  road-building  in  a 
locality  where  more  suitable  material  is  scarce.  The  presence  of 
chert  is  often  revealed  by  the  gullies  in  the  plowed  fields  and  along  the 
streams.  In  some  localities  very  good  roadways  are  constructed 
simply  by  shoveling  these  fragments  from  the  stream  beds  and 
depositing  them  on  the  road.* 

This  cherty  deposit  is  a  valuable  road  material  in  the  southern 
portion  of  the  Appalachian  mountains,  and  along  the  Ozark  foot- 
hills in  southern  Illinois  (particularly  in  Alexander  and  Union 
counties),  in  southern  Missouri,  and  in  northern  Arkansas.  Chert 
is  found  in  some  of  the  states  of  the  Northwest  where  the  glacial 

*  For  a  discussion  of  chert  as  a  road-building  material,  see  §  295. 


156  GRAVEL   ROADS  [CHAP.    V 

erosion  was  small,  so  that  the  rocks  that  had  decayed  before  the 
glacial  time  were  not  entirely  removed.  In  southwestern  Arkan- 
sas the  gravels  consist  of  fragments  of  novaculite  or  razor  stone — a 
material  of  nearly  the  same  geological  origin  and  physical  character- 
istics as  chert.  In  many  places  in  that  state  the  novaculite  gravels 
form  extensive  beds,  20  or  more  feet  thick.  At  the  southern 
extremity  of  the  Appalachian  mountain  system  is  a  wide-spread 
deposit  of  gravel,  termed  the  La  Fayette  formation,  whose  geologi- 
cal origin  is  not  determined.  This  deposit  often  attains  a  thickness 
of  40  to  50  feet,  and  is  a  valuable  source  of  road-building  material. 

280.  If  gravel  be  defined  as  material  prepared  by  nature  ready 
to  be  laid  upon  the  road,  then  a  few  words  are  in  place  here  concern- 
ing  iron   ore.     In   some   localities   there   are   low-grade   iron   ores 
which,  owing  to  the  admixture  of  various  impurities,  are  unfit  for 
use  in  making  iron,  but  may  be  valuable  for  road  building.     These 
low-grade   ores   are   widely   distributed;     and   generally   wherever 
limestone  occurs  below  a  considerable  thickness  of  sandstone,  the 
upper  portion  of  the  limy  layer  will  be  found  to  contain  iron,  and 
will  probably  be  a  fair  road  material.     A  lean  iron  ore  is  frequently 
found  in  marshes;    and  this  variety,  known  as  bog  ore,  usually 
makes  excellent  roads,  since  it  crushes  readily  and  gives  a  smooth 
hard  surface. 

281.  Exploring   for    Gravel.     In    searching    for    gravel    in    the 
glaciated  district,  the  following  suggestions  by  Professor  Shaler  * 
will  be  useful: 

"  In  the  process  of  retreat  of  the  ice,  the  deposits  which  it  left 
were  accumulated  under  several  quite  diverse  conditions.  One  of 
these  produced  the  till,  or  commingled  coarse  and  fine  materials, 
which  had  been  churned  up  into  the  ice  during  the  time  of  its 
motion,  and  came  down,  when  the  melting  occurred,  as  a  broad, 
irregularly  disposed  sheet  which,  with  rare  exceptions,  is  to  be 
found  in  all  parts  of  the  glaciated  district,  save  where  it  has  been 
swept  away  by  streams. 

"  Again,  from  time  to  time  during  the  closing  stages  of  the  ice 
age,  the  prevailing  steadfast  retreat  of  the  ice  was  interrupted  by 
pauses  or  re-advances.  In  these  stages  there  was  formed  along  the 
margin  of  the  ice-field  what  is  called  a  frontal  moraine,  composed  of 
debris  shoved  forward  by  the  glacier  or  melted  out  of  it  along  its 
front.  These  moraines  are  in  most  cases  traceable,  where  they 
have  not  been  washed  away  or  buried  beneath  later  accumulations, 

*  American  Highways,  N.  S.  Shaler,  Professor  of  Geology,  Harvard  University,  p.  71-73. 


ART.    1]  THE    GRAVEL  157 

in  the  form  of  a  ridge-like  heap  of  waste,  which,  as  we  readily  note, 
contains  much  less  clay  and  sand  and  therefore  a  larger  proportion 
of  gravel  and  bowlders,  than  the  sheet-like  deposit  of  till  above 
described.  In  some  cases  these  moraines  are  very  distinct  features 
in  the  landscape,  appearing,  from  the  number  of  large  bowlders 
which  they  expose,  much  like  ruined  walls  of  cyclopean  masonry. 
More  commonly  they  are  found  in  the  form  of  slight  ridges,  which 
may  be  covered  with  fine  material,  but  commonly  exhibit  here  and 
there  projecting  bowlders.  In  general  it  may  be  said  that  the 
moraines  afford  much  better  sites  for  pits  from  which  road  mate- 
rials are  to  be  obtained  than  the  till,  and  this  because  of  the  pre- 
vailing absence  of  clay  and  sand  in  the  deposits. 

"  Here  and  there  in  almost  all  glaciated  districts,  especially  in 
the  valleys  of  the  greater  streams,  there  may  be  found  narrow  ridges, 
often  of  considerable  height,  and  almost  always  extending  in  the 
direction  of  the  ice  movement.  These  ridges  are  generally  termed 
by  geologists  eskars,  and  often  have  a  tolerable  continuity  for 
scores  of  miles  at  right  angles  to  the  ice  front.  A  section  of  them 
shows  generally  a  gravelly  mass,  nearly  always  free  from  clay  and 
often  containing  little  sand,  though  occasionally  there  is  an  abun- 
dance of  large  bowlders,  which  have  a  prevailing  rounded  or  water- 
worn  form.  These  eskars  were  doubtless  formed  in  the  caves  be- 
neath the  ice  through  which  the  ancient  sub-glacial  streams  found 
their  way.  These  under-ice  rivers  were  much  given  to  changing 
their  position,  and  as  a  stream  lost  its  impetus  it  was  apt  to  fill  its 
ancient  arched-way  with  debris,  which  in- its  time  of  freest  flow 
would  have  been  sent  forward  to  the  ice  front.  At  many  places  in 
New  England  and  in  New  York  these  eskars  contain  large  and  use- 
ful deposits  of  gravel,  and  also  occasionally  quantities  of  bowlders 
well  fitted  for  crushing  as  regards  their  size  and  hardness.  In  the 
Western  States,  because  of  the  general  coating  of  deep  soil,  these 
eskars  are  less  easily  found;  but  they  exist  there,  and  should  be 
sought  for. 

"  Where  the  eskars  terminate,  as  they  commonly  do,  on  a 
morainal  line,  there  is  almost  invariably  found,  immediately  in 
front  of  their  southern  terminations,  a  delta-like  deposit  which, 
though  generally  composed  in  large  measure  of  sand,  frequently 
contains  near  the  moraine  extensive  accumulations  of  useful  gravel 
and  small  bowlders  which  are  fit  for  crushing. 

"  Information  may  be  had  from  the  banks  of  streams,  where  by 
chance  they  have  cut  below  the  deep  coating  of  fine  materials.  The 


158  GRAVEL   ROADS  [CHAP.    V 

existence  of  any  distinct  up-rise  of  the  surface  affords  some  reason 
to  expect  that  the  coarse  glacial  waste  may  be  at  that  point  not 
very  deeply  hidden." 

282.  When  a  gravel  road  is  to  be  built,  if  the  local  gravel  resources 
have  not  already  been  thoroughly  explored,  every  reasonable  effort 
should  be  made  to  ascertain  the  extent  and  character  of  the  available 
deposits  of  gravel.     To  test  a  gravel  deposit,  test  holes  or  wells  should 
be  sunk  at  regular  intervals  over  the  deposit.     These  holes  should 
be  large  enough  for  a  man  to  get  down  into  them  and  to  examine 
the  gravel  in  place  and  to  collect  samples.     Care  should  be  taken  that 
the  samples  are  truly  representative.     Note  should  be  made  of  the 
amount  and  character  of  the  overlying  material,  of  the  depth  of  the 
gravel  deposit,  and  of  the  dip  of  the  strata. 

283.  CHARACTERISTICS  OF  DIFFERENT  GRAVELS.    Any  gravel 
which  stands  vertical  in  the  bank,  showing  no  signs  of  slipping 
when  thawing  out  in  the  spring,  requiring  the  use  of  the  pick  to 
dislodge  it,  and  falling  in  large  chunks  or  solid  masses,  is  sufficiently 
clean  and  free  from  clay  for  use  on  the  road,  and  usually  contains 
just  enough  cementing  material  to  cause  it  to  pack  well. 

Pit  gravel  usually  contains  too  much  earthy  material,  and  can 
be  greatly  improved  by  screening.  Gravel  is  still  being  deposited 
in  drifts  and  bars  by  streams,  and  this  will  be  found  to  partake  of 
the  character  of  the  pit  gravel  of  the  locality,  except  that  it  gen- 
erally contains  less  clay,  and  may  have  an  excess  of  sand.  This 
is  often  called  river  gravel,  and  is  one  of  the  best  sources  of  road 
material.  Lake  gravel  varies  greatly  in  character.  It  is  usually 
free  from  earth  and  contains  sufficient  sharp  sand  to  pack  well; 
but  is  liable  to  be  slaty — an  undesirable  quality. 

284.  Composition  of  Representative  Gravels.     In  an  endeavor 
to  determine  the  composition  necessary  for  a  road-building  gravel, 
samples  were  obtained  of  a  number  of  gravels  that  had  given  satis- 
factory service  in  the  road.     The  samples  in  each  case  were  selected 
by  a  person  thoroughly  conversant  with  the  use  of  that  particular 
material,  and  are  believed  to  be  fairly  representative. 

Table  24,  page  160,  shows  the  sieve  analysis  of  these  gravels. 
Each  sample  was  first  washed  in  successive  waters  until  the  water 
remained  clear,  and  then  the  wash  water  was  allowed  to  stand  until 
the  matter  in  suspension  was  precipitated.  The  precipitate  was 
dried  in  an  oven  to  a  constant  weight  and  then  weighed;  and  the 
washed  gravel  was  air-dried,  and  then  sifted  and  weighed.  The 
per  cent  of  voids  in  the  washed  gravel  was  obtained  by  gently  ram- 


ART.    1]  THE   GRAVEL  159 

ming  the  gravel  under  a  measured  quantity  of  water  in  a  small  metal 
cylinder,  the  ramming  not  being  severe  enough  to  crush  any  of  the 
pebbles  or  fragments. 

Table  25,  page  161,  shows  the  results  of  a  mineralogical  analysis 
of  such  of  these  gravels  as  had  passed  a  screen  having  J-inch  meshes. 
The  matter  recorded  in  Table  24  as  being  in  suspension  is  called 
clay  in  Table  25,  although  part  of  it  was  doubtless  organic  matter 
and  part  fine  sand,  but  the  error  is  not  material. 

286.  To  study  these  gravels  further,  each  will  be  considered  in 
order. 

1.  Urbana.     This  is  a  screened  drift  gravel  obtained  near  Ur- 
bana,  Champaign  Co.,  111.,  which  has  been  used  in  a  few  instances 
on  private  driveways.     Table  25  shows  only  3.8  per  cent  of  clay 
present,   which  will  have  only  a  small  binding  effect.     There  is 
7.6  per  cent  of  iron  oxide  (Fe2Os)  in  the  clay,  but  there  is  so  small 
a  proportion  of  clay  in  the  gravel  that  the  iron  contained  in  it  will 
have  an  inappreciable  binding  effect.     The  principal  source  of  binder 
is,  then,  the  65  per  cent  of  ferruginous  limestone.     Limestone  itself 
when  pulverized  makes  an  excellent  binding  material.     However, 
in  this  case  only  a  small  part  of  the  limestone  is  in  the  form  of  flat 
chips  that  may  be  easily  crushed  under  the  wheels,  but  the  most  of 
the  fragments  are  rounded  and  not  easily  crushed  except  by  compara- 
tively heavily  loaded  wheels.     There  is  only  a  small  per  cent  of 
crystalline  rocks  present,  and  these  are  hard  and  not  readily  crushed, 
and  consequently  can  not  materially  affect  the  binding  qualities  of 
the  mass.     The  gravel  also  contains  22.2  per  cent  of  quartz;    but 
this  material  is  very  hard  and  not  easily  crushed,  and  besides  its 
dust  is  almost  wholly  devoid  of  cementing  properties.     Both  the 
quartz  and  the  crystalline  rocks  are  quite  sharp  and  angular,  which 
is  a  very  desirable  condition,  and  aids  the  binding  action  of  the 
clay  and  the  limestone  dust.     This  gravel  packs  slowly  in  the  road, 
particularly  under  the  light  traffic  of  a  private  driveway;  but  under 
moderate  traffic  makes  a  fairly  good  road,  and  is  not  much  affected 
by  freezing  and  thawing. 

2.  Decatur.    This  is  a  gravel  much  used  on  the  country  roads 
near  Decatur,  Macon  County,  111.,  with  satisfactory  results.     This 
gravel  has  a  comparatively  large  amount  of  fine  sand.     An  examina- 
tion of  Table  25  shows  that  it  contains  more  than  twice  as  much 
clay  as  the  Urbana  gravel,  but  only  about  one  third  enough  to 
fill  the  voids.     A  considerable  portion  of  the  limestone  both  of 
the  pure  and  the  ferruginous — a  total  of  30  per  cent, — i»  in  thin 


160 


GRAVEL  ROADS 


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162  GRAVEL  EOADS  [CHAP.   V 

friable  chips,  and  is  easily  crushed  by  the  traffic,  thus  making  an 
excellent  binder.  The  ferruginous  limestone  contains  an  unim- 
portant proportion  of  iron;  but  the  ferruginous  sandstone  is  heavily 
charged  with  iron  oxide,  which  makes  a  good  cementing  material. 
This  gravel  makes  a  smooth,  hard  surface,  reasonably  free  from 
dust  in  the  summer  and  mud  in  the  winter. 

3.  Lexington.     This  gravel  is  used  with  entire  satisfaction  in 
and  around  Lexington,  McLean  County,  111.,  for  country  highways. 
Notice  that  the  clay  is  equal  to  only  one  seventh  of  the  voids. 
Nearly  all  of  the  21  per  cent  of  ferruginous  limestone  consists  of 
thin  chips  which  are  easily  crushed  by  the  traffic.     Some  binder  is 
probably  obtained  from  the  58  per  cent  of  siliceous  limestone.     The 
per  cent  of  crystalline  rocks  present  is  very  small,  and  can  not 
materially  affect  the  quantity  of  the  gravel.     The  amount  of  quartz 
is  less  than  in  the  preceding  gravels,  and  is  an  unimportant  ele- 
ment. 

4.  Rockford.    This  gravel  has  given  satisfactory  service  in  Rock- 
ford,  Winnebago  County,  111.,  probably  under  more  exacting  con- 
ditions than  any  of  the  preceding.     This  is  considerably  the  coarsest 
gravel  in   Table   24.     Notice   that   this    gravel   contains,    roughly 
speaking,  only  about  one  tenth  enough  clay  to  fill  the  voids.     The 
chief  source  of  binder  is  the  limestone  which  exists  in  the  form  of 
pebbles,  but  contains  no  considerable  amount  of  iron  or  silica.     The 
basic  crystalline  rocks  by  decomposing  may  furnish  a  little  binder; 
but  as  they  are  round  hard  pebbles,  not  easily  crushed,  the  binder 
derived  from  this  source  can  be  of  no  practical  importance.     Very 
little  cementing  material  may  be  derived  from  the  iron  conglomerate 
or  from  the  limestone  and  quartz  conglomerate. 

5.  Peekskill.     This  gravel  is  from  Roa  Hook,  a  "  point  "  in  the 
Hudson  river  near  Peekskill,  N.  Y.,  and  is  much  used  in  and  around 
New  York  City,  where  it  is  considered  one  of  the  best  road  gravels. 
Notice  that  the  clay  is  less  than  one  thirtieth  of  the  volume  of  the 
voids.    Considerable  binding  material  is  doubtless  derived  from  the 
ferruginous  limestone,  which  contains  a  comparatively  small  per  cent 
of  iron.     The  iron  in  the  ferruginous  sandstone  is  too  small  in  amount 
to  be  appreciable.     Some  binder  is  doubtless  derived  from  the  meta- 
morphosed rocks  containing  iron,  silica,  and  mica.     Notice  that  there 
are  nearly  30  per  cent  of  crystalline  rocks,  which  upon  being  finely 
pulverized  will  furnish  an  excellent  cementing  material,  particularly 
after  being  decomposed.     This  gravel  requires  considerable  rolling 
with  a  heavy  roller  to  crush  the  several  ingredients  and  liberal 


ART.    1]  THE    GRAVEL  163 

sprinkling  to  work  the  pulverized  material  into  the  voids,  before  the 
mass  binds.  All  other  gravels  in  Table  24  bind  and  make  fair  roads 
under  ordinary  traffic. 

6.  Buck  Hill.     This  gravel  was  obtained  from  the  Buck  Hill  pit 
at   Tuckahoe,    N.    J.     It   was   recommended   as   a   representative 
gravel    by  Hon.   Henry  I.   Budd,   State  Commissioner  of  Public 
Roads  of  New  Jersey.     This  gravel  consists  chiefly  of  clay  and 
partially   rounded    quartz    pebbles.     The   metamorphosed   rock   is 
angular  and  friable.     The  clay  is  probably  enough  to  fill  the  voids 
when  the  gravel  has  been  compacted  by  traffic.     This  is  the  first 
of  the  samples  in  which  the  iron  contained  in  the  clay  is  appreciable, 
and  the  iron  doubtless  has  an  important  part  in  binding  the  road. 
This  gravel  is  used  for  road  building  without  rolling. 

7.  Rock  Hill.     This  sample   was  obtained  from  the  Rock  Hill 
pit  at  Tuckahoe,  N.  J.,  and  is  substantially  the  same  as  No.  6, 
except  in  having  a  greater  per  cent  of  voids  and  in  containing  some 
sandstone  which  crushes  easily  and  materially  reduces  the  voids 
of  the  gravel  after  it  has  been  compacted  in  the  road.     It  is  said 
that  the  best  results  are  obtained  by  mixing  this  and  the  preceding 
gravel  half  and  half. 

8.  Shark  River.     This  gravel  was  obtained  from  the  Manasquan 
Gravel  Co.  of  Asbury  Park,  N.  J.,  and  is  much  used  in  southern 
New  Jersey  and  around    New  York  City.     It  consists  wholly  of 
clay  and  small  rounded  pebbles  of  pure  white  quartz,  and  conse- 
quently the  only  binding  material  is  the  clay  and  the  2  per  cent 
of  iron  contained  in  it. 

9.  Oaktown.     This  is  a  gravel  obtained  from  the  Wabash  river 
by  dredging,  a  few  miles  above  Vincennes,  Ind.,  which  has  been 
used  on  the  roads  entering  Oaktown,   Knox  Co.,  Ind.     There  is 
very  little  clay  in  this  gravel, — only  7.1  per  cent,  if  the  shale  be 
considered  as  clay,  as  it  is  practically.     The  chief  source  of  binding 
material  is  the  18.9  per  cent  of  carbonate  of  lime,  much  of  which 
is  in  the  form  of  flat  chips.     The  metamorphic  rocks  are  also  in 
thin  chips,  and  are  easily  pulverized.     The  crystalline  rocks  and 
the  quartz  are  comparatively  rough  and  angular.     In  service  the 
limestone  pebbles  grind  up  under  traffic,  and  the  road  becomes 
hard  and  firm,  and  is  not  much  affected  by  freezing  and  thawing. 

10.  Shaker  Prairie.     This  gravel  is  found  in  a  pit  on  Shaker 
Prairie,  west  of  Oaktowp,  Knox  Co.,  Ind.,  and  consolidates  under 
traffic  much  more  quickly  than  the  preceding.     This  gravel  contains 
a  comparatively  small  amount  of  fine  sand,  being  in  this  respect 


164  GRAVEL   ROADS  [CHAP.    V 

about  on  a  par  with  the  Peekskill  gravel — see  No.  5,  Table  24. 
It  contains  a  comparatively  large  amount  of  clay,  being  in  this  respect 
similar  to  the  New  Jersey  gravels — No.  7,  8,  and  9  in  Tables  24 
and  25.  This  gravel  has  more  iron  in  the  clay  than  any  of  the 
samples  except  the  Tuckahoe  gravels — No.  6  and  7.  The  limestone 
is  in  comparatively  large  rounded  pebbles,  and  not  easily  crushed 
under  traffic.  The  road  is  bound  almost  wholly  by  the  clay  and 
the  iron  in  it,  and  by  the  pulverized  limestone. 

11.  Paducah.     This  gravel  came  from  a  pit  about  2  miles  west 
of  Paducah,  Ky.,  on  the  Ohio  river  at  the  mouth  of  the  Tennessee 
river.     It  makes  excellent  roads  that  pack  quickly  under  traffic 
and  are  not  much  affected  by  freezing  and  thawing.     The  coarse 
material   consists  of  water-worn   chert   pebbles,   and  is   cemented 
by  ferruginous  clay.     The  chert  is  brittle  and  crushes  with  a  sharp 
splintery  fracture,  and  consolidates  readily  under  traffic,  the  sharp 
angular  fragments  giving  an  immobile  mass  and  offering  excellent 
surfaces  for  the  cementing  action  of  the  binder. 

12.  Rosetta.     This  gravel  comes  from  the  Rosetta  pit  at  Fort 
Gibson;  near  Vicksburg,  Miss.,  and  is  much  used  by  the  Illinois 
Central  Railroad  as  ballast.     It  is  here  included  under  the  belief 
that  it  will  also  make  good  wagon  roads.     The  quartz  pebbles  are 
quite  rough  and  angular,  and  in  the  pit  seem  to  be  quite  firmly 
cemented  together  by  ferruginous  clay. 

286.  Conclusion.  From  the  preceding,  the  following  conclusions 
may  be  drawn.  1.  The  relation  between  the  proportion  of  voids 
and  the  per  cent  of  clay  is  no  indication  of  the  road-building  qual- 
ities of  a  gravel,  for  under  traffic  some  of  the  fragments  may  crush 
and  decrease  the  per  cent  of  voids  and  at  the  same  time  increase 
the  amount  of  the  binding  material.  2.  The  friability  of  the  pebbles 
has  a  greater  effect  upon  the  road-building  qualities  of  a  gravel 
than  the  per  cent  of  the  voids.  3.  The  binding  material  may  be 
clay,  or  clay  and  iron,  or  pulverized  limestone,  or  all  of  these  com- 
bined. The  less  clay  the  more  slowly  will  the  road  bind,  but  the 
less  it  will  be  affected  by  frost. 

A  study  similar  to  the  preceding  will  not  certainly  determine 
the  suitability  of  a  gravel  for  road  purposes,  but  it  will  throw  valu- 
able light  upon  its  probable  behavior  in  the  road.  The  only  sure 
way  to  determine  the  road-building  qualities  of  a  gravel  is  to  test 
it  by  actual  service,  for  much  depends  upon  the  friability  of  the 
pebbles,  the  weight  of  the  traffic,  the  climatic  conditions,  etc.  In 
applying  the  test  of  actual  service,  particularly  to  determine  the 


ART.    2]  CONSTRUCTION  165 

relative  merits  of  two  gravels,  account  should  be  taken  of  (1)  the 
nature  of  the  soil,  (2)  the  care  employed  in  preparing  the  founda- 
tion, (3)  the  quantity  of  material  used,  (4)  the  amount  and  char- 
acter of  the  traffic,  (5)  the  care  given  to  maintaining  the  road,  and 
(6)  the  length  of  time  the  material  has  been  in  service.  The  char- 
acter of  a  gravel  road  is  generally  indicated  by  the  sound  of  the 
metal  tires  of  the  wheels  of  the  vehicles  passing  over  it.  If  the  wheel 
makes  a  continuous  crisp  gritty  sound,  the  road  is  reasonably  good; 
if  the  gritty  sound  is  absent,  there  is  probably  too  much  earthy 
matter  on  the  surface;  and  if  the  sound  is  intermittent,  there  are 
probably  too  many  large  pebbles  in  the  surface. 

ART.  2.     CONSTRUCTION 

288.  The  subgrade  for  a  gravel  road  should  be  prepared  in  sub- 
stantially the  same  manner  as  for  an  earth  road  (see  Art.  1,  Chapter 
III).     Indeed  a  first-class  earth  road  is  the  best  foundation  for  a 
gravel  road. 

289.  DRAINAGE.     In  no  case  should  the  drainage  be  neglected 
— neither  the  side  ditches  nor  the  underdrainage.     With  the  hard, 
impervious  surface  of  a  gravel  road,  the  water  reaching  the  side 
ditches  is  greater  than  with  an  earth  surface;    and  therefore  the 
side  ditches  should  be  larger  for  gravel  and  broken-stone  roads  than 
for  earth  ones. 

A  gravel  road  upon  an  undrained  soil  entails  a  needless  expense 
for  maintenance,  and  is  never  so  good  as  if  the  road-bed  had  been 
thoroughly  underdrained.  Not  infrequently  a  thin  coating  of 
gravel  has  been  thrown  upon  an  undrained  foundation,  only  to 
sink  out  of  sight  in  a  year  or  two,  and  the  attempt  to  secure  a 
gravel  road  has  been  abandoned.  In  such  cases  a  comparatively 
small  expense  for  underdrainage  would  probably  have  resulted  in  a 
fair  road  instead  of  a  failure.  The  total  amount  of  good  road-build- 
ing material  in  the  world  is  small  in  comparison  with  the  possible 
future  demand,  and  therefore  it  is  a  public  misfortune  to  have  any  of 
it  wasted  in  bungling  attempts  at  road  building.  One  purpose  of 
gravel  is  to  give  a  more  or  less  rigid  layer  which  will  distribute  the 
concentrated  pressure  of  the  wheels  over  a  sufficiently  large  area 
of  the  earth  foundation  to  enable  it  to  support  the  load  without 
indentation.  The  thickness  of  gravel  required  to  support  the 
load  depends  upon  the  degree  of  the  drainage,  since  the  more  water 
in  the  earth  the  less  load  it  can  support.  Underdrainage  costs 


166  GKAVEL   ROADS  [CHAP.    V 

nothing  for  maintenance,  and  decreases  the  amount  of  gravel  re- 
quired, as  well  as  the  cost  of  maintaining  the  surface. 

290.  The  tile  should  be  placed  under  the  side  ditches — as  de- 
scribed for  earth  roads  (§116).     Some  writers  recommend  that  a 
tile  be  laid  under  the  middle  of  the  gravel  with  the  earth  surface 
sloping   both  ways  to  the  tile.     There  are  several  objections  to 
this  construction:    (1)   sloping  the  earth  surface  is  not  of  much 
advantage,  and  (2)  it  needlessly  increases  the  depth  of  the  gravel; 
and  (3)  if  the  road  is  otherwise  well  made,  the  surface  should  be 
practically  impervious  to  water.     See  §  124. 

Some  writers  advocate  a  tile  each  side  of  the  graveled  portion, 
with  short  lines  of  tile  running  each  way  from  the  center  of  the 
roadway  obliquely  to  the  side  tile,  these  "miter  drains"  to  be 
placed  15  feet  apart  in  wet  places.  Clearly  this  construction  is 
based  upon  a  misapprehension  of  the  source  of  the  water  reaching 
a  drain  tile.  The  water  that  enters  a  tile  comes  from  below  and 
not  directly  down  from  above.  It  is  abundantly  proven  that  in 
an  earth  road  needing  underdrainage,  little  or  no  water  penetrates 
the  surface;  and  with  good  gravel  roads  there  will  be  still  less. 
Therefore  "miter  underdrains  "  below  the  graveled  portion  of  the 
roadway  are  absolutely  worthless,  and  tiles  at  the  edges  of  the  hard- 
ened way  are  no  better  than  tiles  under  the  side  ditches'. 

291.  WIDTH.    "For  a  discussion  of  the  principles  governing  the 
width  of  improved  way  and  also  whether  it  shall  be  located  in  the 
center  or  at  one  side  of  the  wheel  way,  see  §  95-98.     For  a  consider- 
ation of  the  excess  width  on  curves,  see  §  97. 

292.  MAXIMUM    GRADE.     For  a    general    discussion  of  maxi- 
mum grades,   see  §  79-86.     A  committee  of  the  American  Society 
of  Civil  Engineers  recommends  that  the  maximum  grade  permissible 
be  12  per  cent — see  Table  15,  page  57.     In  the  matter  of  permissible 
maximum  grades,  gravel  and  macadam  roads  are  in  the  same  class. 
For  data  concerning  existing  maximum  grades  on  water-bound  mac- 
adam roads,  see  §  112. 

293.  CROWN.     The    same    general    principles    concerning    the 
crown  apply  in  gravel  roads  as  in  earth  roads — see  §  130-31.     The 
slope  of  the  gravel  surface  from  the  center  to  the  side  should  be 
at  least  one  quarter  of  an  inch  per  foot,  and  it  should  not  be  more 
than  three  quarters  of  an  inch  per  foot.     The  first  is  about  right 
for  park  drives,  which  have  light  traffic  and  are  well  cared  for.     If 
the  drive  is  narrow,  the  crown  may  be  a  little  greater  than  this; 
but  if  it  is  broad,  the  crown  should  be  less,  to  prevent  the  surface 


ART.    2]  CONSTRUCTION  167 

from  being  gullied  out  near  the  gutters  by  the  water  running  from 
the  center  to  the  tides.  The  maximum  crown,  as  above,  would  be 
about  right  for  a  country  gravel  road  with  heavy  traffic,  or  for  a 
street.  If  the  gravel  contains  an  excess  of  clay,  the  crown  should 
be  greater  than  the  above  maximum,  as  the  surface  will  be  liable 
to  rut  up. 

Frequently  gravel  roads  have  an  excessive  crown,  which  forces 
travel  to  use  a  narrow  strip  in  the  center — see  §  129.  This  results 
from  the  fact  that  the  gravel  is  placed  thicker  at  the  center  than 
at  the  edges;  and  thus  the  surface  of  the  gravel  is  given  a  greater 
crown  than  the  original  earth  road,  while  a  gravel  road  should  have 
a  less  crown  than  an  earth  one. 

A  committee  of  the  American  Society  of  Civil  Engineers  recom- 
mends a  maximum  crown  of  1  inch  per  foot  of  half  width  and  a 
minimum  of  half  that  amount — see  Table  16,  page  65. 

294.  For  a  rule  for  super-elevation  on  curves,  see  §  90. 

295.  FORMS  OF  CONSTRUCTION.     There  are  two  forms  of  con- 
struction of  country  gravel  roads,  which  differ  as  to  the  manner 
of  preparing  the  subgrade  to  receive  the  gravel.     In  one  form  the 
gravel  is  simply  deposited  on  the  surface  in  a  strip  along  the  middle 
of  the  former  earth  road;    and  in  the  other  a  trench  is  excavated 
in  which  the  gravel  is  placed.     For  convenience  of  reference  the 
former  will  be  called  Surface  Construction,  and  the  latter  Trench 
Construction. 

296.  Surface  Construction.     The  crudest  form  of  this  method 
of  construction  consists  in  dumping  gravel,  as  it  comes  from  the 
bank,  in  piles  in  line  on  an  earth  road.     The  quantity  of  gravel  is 
gaged  by  dumping  a  load  in  one,  or  two,  or  three  lengths  of  the 
wagon.     Little  or  no  attention  is  given  to  leveling  off  the  top  of 
the  piles,  and  it  is  not  rolled  except  as  travel  is  forced  upon  the 
ridge  when  the  earth  upon  the  sides  gets  muddy.     For  the  first  year 
or  two  after  construction,  such  a  gravel  road  is  little  if  any  better 
than  an  earth  one.     The  surface  is  full  of  cradle  holes  and  is  easily 
cut  into  ruts;    and  the  loose  material  absorbs  the  rain,  and  be- 
comes mixed  with  the  soil  below.     If  the  gravel  is  good,  the  road 
becomes  fairly  good  after  the  gravel  has  been  packed  by  travel  and 
after  the  holes  have  been  filled  up  by  the  addition  of  new  material. 
This  form  of  construction  is  common  where  gravel  is  plentiful,  the 
work  usually  being  done  by  labor  road-tax. 

297.  Another  form  of  surface  construction  consists  in  setting 
up  two  lines  of  plank  on  edge  and  filling  the  space  between  them 


168  GRAVEL   ROADS  [CHAP.    V 

with  gravel.  The  gage  planks  are  set  en  edge,  8,  10,  or  12  feet 
apart  according  to  the  importance  of  the  road,  and  the  gravel  is 
filled  in  between  the  planks,  8  or  10  inches  deep  at  the  sides  and 
12  or  15  at  the  center.  Of  course,  when  the  boards  are  moved 
forward  to  be  used  again,  the  edge  of  the  gravel  spreads  out  and 
takes  the  natural  slope,  and  under  traffic  it  spreads  out  still  further. 
Ordinarily  in  this  form  of  construction  the  gravel  is  not  rolled, 
and  there  is  little  or  no  driving  over  it  by  teams  engaged  in  the 
construction.  The  only  advantage  of  this  method  over  the  pre- 
ceding one  is  that  it  affords  a  means  of  gaging  the  depth  of  gravel 
and  of  determining  the  quantity  used;  and  the  chief  objection  to 
it  is  that  when  gravel  is  put  on  in  a  thick  layer,  the  lower  part  is 
not  consolidated  well,  at  least  not  for  a  considerable  time,  and 
therefore  the  surface  is  liable  to  break  up.  This  form  of  construc- 
tion is  very  common. 

298.  In  the  best  form  of  surface  construction,  the  former  earth 
road  is  first  smoothed  up  with  the  scraping  grader  and  if  necessary 
the  crown  is  reduced.  If  after  smoothing  the  surface  with  the 
grader,  the  foundation  is  not  already  firm  and  solid,  it  should  be 
rolled.  Next  a  layer  of  gravel  4,  or  at  most  6,  inches  deep  is  spread 
upon  the  prepared  subgrade,  and  leveled — either  by  hand  with  a 
shovel  and  rake,  or  with  a  harrow  or  scraping  grader.  In  dump- 
ing from  a  wagon  or  cart,  the  larger  stones  will  roll  to  the  outer 
edge  of  the  heap;  and  hence  in  leveling  the  gravel  care  should 
be  taken  that  these  are  scattered  and  covered  deeply  with  fine 
material,  for  otherwise  the  road  will  not  have  an  uniform  texture 
and  will  wear  unevenly  and  the  large  stones  are  liable  to  work  to 
the  top. 

If  the  teams  hauling  the  gravel  are  required  to  drive  over  that 
already  placed,  the  road  will  be  consolidated  much  sooner,  but 
as  the  tractive  resistance  on  loose  gravel  is  very  great,  there  is 
some  disadvantage  in  this  requirement.  If  it  is  to  be  insisted 
upon,  the  construction  of  the  road  should  begin  at  the  end  nearest 
the  gravel  pit.  The  gravel  can  be  consolidated  with  a  roller,  but  not 
as  effectively  as  by  traffic,  since  no  roller  gives  so  great  a  pressure 
as  the  wheels  of  loaded  wagons  (see  §  378).  But  heavy  loads  should 
not  be  permitted  to  go  over  the  road  while  the  surface  is  wet  and 
soft,  for  fear  the  wheels  will  cut  through  and  mix  the  earth  and  the 
gravel.  While  the  gravel  is  being  consolidated  by  the  passage  of 
the  teams  employed  in  the  construction  or  by  ordinary  traffic,  all 
ruts  should  be  filled  as  soon  as  formed,  by  the  use  of  a  garden  rake, 


ART.    2] 


CONSTRUCTION 


169 


and  all  saucer-like  depressions  should  be  filled  by  shoveling  in  fresh 
gravel.  The  cost  of  filling  ruts  and  depressions  will  be  more  than 
saved  in  future  repairs,  and  besides  a  much  better  road  will  be  the 
result. 

After  one  layer  has  been  thoroughly  consolidated  add  a  second, 
and  so  on  until  the  desired  depth  is  reached.  The  first  layer  may 
be  the  poorer  gravel,  the  best  being  reserved  for  the  top.  All  the 
layers  should  be  added  in  time  to  get  well  packed  before  the  rains 
and  frosts  of  winter  soften  the  road-bed. 

When  finished  the  gravel  should  be  deepest  at  the  center  and 
taper  off  to  the  sides.  It  is  immaterial  whether  the  first  layer  is 
the  widest  or  the  narrowest — there  is  a  little  advantage  either  way. 
The  depth  necessary  will  depend  upon  the  nature  of  the  soil,  the 
quality  of  the  gravel,  the  amount  of  travel,  the  maximum  weight 
per  wheel,  and  the  care  given  to  maintenance;  but  under  ordinary 
conditions,  a  depth  of  8  or  10  inches  of  compacted  gravel  at  the 
center  is  sufficient.  The  width  should  vary  with  the  amount  of 
travel,  but  for  a  country  road  a  depth  of  6  inches  at  4  or  5  feet 
from  the  center  is  sufficient. 

Fig.  43  shows  the  dimensions  required  in  good  practice. 


4             "t'fi*          h 

rf      &  G*      \ 

(S/rface  of  Loose  ^Material 

x^ 

V" 

0^          V 

^r^^J 

Fia.  43. — CROSS  SECTION  OF  GRAVEL  ROAD.     SURFACE  CONSTRUCTION. 

For  data  on  the  width  of  the  actually  traveled  way  on  gravel 
roads,  see  §  95-96. 

299.  Trench  Construction.  In  this  form  of  construction,  a 
trench  is  excavated,  10  or  12  inches  deep  and  of  the  required  width 
for  the  reception  of  the  gravel.  The  bottom  of  the  trench  is  usually 
made  parallel  to  the  finished  road  surface  by  sloping  it  from  the  center 
toward  the  sides  (see  §  351).  Fig.  44  shows  the  form  when  the 
finished  surface  is  an  arc.  Fig.  44  is  the  standard  form  for  state- 
aid  roads  in  Connecticut,  except  that  the  width  of  the  graveled  way 
may  be  12,  14,  or  16  feet.  The  crown  is  f  inch  per  foot  of  distance 
from  side  to  center,  or  6  inches  for  a  16-foot  roadway.  There  is 
not  much  difference  whether  the  road  surface  is  an  arc  or  two  planes 
meeting  in  the  center.  The  latter  is  probably  a  little  the  better  for 


170 


GRAVEL   ROADS 


[CHAP,  v 


country  roads,  although  the  former  is  the  more  common.  Notice 
that  in  Fig.  44  the  intersection  of  the  road  surface  with  the  side  slope 
of  the  embankment,  is  rounded  off  somewhat  as  recommended  in 


Half  Section  in  Cut  Half  Section  in  Fill 

• 

FIG.  44.  —  CONNECTICUT  GRAVEL  ROAD.     THENCH  CONSTRUCTION. 


Fig.  15  and  16,  page  85.  The  exact  method  of  rounding  off  the  cor- 
ners in  Fig.  44  is  not  specified.  The  thickness  of  the  layers  as  shown 
is  after  consolidation. 

Fig.  45  shows  the  standard  form  of  construction  adopted  by 
the  Texas  Highway  Commission.  Notice  the  wings  in  Fig.  45. 

The  bottom  of  the  trench  should  be  rolled  to  consolidate  it  and 
to  discover  any  soft  places  in  the  foundation.  After  rolling,  any 
depressions  should  be  filled  and  the  foundation  then  re-rolled.  The 
steam  roller  is  better  for  this  purpose  than  the  horse  roller,  since  it  is 
heavier  and  since  the  horses'  feet  do  not  dig  up  the  subgrade.  For  a 
discussion  of  rollers,  see  §  378.  For  precautions  to  be  taken  in  roll- 
ing the  subgrade,  see  §  369. 

A  layer  of  3  or  4,  or  at  most  6,  inches  of  gravel  is  placed  in  the 
trench,  and  the  gravel  is  harrowed  with  a  tooth  harrow,  and  is  then 
consolidated  either  by  throwing  the  road  open  to  travel  or  by  rolling. 
The  latter  is  preferable,  since  teams  in  passing  each  other  are  liable 
to  break  down  the  edges  of  the  trench  and  mix  the  earth  with  the 
gravel,  and  since  the  wheels  are  liable  to  break  through  the  thin 
layer  of  gravel  —  particularly  if  a  wet  time  intervenes.  If  the  only 


FIG.  45.— TEXAS  GRAVEL  ROAD.     TRENCH  CONSTRUCTION. 

gravel  available  contains  an  excess  of  large  pebbles,  they  may  be 
used  in  the  lower  layer,  in  which  case  the  layer  can  not  be  compacted 
either  by  the  wheels  or  by  rolling.  If  the  gravel  is  only  slightly 


ART.    2]  CONSTRUCTION  171 

deficient  in  binding  material,  it  will  be  impossible  to  use  a  heavy 
roller,  since  the  gravel  will  push  along  in  front  of  it. 

Additional  layers  are  added  as  rapidly  as  the  preceding  one  is 
compacted,  until  the  desired  depth  is  reached.  Before  rolling  the 
last  layer  the  earth  at  the  sides  of  the  trench,  i.  e.,  the  "  shoulders  " 
or  "  wings,"  should  be  thoroughly  rolled;  and  then  the  rolling  of  the 
gravel  should  proceed  from  the  sides  toward  the  center,  to  prevent 
the  gravel  from  slipping  outward.  The  gravel  will  compact  much 
better  when  damp;  but  if  it  is  sprinkled,  care  should  be  taken  that 
(1)  the  gravel  is  not  made  so  wet  that  the  earthy  binding  material 
becomes  semi-fluid  and  collects  on  the  surface,  and  (2)  that  the  sub- 
grade  is  not  unduly  softened. 

No  practical  amount  of  rolling  will  cause  a  gravel  road  to  "  come, 
down  "  in  the  sense  that  a  water-bound  macadam  road  does;  that 
is,  a  gravel  road  can  not  be  rolled  until  the  surface  is  as  hard  as  it 
will  probably  be  after  it  has  been  opened  to  traffic  for  a  time,  since 
even  the  heaviest  rollers  do  not  give  as  much  pressure  as  the  wheels 
of  heavily  loaded  wagons.  This  difference  between  gravel  and 
water-bound  macadam  roads  is  due  to  the  fact  that  gravel  has  the 
binding  material  uniformly  distributed  throughout  the  mass,  while 
with  broken  stone  the  binder  is  spread  upon  the  top  and  worked  in 
by  rolling  and  sprinkling. 

300.  Surface    vs.    Trench    Construction.     Surface    construction 
is  cheaper  and  seems  to  be  much  more  common  than  trench  con- 
struction.    Surface  construction  is  the  better,  since  the  depth  of 
the  gravel  at  different  distances  .from  the  center  is  approximately 
proportional  to  the  amount  of  traffic;   while  in  the  trench  construc- 
tion, if  the  graveled  portion  is  wide  the  sides  are  liable  not  to  be  much 
used,  and  if  the  graveled  portion  is  narrow  passing  vehicles  are 
forced  upon  the  earth  shoulders.     Therefore  it  appears  that  surface 
construction  is  best  for  roads  having  a  large  amount  of  traffic.     In 
park  drives  and  streets,  the  whole  width  of  the  roadway  is  excavated 
and  filled  with  gravel. 

Trench  construction  is  a  little  more  economical  of  gravel,  and 
is  therefore  most  suitable  where  gravel  is  expensive. 

301.  Earth  Road  beside  the  Graveled  Way.     It  is  sometimes 
advocated  that  there  should  be  two  tracks,  an  earth  road  for  sum- 
mer travel  and  a  graveled  way  for  winter  use.     This  plan  has  some 
advantages  and  also  some  disadvantages.     When  the  earth  track 
is  dry,  it  is  preferred  by  most  teamsters  to  the  hard  gravel  road; 
and  the  use  of  the  earth  roadway  decreases  the  wear  on  the  gravel, 


172  GRAVEL   ROADS  [CHAP.    V 

—which  is  clearly  an  advantage,  for  a  gravel  road  like  most  other 
things  will  wear  out.  On  the  other  hand,  if  the  summer  track 
is  immediately  adjacent  to  the  hardened  way,  the  earth  of  the 
former  will  become  mixed  with  the  gravel  of  the  latter,  much  to 
the  detriment  of  the  gravel.  The  chief  source  of  expense  in  the 
maintenance  of  gravel  roads  is  due  to  the  damage  done  by  the 
mixing  of  earth  from  the  side  of  the  road  with  the  gravel,  thus 
forming  a  mixture  that  will  hold  water  and  cause  the  road  to  cut 
up.  It  has  been  suggested  that  the  objection  to  the  two  tracks 
could  be  obviated  by  constructing  a  ditch,  or  sodding  a  narrow 
space  between  the  two;  but  this  is  impracticable.  The  two  tracks 
require  a  wider  right-of-way,  and  therefore  for  this  reason  are  fre- 
quently impossible. 

302.  For  a  discussion  as  to  whether  the  gravel  road  shall  be  in  the 
center  of  the  right-of-way  or  at  one  side,  see  §  98. 

303.  BOTTOM    COURSE.    The    gravel  usually   contains    many 
stones  too  large  to  be  used  in  or  near  the  wearing  surface,  and 
therefore  it  is  economy  to  screen  the  material   and  lay  the  larger 
pebbles  in  the  bottom.     Some  writers  object  to  using  pebbles  larger 
than  1  or  1|  inches  in  diameter  for  the  bottom  course,  on  the  ground 
that  the  heaving  effect  of  frost  and  the  vibration  due  to  the  pass- 
ing wheels  will  cause  the  larger  stones  to  rise  to  the  surface  and 
the  smaller  ones  to  descend — like  the  materials  in  a  shaken  sieve. 
Unquestionably,  if  a  vehicle  is  driven  over  a  layer  of  loose  stones 
of  all  sizes,  the  larger  ones  will  tilt  up  when  the  weight  comes  upon 
them  and  the  smaller  ones  will  roll  down  into  the  space  made  vacant 
by  such  tipping;  and  by  a  repetition  of  this  process,  the  large  stones 
will  gradually  reach  the  surface.     The  heaving  action  of  the  frost 
acts  in  a  similar  way.     But  it  does  not  follow  that  a  layer  of  coarse 
stones  at  the  bottom  of  a  gravel  road  will  thus  work  to  the  top 
when  the  interstices  of  the  gravel  above  are  filled  with  binding  mate- 
rial and  all  is  compacted  by  traffic  or  by  rolling.     Experience  has 
shown  that  if  2  to  4  inches  of  the  top  dressing  has  suitable  binding 
material,  it  is  extremely  improbable  that  pebbles  2  to  2|  inches  in 
diameter  in  the  bottom  course  will  ever  work  to  the  surface. 

304.  Other  materials  than  coarse  pebbles  may  be  used  for  the 
lower   course.     In   many   localities   there   are   large   quantities   of 
coal  slack,  which  is  useless  as  fuel  and  is  too  friable  for  the  wearing 
surface  of  a  road,  but  which  can  be  used  for  the  bottom  course  of 
a  gravel  road.     Coal  slack  has  thus  been  successfully  employed, 
and  is  often  cheaper  than  gravel.     Blast-furnace  slag  has  also  been 


ART.    2]  CONSTRUCTION  173 

used  for  this  purpose.  Sometimes  broken  stone  is  used  for  a  bot- 
tom course;  but  on  account  of  the  expense  of  breaking,  only  a 
stone  found  already  broken  in  the  quarry  is  suitable  for  this  pur- 
pose. A  "  flake  "  stone  or  quarry  chips  are  the  forms  generally 
used.  The  celebrated  gravel  roads  of  Central  Park,  New  York 
City,  have  a  "  rubble  foundation  " — not  a  Telford  foundation 
(§  349).  The  rubble  layer  is  10  to  12  inches  thick,  and  the  gravel 
4  to  6  inches  after  being  thoroughly  compacted.  The  stones,  none  of 
which  exceeded  9  inches  in  greatest  dimensions,  were  dumped  upon 
the  subgrade  from  carts  and  "  evenly  adjusted  by  a  little  labor 
of  the  hand." 

305.  SCREENING  THE  GRAVEL.    As  a  rule  gravel  should  be 
screened  to  exclude  that  which  is  too  fine,  and  also  to  insure  an 
even  distribution  of  the  fine  and  coarse  material  when  placed  upon 
the  road.     Where  a  small  amount  of  gravel  is  required,  the  ordi- 
nary stationary  inclined  screen  is  used,  the  gravel  being  thrown 
against  it  with  a  shovel;    but  where  a  considerable  amount  is  re- 
quired, it  is  much  cheaper  to  use  a  rotary  screen  driven  by  power. 

If  the  gravel  contains  a  considerable  quantity  of  stones  more 
than  2j  or  3  inches  in  diameter,  a  stone  crusher  can  be  profitably 
employed,  in  which  case  it  may  be  economical  to  use  an  elevator, 
rotary  screen,  and  elevated  storage  bins,  and  to  put  all  of  the 
gravel  through  the  crusher,  rotary  screen,  and  storage  bin  (see  Fig.  62, 
page  205). 

Under  favorable  circumstances,  the  cost  per  cubic  yard  of  screen- 
ing by  hand  will  be  about  an  hour's  wages  for  a  man  for  each  time 
the  material  is  handled  with  a  shovel;  while  with  the  rotary  screen, 
it  can  be  screened  to  three  sizes  and  be  placed  in  elevated  bins  for  the 
same  amount. 

306.  The  Michigan  State  Highway  Department,  which  builds 
many  good  gravel  roads,  requires  the  use  of  a  rotary  screen  not  less 
than  9  feet  long  and  30  inches  in  diameter,  divided  into  three  sec- 
tions  having   perforations   3,  2,  and  f  inches  in   diameter.      The 
Department  divides  road-building  gravel  into  two  classes  as  follows: 
The  best  contains  at  least  60  per  cent  of  material  passing  a  2j-inch 
screen  and  caught  on  a  J-inch  screen;  and  the  second  class  contains 
at  least  40  per  cent  caught  between  these  screens.     The  Department 
limits  the  clay  binder  to  10  per  cent  of  the  entire  mass. 

The  specifications  of  the  American  Society  for  Municipal  Im- 
provements for  1916  divide  mixtures  of  gravel,  sand,  and  clay  as 
follows:  No.  1  contains  60  to  75  per  cent  caught  between  the  l?-inch 


174  GRAVEL   ROADS  [CHAP.    V 

and  the  J-inch  screen,  and  of  this  portion  from  25  to  75  per  cent 
shall  be  retained  on  the  f-inch  screen,  and  of  the  portion  passing 
the  J-inch  screen  from  65  to  85  per  cent  shall  be  retained  on  the 
200-mesh  sieve.  No.  2  contains  60  to  75  per  cent  caught  between  the 
2i-mch  and  the  J-inch  screen,  and  of  this  portion  from  25  to  75  per 
cent  shall  be  retained  on  the  1-inch  screen,  and  of  the  portion  passing 
the  J-inch  screen  65  to  85  per  cent  shall  be  retained  on  the  200-mesh 
sieve.  No.  2  is  to  be  used  for  the  two  lower  courses  of  the  road,  and 
No.  1  for  the  top  course. 

307.  HAULING  THE  GRAVEL.    Gravel  is  usually  obtained  from 
pits,  and  is  generally  overlaid  with  more  or  less  earth,  which  should 
be  entirely  removed  before  beginning  to  haul  the  gravel.     Not  infre- 
quently this  earthy  material  is  allowed  to  tumble  into  the  pit  and 
mix  with  the  gravel,  greatly  to  the  detriment  of  the  finished  road. 

The  loading  of  the  gravel  can  be  greatly  facilitated  by  using  a 
board  platform  8  to  10  feet  long  and  4  to  6  feet  wide.  This  plat- 
form is  placed  against  the  bottom  of  the  bank  in  such  a  manner 
that  when  the  gravel  above  is  dislodged  it  falls  upon  the  platform, 
from  which  it  is  easily  shoveled  into  the  wagon.  Often  the  plat- 
form can  be  supported  upon  legs  at  a  height  above  the  top  of  the 
wagon,  and  the  gravel  can  be  simply  pushed  off  into  the  wagon 
with  the  shovel.  Sometimes  the  circumstances  justify  the  use 
of  a  drag  scraper  (§  150) — drawn  by  a  horse  attached  to  a  cable 
passing  through  a  block — to  drag  the  gravel  to  the  edge  of  the  plat- 
form, whence  it  drops  into  the  wagon;  and  sometimes,  if  a  large 
quantity  is  to  be  loaded  and  a  large  number  of  teams  are  engaged 
in  the  hauling,  the  wagons  can  be  loaded  with  a  trap — an  elevated 
platform  upon  which  the  gravel  is  hauled  with  a  drag  or  a  wheel 
scraper,  and  through  which  it  drops  into  the  wagon  below. 

308.  MEASURING  THE  GRAVEL.    When  gravel  roads  are  built 
by  public  officials,  the  gravel  is  usually  measured  in  the  pit  or  in 
the  wagon.     The  former  is  the  better  practice,  since  it  is  more  definite. 
When  the  road  is  built  by  contract,  the  gravel  is  measured  (1)  in 
the  wagons,  or  (2)  loose  in  the  road  by  means  of  gage  boards,  or  (3) 
compacted  in  the  road  by  means  of   established  grades.     The  first 
or  second  method  is  generally  used  with  surface  construction,  and  the 
third  with  trench  construction.     With  the  last,  it  is  customary  to 
require  that  the  finished  surface  shall  conform  to  an  established  grade; 
and  consequently  a  considerable  quantity  of  gravel  is  liable  to  be 
forced  into  the  subgrade, — particularly  if  the  earth  foundation  is 
made  to  conform  to  the  grade  established  for  it.     The  specifications 


ART.    2]  CONSTRUCTION  175 

for  state-aid  roads  in  New  Jersey  specify  that  "  the  contractor  is  to 
place  sufficient  gravel  on  the  road  to  allow  it  to  shrink  33  per  cent 
in  rolling  and  settling."  Loose  gravel  with  clay  or  loam  binder  will 
shrink  12  to  15  per  cent  in  rolling,  and  gravel  in  which  the  binder  is 
produced  by  crushing  part  of  the  material  will  shrink  still  more — 
possibly  twice  as  much;  the  above  specifications  provide,  therefore, 
for  the  possibility  of  forcing  18  to  21  per  cent  of  the  gravel  into  the 
subgrade. 

If  it  is  expected  that  part  of  the  gravel  may  be  forced  into  the 
soil,  the  subgrade  may  be  left  a  little  higher  than  the  established 
grade;  and  then  the  addition  of  the  stipulated  amount  of  gravel  will 
bring  the  finished  surface  to  the  specified  grade.  Or,  a  thin  layer 
of  sand  on  the  subgrade  will  sometimes  prevent  the  gravel  from 
being  forced  into  the  soil.  For  a  further  discussion  of  this  subject, 
see  §  377. 

309.  COST.     The  cost  of  gravel  roads  varies  greatly  with  the 
form  of  construction,  the  cost  of  gravel,  the  amount  of  grading  and 
drainage  required,  the  width  and  thickness  of  the  gravel,  etc.     An 
average  depth  of  1  foot  over  a  width  of  13J  feet  requires  half  a 
cubic  yard  per  linear  foot  of  road,  or  2,640  cubic  yards  per  mile. 
The  gravel  usually  costs  from  5  to  10  cents  per  cubic  yard  stripped 
in  the  bank.     The  cost  of  loading  will  vary  from  5  to  10  cents  per 
cubic  yard,  not  including  the  time  lost  by  the  team  in  waiting  for 
a  load.     Setting  gage  plank,  leveling,  etc.,  may  cost  from  2  to  10 
cents  per  cubic  yard.     The  cost  of  hauling  varies  materially  with 
the  time  of  year  (see  §  4),  and  including  the  time  lost  in  loading 
and  unloading,  will  usually  be  at  least  15  cents  per  cubic  yard 
(about   1J  tons)   per  mile  and  seldom  more  than  30  cents — the 
former  when  done  by  farmers  in  the  slack  season  and  the  latter 
when  done  by  teamsters.     For  a  haul  of  1  mile  the  total  cost  in 
place  is  40  to  50  cents  per  cubic  yard. 

310.  Reports  from  forty-four  counties  in  Indiana  show  that  the 
total  cost  of  construction  of  gravel  roads  in  that  state  varies  from 
$800  to  $3,500  per  mile;    and  except  in  a  few  counties,  the  cost 
varies  from  $1,000  to  $2,500,  and  is  generally  from  $1,000  to  $2,000. 
The  cost  varies  with  the  distance  about  as  follows:  when  the  gravel 
is  hauled  1  mile,  the  total  cost  of  the  road  is  $1,000  per  mile;  when 
the  haul  is  2  miles,  $1,250  per  mile;  when  the  haul  is  3  miles,  $1,500 
per  mile;  when  the  haul  is  4  miles,  $1,750  per  mile;  and  if  5  miles, 
$2,000  per  mile.     Numerous  data  from  Ohio  and  Illinois  seem  to 
show  that  the  above  prices  are  fairly  representative. 


176  GRAVEL   ROADS  [CHAP.    V 

In  Missouri  in  1912  one-course  gravel  roads  10  feet  wide  cost 
$800  to  $1,800  per  mile,  and  15  feet  wide  from  $1,500  to  $2,500, 
exclusive  of  grading,  drainage,  culverts,  interest,  and  profits. 

In  Michigan  in  1913  the  average  cost  of  68  miles  of  state-aid 
gravel  roads  was  as  follows: 

ITEMS.  AVERAGE         COST 

Per  Mile.  Per  Sq.  Yd. 

Shaping  and  draining $455.46  $0.086 

Gravel,  loading  and  hauling,  1,649  cu.  yd 247 . 34  0 . 046 

Culverts,  etc 71.93  0.013 

Surfacing 1661.85  0.316 

Total * $2  436.58  $0.460 

311.  ECONOMIC  VALUE  OF  A  GRAVEL  SURFACE.  The  value 
to  a  community  of  covering  an  earth  road  with  gravel  is  a  subject 
the  discussion  of  which  leads  different  persons  to  widely  different 
conclusions,  depending  upon  the  point  of  view  and  upon  the  data 
assumed. 

The  advantage  of  a  gravel  surface  over  one  of  earth  is  that  the 
hard  and  impermeable  surface  of  the  former  is  equally  good  at  all 
seasons  of  the  year.  The  financial  value  of  a  road  which  is  good  at  all 
seasons  of  the  year  varies  greatly  with  the  locality  and  the  occupa- 
tion of  those  who  use  it.  Near  a  large  city  such  roads  are  nearly 
indispensable  to  dairymen,  fruit  growers,  and  truck  farmers;  but 
permanently  hard  roads  are  not  of  great  financial  advantage  to 
grain  growers  and  stock  raisers,  except  in  the  immediate  vicinity 
of  a  large  city.  A  road  which  is  uniformly  good  at  all  seasons  of 
the  year  is  of  some  economic  advantage  to  a  farming  community, 
since  it  permits  hauling  to  be  done  at  times  when  other  work  is 
impossible,  and  since  it  makes  possible  the  marketing  of  commodi- 
ties when  the  price  is  most  favorable.  It  is  impossible  to  compute 
the  money  value  of  these  factors;  but,  in  general,  it  is  not  very 
great  (see  §  4-7).  The  chief  advantage  of  a  road  good  at  all  seasons 
of  the  year  is  its  effect  upon  the  social  life  of  the  rural  district 
(§  1-3). 

The  amount  of  a  load  than  can  be  hauled  on  an  earth  road  is 
often  determined  by  the  grades  rather  than  by  the  nature  of  the 
surface;  and  unless  the  grades  are  light,  the  maximum  load  for  a 
gravel  road  is  not  much  greater  than  that  for  a  dry  earth  road. 
Therefore,  before  adding  a  gravel  surface  to  an  earth  road,  the 
gradients  should  be  carefully  studied  with  a  view  of  deriving  the 


ART.    2] 


CONSTRUCTION 


177 


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178  GRAVEL   ROADS  [CHAP.    V 

utmost  benefit  of  the  improved  surface  by  securing  easy  ruling 
grades  (see  §  74). 

312.  The  cost  of  the  improvement  is  the  sum  of  (1)  the  annual 
interest  on  the  cost  of  construction,  (2)  the  excess  of  the  annual  cost 
of  maintaining  the  gravel  road  over  that  of  maintaining  the  earth 
road,  and  (3)  the  annual  payment  necessary  to  accumulate  a  fund 
sufficient  to  make  periodic  repairs,  i.  e.,  to  add  a  new  surface  at 
intervals.  The  money  spent  in  road  improvements  is  to  be  con- 
sidered as  an  investment  which  will  return  annual  interest  in  the 
reduced  cost  of  transportation  and  in  the  greater  freedom  of  traffic 
and  social  intercourse. 

313.  DURABILITY.  On  account  of  the  low  first  cost  of  the  gravel, 
and  the  fact  that  reasonably  good  gravel  roads  can  be  built  without 
any  investment  of  money  in  rollers,  crushers,  and  other  costly 
machinery,  they  are  well  suited  to  light  traffic  roads,  to  residence 
streets  in  small  cities,  and  to  park  drives.  A  gravel  road  well  built 
of  good  material  is  excellent  for  automobiles,  and  will  safely  carry 
a  considerable  number  daily — see  Table  26,  page  177. 

314.  SPECIFICATIONS.  The  American  Society  of  Municipal 
Improvements  publishes  standard  specifications  for  the  material 
and  workmanship  of  gravel  roads.  The  specifications  are  modified 
from  time  to  time  as  is  necessary  to  keep  them  up  to  date.  Printed 
copies  may  be  had  of  the  secretary  for  a  nominal  sum. 

The  various  State  Highway  Departments  also  publish  standard 
specifications 

ART.  3.    MAINTENANCE 

316.  There  are  more  miles  of  gravel  roads  in  this  country  than 
of  any  other  type  of  improved  roads;  and  therefore  the  proper  main- 
tenance of  these  roads  is  an  important  matter.     If  properly  con- 
structed and  reasonably  maintained,  a  gravel  road  is  quite  satisfac- 
tory except  under  very  heavy  travel. 

The  proper  care  of  side  ditches,  culverts,  shoulders,  and  the 
surface  has  been  considered  in  the  discussion  on  earth  roads — 
Art.  2,  Chapter  III; — and  all  that  has  been  said  there  applies  equally 
well  to  gravel  roads.  The  maintenance  of  a  gravel  road  is  more  im- 
portant than  that  of  an  earth  road,  because  more  is  rightly  expected 
of  the  former  than  of  the  latter;  and  since  the  gravel  road  represents 
a  greater  investment,  neglect  may  result  in  greater  damage. 

317.  DESTRUCTIVE   AGENTS.     The  destructive  agents  are  the 
same  for  gravel  as  for  earth  roads  (see  §  198-204),  except  that  for 


ART.    3]  MAINTENANCE  179 

gravel  roads  a  gradient  is  an  element  of  destruction  whose  impor- 
tance varies  with  its  steepness.  Horses  in  drawing  a  load  up  a  hill 
or  in  holding  back  a  load  in  coming  down,  are  liable  to  displace 
pebbles  with  the  calks  of  their  shoes,  and  after  the  first  stone  is 
displaced  it  is  easier  to  loosen  others.  The  locking  of  the  wheel, 
until  it  slides  in  going  down  hill,  is  also  hard  on  a  gravel  road. 

Grades  have  a  further  disadvantage.  Automobiles  are  likely 
to  speed  up  at  the  bottom  of  the  grade  so  as  to  reach  the  top  at  a 
fair  velocity,  and  the  sudden  acceleration  of  the  speed  is  likely  to 
dislodge  pebbles  and  cause  the  road  to  ravel. 

318.  THE  METHOD  OF  MAINTENANCE.  The  three  methods  of 
maintenance  employed  in  caring  for  earth  roads  (§  222-26)  are  also 
employed  for  gravel  roads.  Since  gravel  roads  represent  a  greater 
investment,  and  since  more  is  expected  of  them,  it  is  desirable  that 
they  shall  be  cared  for  under  the  patrol  system,  that  is,  the  system 
of  continuous  maintenance. 

When  a  gravel  road  is  first  thrown  open  to  traffic,  it  should  be 
carefully  watched  and  all  incipient  ruts  and  depression  should  be 
filled  as  soon  as  formed,  either  by  raking  in  gravel  from  the  sides  of 
the  depression  or  by  adding  fresh  gravel — in  the  earlier  stages  of  this 
work  the  former  is  the  better,  and  in  the  later  stages  the  latter  is 
necessary.  The  new  gravel  should  be  finer  and  contain  more  bind- 
ing material  than  that  employed  in  the  original  construction.  If  the 
depression  is  very  shallow,  it  is  wise  to  roughen  the  surface  with  a 
garden  rake  before  adding  the  new  material.  It  is  important  that 
ruts  and  shallow  holes  should  be  filled  as  soon  as  they  appear, 
for  they  will  hold  water,  which  will  soften  the  gravel  bed  and  cause 
the  road  to  wear  rapidly.  At,  say,  every  f  mile  a  small  pile  of  gravel 
should  be  stored  to  be  used  in  filling  depressions. 

If  the  road  surface  becomes  muddy  when  wet,  there  is  an  excess 
of  clay  binder;  and  therefore  a  thin  layer  of  coarse  clean  sand  or  fine 
clean  gravel,  preferably  the  latter,  should  be  added.  On  the  other 
hand,  if  the  surface  shows  a  tendency  to  disintegrate  or  ravel,  there 
is  not  enough  binder;  and  therefore  a  layer  of  clay  should  be  added 
and  harrowed  into  the  gravel.  However,  if  the  surface  does  not  at 
once  set  up,  it  should  not  be  concluded  that  there  is  not  enough 
binder,  for  a  road  that  binds  quickly  is  likely  to  cut  up  badly,  par- 
ticularly when  wet. 

During  this  stage,  all  loose  stones  should  be  removed  from  the 
roadway,  both  for  the  comfort  of  travelers  and  the  good  of  the  road. 

After  the  gravel  has  become  thoroughly  consolidated,  i.  e.,  after 


180  GRAVEL   ROADS  [CHAP.   V 

the  wheels  no  longer  make  even  shallow  ruts,  the  only  care  the 
road  is  likely  to  need  for  several  years  is  to  keep  the  side 
ditches  and  culverts  free  from  weeds  and  floating  trash,  and  to 
attend  to  the  drainage  of  the  surface  when  the  snow  is  melting 
(§  219). 

After  a  time  the  gravel  will  work  out  to  the  sides  of  the  road  too 
far,  and  the  center  will  wear  hollow.  It  will  then  be  necessary  to  use 
a  scraping  grader  (§  155-56)  to  push  the  gravel  back  to  the  center. 
In  doing  this  care  should  be  taken  not  to  scrape  up  the  earth  with  the 
gravel.  A  good  time  to  use  the  grader  is  just  after  a  rain,  when  the 
road  is  soft  and  easily  scraped,  and  when  the  gravel  scraped  to  the 
center  is  in  the  best  condition  to  pack  again.  The  road  should  never 
-be  allowed  to  wear  so  hollow  in  the  center  as  to  interfere  with  the  flow 
of  water  from  the  surface  to  the  side  ditches. 

319.  Sprinkling.     A  gravel  road  with  clay  binder  needs  a  little 
moisture  to  hold  it  together,  since  the  clay  shrinks  and  cracks  under 
excessive  drought,  loses  its  binding  power,  and  permits  the  road  to 
break  to  pieces.     Under  such  circumstances  a  sprinkling  with  water 
is  a  means  of  preserving  the  road  from  serious  damage,  although 
on  account  of  the  expense  this  is  seldom  done  except  on  park 
drives. 

320.  Re-Surfacing.     It  will  finally  be  necessary  to  repair  the 
surface  by  adding  a  coating  of  new  gravel.     For  this  purpose  the  size 
of  the  largest  pebbles  should  vary  with  the  thickness  of  the  coat.     It  is 
usual  to  put  the  gravel  on  by  making  two  or  three  dumps  of  a  wagon 
load,  i.  e.,  by  stretching  a  cubic  yard  over  15  to  25  linear  feet,  accord- 
ing to  the  thickness  of  layer  required,  and  spreading  the  gravel  just  a 
little  wider  than  the  wagon  track.     Traffic  will  spread  it  still  wider, 
and  also  pack  it. 

In  making  repairs,  it  is  better  to  apply  a  thin  coat  often  than  a 
thicker  coat  less  frequently,  since  a  thick  coating  does  not  pack 
well.  A  layer  of  2  inches  of  gravel  is  better  than  more — unless  on  a 
spot  that  has  cut  through. 

321.  COST.    The  cost  of  maintenance  varies  with   the  climate, 
the  amount  and  nature  of  the  traffic,  the  quality  of  the  gravel,  etc. 
Data  from  Indiana  and  Ohio  show  that  it  varies  from  $40  to  $100 
per  mile  per  annum — the  former  where  the  traffic  is  light,  the  gravel 
good,  and  the  snow  light;  and  the  latter  where  the  traffic  is  heavy, 
the  gravel  poor,  and  the  snow  heavy.     In  New  Hampshire  the  cost 
is  $20  to  $100,  usually,  $20  to  $50  per  mile  per  year  exclusive  of  re- 
surfacing; or  including  re-surfacing,  $150  to  $300  per  mile  per  year 


ART.    4]  DUST   PALLIATIVES  181 

for  all  expense.*     In  Vermont  the  average  cost  of  maintaining  175 
miles  in  1912  varied  from  $10.97  to  $32.33,  the  average  being  $20.71. 

ART.  4.     DUST  PALLIATIVES 

323.  The  surfaces  of  gravel  roads  are  treated  for  two  distinct 
purposes, — to  lay  the  dust  and  to  bind  the  surface  materials.     The 
agents  that  accomplish   the  first  are   called  dust  palliatives;    and 
those  that  secure  the  second  are  called  road  binders,  protective 
coatings,  or  bituminous  carpets.     Only  the  first  will  be  considered 
here. 

324.  DU.ST    PREVENTIVES.     The  suppression  of  the  dust  from 
an  earth  road  is  in  the  interests  of  the  people  using  the  road  or  re- 
siding adjacent  to  it;  but  the  prevention  of  dust  on  a  gravel  road  is 
important  not  only  to  the  interests  of  those  using  the  road  or  living 
near  it,  but  also  to  the  very  life  of  the  road  itself.     The  simplest 
materials  used  for  this  purpose  are:    Fresh  water,  salt  water,  deli- 
quescent salts,  proprietary  compounds,  oil,  and  tar.     The  two  latter 
are  the  most  common,  and  are  used  primarily  as  road  binders,  al- 
though they  incidentally  prevent  the  binder  of  the  gravel  from  being 
blown  away  as  dust. 

325.  Sprinkling  with  Fresh  Water.     This  is  the  simplest  method 
of  preventing  dust.     Sprinkling  a  gravel  road  with  water  not  only 
suppresses  the  dust,  but  prevents  the  disintegration  of  the  surface  by 
raveling  (see  §  397).     The  water  should  be  applied  in  a  fine  spray, 
and  in  such  quantities  as  not  to  run  in  streams  on  the  surface;  that 
is,  several  light  sprinklings  are  better  than  a  single  flooding.     If 
sprinkled  too  heavily  or  too  often,  the  road  is  softened  and  breaks 
up  easily. 

Reliable  and  definite  data  concerning  the  cost  of  sprinkling  are 
rather  meager.  Dust  may  usually  be  kept  down  on  a  gravel  road 
carrying  a  moderate  amount  of  heavy  travel,  by  sprinkling,  for  about 
2  to  3  cents  per  square  yard  per  annum. 

326.  Sprinkling  with  Sea  Water.     This  is  a  simple  remedy,  but 
obviously  is  applicable  only  to  roads  located  near  the  sea  coast.     Sea 
water  is  more  effective  in  laying  dust  than  fresh  water  owing  to  cer- 


*  From  an  instructive  account  of  the  patrol  system  of  maintenance  employed  by  the  New 
Hampshire  Highway  Department,  in  Engineering  News,  Vol.  74  (1915),  p.  1110.  For  an 
article  explaining  the  use  of  gasoline  motor  trucks  and  trailers  in  maintaining  gravel  roads  in 
Alabama  and  giving  data  on  the  cost  of  the  work,  see  Engineering  Record,  Vol.  74  (1916),  p, 
73-74. 


182  GRAVEL    ROADS  [CHAP.    V 

tain  deliquescent  salts  which  it  contains;  but  the  presence  in  the 
sea  water  of  salts  not  possessing  hygroscopic  properties  causes  disa- 
greeable and  destructive  mud  in  wet  weather,  and  renders  this  form 
of  treatment  rather  unsatisfactory. 

327.  Moistening  with  Deliquescent  Salts.     Solutions  of  water 
and  various  deliquescent  salts  have  been  used  to  moisten  the  surface 
of  roads  to  prevent  dust.     The  effect  of  these  solutions  is  more  lasting 
than  that  of  fresh  water  alone,  and  they  are  easily  applied  by  the 
ordinary  sprinkling  wagon.     However,   some  difficulty  is  encoun- 
tered in  obtaining  a  solution  of  constant  strength,  and  its  cost  is 
considerable. 

Among  the  most  common  of  these  salts  is  calcium  chloride, 
which  may  be  obtained  commercially  in  a  granular  condition  or  in  a 
concentrated  solution.  The  granular  salt  may  be  applied  with  an 
ordinary  agricultural  drill.  When  applied  in  granular  form  about 
three  fourths  of  a  pound  per  square  yard  is  used  for  the  first  applica- 
tion, and  slightly  less  for  succeeding  ones.  When  applied  in  liquid 
form,  a  15  per  cent  solution  is  ordinarily  used  for  the  first  application, 
and  for  successive  applications  an  8  or  10  per  cent  solution  is  em- 
ployed. Of  course,  such  salts  are  not  suitable  in  arid  or  semi-arid 
regions,  since  there  is  but  little  moisture  in  the  air  to  be  absorbed; 
nor  in  an  extremely  humid  climate,  since  the  salt  is  likely  to  be 
washed  away  by  the  rains.  Calcium  chloride  is  odorless  and  clean, 
but  has  no  permanent  effects  upon  the  road.  It  may  cause  soreness  in 
horses'  feet,  if  used  in  quantities  much  in  excess  of  those  stated  above. 

328.  Sprinkling    with    Proprietary    Compounds.     There    are    a 
number  of  proprietary  dust-laying  compounds  upon  the  market. 
Most  of  them  consist  of  deliquescent  salts  dissolved  in  water,  and 
some  consist  of  by-products  from  manufacturing.     The  names  of  a 
few  of  the  first  class  are:    Aconia,  calcite,  and  panscale.     Several  of 
them  seem  to  be  no  more  efficient  than  calcium  chloride,  but  are  con- 
siderably more  expensive. 

329.  Sprinkling  with  Light  Oil.     The  dust  of  a  gravel  road  may 
be  laid  by  sprinkling  it  with  oil  much  as  was  described  for  an  earth 
road  (§  236-39);   but  it  is  usually  more  economical  to  apply  a  pro- 
tective coating  or  bituminous  carpet  (Art.  1  and  2  of  Chapter  IX) 
which  not  only  prevents  to  a  certain  degree  the  formation  of  dust, 
but  protects  the  surface  of  the  road. 

Oil  as  a  dust  layer  was  used  chiefly  on  park  drives  and  suburban 
roads,  but  has  been  abandoned  where  there  is  any  considerable 
amount  of  automobile  travel.  The  following  account  from  the  pre- 


ART.   4]  DUST   PALLIATIVES  183 

vious  edition  of  this  volume  describes  the  former  practice  in  Wash- 
ington, D.  C. 

330.  Practice  in  Washington,  D.  C.  The  city  of  Washington, 
D.  C.,  formerly  sprinkled  the  gravel  park  drives  with  the  light  as- 
phaltic  oil  described  in  §  552.  The  following  is  a  description  of  the 
method  then  employed  in  applying  the  oil.* 

"  All  ruts  and  holes  in  the  surface  of  the  road  are  first  repaired 
by  cleaning  out  the  cavity,  filling  it  with  coarse  stone  which  is  cov- 
ered with  a  coating  of  hot,  heavy,  asphaltic  oil,  then  sprinkling  a 
light  coat  of  screenings  over  the  oil,  and  finally  compacting  the 
patch  by  ramming.  When  all  holes  have  been  thus  repaired,  the 
surface  of  the  road  is  thoroughly  cleaned  with  rattan  brooms,  care 
being  taken  to  remove  all  loose  materials  and  caked  dirt  or  dust  so 
that  the  stone  forming  the  wearing  surface  of  the  road  shall  be 
exposed  and  clean. 

"  When  the  road  is  entirely  free  from  moisture,  and  during  warm 
dry  weather,  if  possible,  a  light  asphaltic  oil  is  spread  (without  being 
heated)  by  means  of  special  sprinkling  wagons.  One  third  to  one 
half  gallon  of  oil  to  the  square  yard  usually  forms  the  first  applica- 
tion. To  allow  it  to  penetrate  into  the  surface,  the  road  is  closed  to 
traffic  at  least  48  hours  after  the  first  application. 

"At  the  end  of  this  time  the  surface  of  the  road  is  covered  with 
a  thin  coating  of  clean,  coarse,  sharp  sand  or  stone  screenings,  free 
from  dust ;  and  is  then  rolled  and  traffic  allowed  to  go  over  it.  A 
cubic  yard  of  sand  or  screenings  usually  covers  from  75  to  125  square 
yards  of  road  surface. 

"  The  oiling  described  above  keeps  the  surface  in  excellent  con- 
dition for  a  year.  It  is  never  dusty,  and  is  muddy  for  only  a  few 
hours  after  a  heavy  thaw  when  the  skid-chains  of  automobiles  tear 
up  the  surface.  The  subsequent  passage  of  automobiles  without 
chains  soon  irons  out  the  roadway.  At  the  end  of  the  year  the  sur- 
face of  the  road  is  again  thoroughly  cleaned,  from  \  to  J  gallon  of  oil 
to  the  square  yard  under  normal  conditions  being  spread  over  it,  and 
the  road  closed  for  48  hours  and  covered  with  sand  or  screenings 
as  before.  This  treatment  is  continued  from  year  to  year." 

"  The  cost,  for  the  first  application,  from  2.8  to  4.6  cents  per 
square  yard;  and  for  the  second  application  from  1.3  to  2.8  cents  per 
square  yard." 

*  Paper  by  Col.  Spencer  Cosby,  U.  S.  Army,  in  Charge  of  Buildings  and  Grounds,  Wash- 
ington, D.  C.,  presented  before  Section  D'of  the  American  Association  for  the  Advancement 
of  Science,  on  Dec.  29,  1911. 


184  GRAVEL   ROADS  [CHAP.    V 

331.  The  amount  of  oil  applied  is  often  considerably  greater  than 
that  employed  in  Washington,  D.  C.,  as  mentioned  above,  being 
from  0.3  to  0.4  gallon  per  square  yard,  in  which  case  the  total  cost  of 
material  and  labor  is  from  4.0  to  7.0  cents  per  square  yard.  When 
the  quantity;required  is  as  great  as  this,  it  is  probable  that  the  ex- 
pense is  not  justifiable,  since  instead  of  spending  the  larger  sum  for  a 
light  oil,  it  is  more  economical  to  spend  a  little  greater  amount  for 
a  heavier  oil  or  for  a  stronger  binding  material,  and  construct  a  more 
durable  protective  coating  (see  §  583). 


CHAPTER  VI  > 
WATER-BOUND  MACADAM  ROADS 

334.  Throughout  the  entire  nineteenth  century  a  road  built 
by  placing  small  fragments  of  broken  stone  on  the  ground  and  com- 
pacting them  into  a  solid  mass  by  rolling  or  by  travel  was  called  a 
macadam  road,  after  John  Loudon  MacAdam  (1756-1836),  a  famous 
English  builder  of  broken-stone  roads  and  one  of  the  first  to  build 
such  roads.  The  broken  stone  is  called  macadam,  and  the  work  of 
construction  macadamizing. 

A  broken-stone  road  is  sometimes  called  a  telford  road  after 
Thomas  Telford  (1757-1834),  a  famous  English  engineer;  but  the 
term  telford  is  usually,  and  appropriately,  restricted  to  a  particular 
form  of  the  foundation  of  a  broken-stone  road  (§  349). 

The  fragments  of  stone  in  the  road  referred  to  above  were  held 
together  by  the  cementing  power  of  the  dust  of  the  stone;  but  the 
use  of  the  automobile  has  shown  the  desirability  of  a  broken-stone 
road  having  a  binder  stronger  than  stone  dust;  and  this  led  in  the 
early  years  of  the  twentieth  century  to  the  introduction  of  a  new 
type  of  broken-stone  road — one  bound  with  a  bituminous  cement,  such 
as  tar  or  asphalt.  Such  a  road  is  called  a  bituminous  madacam  road ; 
and  consequently  the  broken-stone  road  having  a  stone-dust  binder 
is  now  called  a  water-bound  macadam  road. 

For  more  than  a  hundred  years  the  water-bound  macadam  was  a 
leading  form  of  improved  road  all  over  the  world,  and  even  now  it 
is  exceeded  in  mileage  only  by  earth  and  gravel  roads;  but,  since  the 
early  years  of  this  century,  it  is  much  less  frequently  built  than 
formerly.  It  is  used  now  only  in  rural  roads  having  comparatively 
little  motor-driven  travel  and  on  residence  streets  having  but  little 
through  travel.  In  some  particulars  the  water-bound  macadam 
road  will  be  discussed  a  little  more  fully  than  its  individual  merits 
may  warrant,  because  many  of  the  principles  of  its  construction  are 
applicable  to  bituminous  roads,  which  are  becoming  increasingly 
important. 

185 


186  WATER-BOUND   MACADAM   ROADS  [CHAP.   VI 


ART.  1.    THE  STONE 

335.  REQUISITES  FOR  ROAD  STONE.     The  principal  requisites 
of  a  material  for  a  water-bound  macadam  road  are  hardness,  tough- 
ness, cementing  or  binding  power,  and  resistance  to  the  weather. 
Usually  any  stone  that  is  hard  and  tough  will  resist  the  weather 
reasonably  well;  but  shales  and  slates,  though  hard  and  tough  when 
first   quarried,   often   disintegrate   when   exposed   to   the   weather. 
The  material  for  a  road  surface  should  also  be  uniform  in  quality 
or  the  surface  will  wear  unevenly;  and  the  depressions  which  occur 
where  the  material  is  comparatively  soft  will  hold  water,  thus  soften- 
ing the  road-bed  and  occasioning  damage  difficult  to  repair. 

336.  Hardness  and  Toughness.     These  two  qualities  are  closely 
related.     Hardness  is  that  property  of  a  solid  which  renders  it  dif- 
ficult to  displace   its   parts   among   themselves;    while   toughness 
enables  the  parts  to  yield  somewhat  without  being  separated  or 
broken.    For  road  purposes,  hardness  is  the  power  possessed  by 
a  rock  to  resist  the  rubbing  or  the  abrasive  action  of  wheels  and 
horses'  feet;    while  toughness  is  the  adhesion  between  particles  of 
a  rock  which  gives  it  power  to  resist  fracture  when  subjected  to 
blows.     A  stone  may  be  hard  and  brittle,  and  be  quickly  pounded  to 
pieces  in  the  road,  as  quartz;  or  it  rr  ay  ha\ e  a  high  crushing  strength 
and  yet  be  deficient  in  toughness,  and  grind  away  speedily  under  the 
abrasion  of  traffic,  as  some  \arieties  of  sandstones.     A  road  metal 
should  have  enough  resistance  to  crushing  to  support  the  load  brought 
upon  it  by  the  wheels,  and  enough  toughness  to  prevent  its  being 
readily  ground  into  powder.     A  large  part  of  the  fine  material  is 
inevitably  swept  away  by  the  rains  and  winds,  or  is  removed  by 
scrapers  to  keep  the  road  in  good  condition  during  wet  weather; 
and  therefore  it  is  important  that  the  fragments  should  be  tough 
enough  not  to  be  unduly  pulverized  by  travel.     Toughness  is  incom- 
patible with  a  high  degree  of  hardness,  and  in  a  measure  makes  up 
for  a  deficiency  in  resistance  to  crushing.     Hardness  could  be  meas- 
ured by  the  resistance  offered  by  a  rock  to  the  grinding  of  an  emery 
wheel;   and  toughness  would  be  measured  by  the  resistance  to  frac- 
ture when  struck  with  a  hammer. 

337.  Cementing  or  Binding  Power.     Binding  power  is  the  prop- 
erty possessed  by  rock  dust  to  act  as  a  cement  between  the  coarser 
fragments  composing  a  stone  road.     This  property  is  of  the  highest 
value,  for  the  strength  of  the  binder  determines  the  resistance  of  the 


AJRT.    1]  THE    STONE  1S7 

road  to  the  wear  and  tear  of  travel  more  than  does  the  strength  of 
the  fragments  themselves.  It  is  possessed  in  a  very  much  higher 
degree  by  some  varieties  of  rocks  than  by  others,  and  its  absence  is 
so  pronounced  in  some  varieties  that  they  can  not  be  made  to  com- 
pact under  the  roller  or  under  traffic  without  the  addition  of  some 
cementing  agent. 

338.  METHODS  OF  TESTING  STONE.     There  are  two  methods 
of  determining  the  qualities  of  a  stone  for  road-building  purposes: 
(1)  by  using  the  stone  in  the  road  and  keeping  an  account  of  the 
cost  of  repairs  over  a  series  of  years,  or  (2)  by  laboratory  experi- 
ments.    The  first  is  uncertain  owing  to  the  variations  in  climatic 
conditions,  and  in  the  amount  and  nature  of  the  traffic,  etc.;  and 
would  be  very  expensive  and  take  a  long  time.     In  the  second 
method  of  testing,  it  is  difficult  to  duplicate  in  the  laboratory  the 
conditions  of  actual  service;   but  nevertheless  much  valuable  infor- 
mation may  thus  be  obtained  at  a  moderate  expense  and  in  a  com- 
paratively short  time. 

Systematic  laboratory  tests  of  road  metal  are  of  comparatively 
recent  origin,  and  may  be  said  to  have  been  started  about  1880  by 
the  French  governmental  engineers,  who  have  made  extensive  use 
of  this  method  in  determining  the  quality  of  the  rock  used  in  con- 
tract work  and  in  selecting  new  quarries.  Only  a  little  such  labora- 
tory work  has  been  done  in  England  and  Germany.  From  1894  to 
1899  the  Massachusetts  Highway  Commission  conducted  a  series  of 
tests  of  road-making  materials,  and  developed  new  and  important 
methods  of  testing,  and  deduced  much  valuable  information. 

339.  Since  the  latter  date  the  U.  S.  Office  of  Public  Roads  and 
Rural  Engineering  has  conducted  extensive  laboratory  tests  of  the 
road-building  stones.     Bulletin  No.  370  (1916)  describes  the  methods 
employed  and  gives  the  results  obtained  for  3,650  samples  from  forty 
states   and   three   foreign    countries.     The  Laboratory  tests  road 
materials  for  public  officials  without  cost;  and  also  gives  advice  as  to 
the  value  of  the  material  for  road-building  purposes. 

340.  The  principal  tests  applied  to  stone  for  water-bound  mac- 
adam roads  will  be  very  briefly  described. 

341.  Hardness  Test.     This  test  determines  the  hardness  of  the 
stone;    and  virtually  consists  in  measuring  the  amount  ground  off 
under  certain  conditions.     The  loss  in  the  above  tests  varied  from 
1.0  to  32.8  per  cent,  usually  from  2  to  10.     The  test  is  made  with  the 
Dorry  machine. 

342.  Toughness  Test.     This  test  is  to  determine  the  resistance  to 


188 


WATER-BOUND    MACADAM    ROADS 


CHAP.    VI 


impact.  It  is  made  by  finding  the  number  of  hammer-like  blows 
required  to  break  a  cylindrical  specimen.  The  results  range  from  3 
to  43,  about  half  of  the  materials  requiring  from  10  to  20  blows. 
The  test  is  made  with  the  Page  impact  machine. 

343.  Abrasion  Test.     The  results  of  this  test  depend  upon  both 
hardness  and  the  resistance  to  abrasion.     Fragments  of  the  stone  are 
rotated  in  a  cylinder  inclined  to  the  axis  of  rotation,  and  the  amount 
worn  off  is  determined.     The  result  is  expressed  either  in  per  cent  of 
wear  or  in  the  arbitrary  French  coefficient  of  wear,  which  equals 
40  divided  by  the  per  cent  of  wear.     The  latter  is  in  more  common 
use.     The  French  coefficient  of  wear  for  the  test  referred  to  in  §  339 
ranges  from  1.4  to  41.7,  usually  from  10  to  30.     The  test  is  made  with 
a  Deval  machine. 

344.  Cementation  Test.     This  test  determines  the  binding  power 
or  cementing  value  of  the  stone  dust  to  hold  together  the  coarser 
fragments   of  a  water-bound  macadam  road.     This   is   the  most 
important  quality  of  a  stone  for  such  a  road.     The  test  is  made  by 
placing  small  fragments  of  stone  and  water  in  a  ball  mill  and  grinding 
them  to  a  stiff  paste,  which  is  then  moulded  into  a  briquette  under 
heavy  pressure.     The  briquette  is  dried  and  tested  in  an  impact 
machine  to  determine  the  number  of  blows  required  to  break  it. 
The  results  range  from  0  to  over  500.     Values  below  10  are  called 
low;  from  10  to  25,  fair;  from  25  to  75,  good;  from  75  to  100,  very 
good;  and  above  100,  excellent.    The  results  for  a  few  stones  are  as 
follows: 


NAME. 

MAX. 

MlN. 

NAME. 

MAX. 

Mm. 

Andesite 

500+ 

9 

Gravel 

500 

3 

Basalt                  ...    . 

500+ 

2 

Limestone 

500 

9 

Chert  

500  + 

2 

Marble 

85 

10 

Conglomerate  

500+ 

20 

Quartzite 

45 

o 

Diabase 

500+ 

2 

Sandstone 

500  + 

1 

Diorite. 

148 

5 

Shale 

367 

28 

Granite 

255 

2 

Slate 

500+ 

1 

345.  Conclusion.  The  principal  rocks  used  for  water-bound 
macadam  roads  are  traps  (a  popular  term  which  includes  diabase, 
diorite,  and  several  other  igneous  rocks),  granites,  and  limestones. 
Their  value  for  road-building  purposes  is  in  the  order  named.  In 
the  Eastern  States  the  traps  are  the  ones  most  used,  in  the  Missis- 
sippi Valley  limestone  is  the  most  common. 

For  information  concerning  the  road-building  materials  of  the 


ART.    2]  CONSTRUCTION  189 

United  States,  see  Preliminary  Report  on  the  Geology  of  the  Common 
Roads  of  the  United  States,  by  Nathaniel  Southgate  Shaler,  in  U.  S. 
Geological  Survey,  Fifteenth  Annual  Report,  1893-94,  p.  255-306. 

ART.  2.    CONSTRUCTION 

347.  The  principles  of  construction  for  earth  roads  apply  also  to 
the  construction  of  the  subgrade  for  broken-stone  roads  (see  Art. 
1,  Chapter  III).     The  drainage  of  the  foundation  by  tile  drains  and 
side  ditches  should  not  be  neglected  (see  §  114-24  and  §  125-28). 

348.  FORMS  OF  CONSTRUCTION.     With  reference  to  the  method 
of  preparing  the  subgrade  to  receive  the  stone,  there  are  two  forms 
of  construction — surface  construction  and  trench  construction.     The 
surface  construction  consists  simply  in  placing  a  layer  of  broken 
stone  upon  the  earth  road  and  leaving  it  to  be  compacted  by  travel. 
In  the  West  many  miles  of  road  are  constructed  on  this  plan  with 
limestone.     As  a  rule  this  material  readily  pulverizes  under  the 
traffic,  and  the  powder  cements  well;    consequently  the  road  soon 
binds  together.     Such  roads  are  not  first  class,  but  they  give  good 
returns  on  their  cost.     On  account  of  the  simplicity  of  the  con- 
struction, this  form  will  not  be  considered  further. 

The  trench  construction  consists  in  excavating  a  trench  of  the 
required  width  and  depth,  and  depositing  the  broken  stone  in  it. 

349.  With  reference  to  the  lower  course  of  stone  there  are  two 
systems  of  construction, — the  macadam  and  the  telford  (§  334), 
The  macadam  road  consists  of  two  or  more  layers  of  crushed  stone, 
its  distinguishing  characteristic  being  that  the  lower  course  of  crushed 
stone  is  placed  directly  upon  the  earth  road-bed.     The  telford  road 
consists  of  a  foundation  or  pavement  of  rough  stone  blocks  set  upon 
the  road-bed,  covered  with  one  or  more  layers  of  crushed  stone,  the 
distinguishing  feature  being  the  paved  foundation. 

350.  Telford  vs.  Macadam  Roads.     Each  of  these  systems  has 
its  earnest  advocates  who  contend  for  its  exclusive  use. 

The  most  important  claims  of  the  advocates  of  the  telford  con- 
struction are  (1)  that  the  open  foundation  is  necessary  for  drainage; 
(2)  that  the  sub-pavement  is  necessary  on  soft  or  poorly  drained  soil 
to  prevent  the  small  fragments  of  broken  stone  from  working  down 
into  the  soil  and  the  soil  from  working  up  into  the  stone;  and  (3) 
that  the  telford  is  the  cheaper,  since  the  expense  of  crushing  is  saved. 

The  most  important  claims  of  the  advocates  of  the  macadam 
system  are:  (1)  that  the  drainage  afforded  by  the  telford  construe- 


190  WATER-BOUND   MACADAM   ROADS  [CHAP.    VI 

tion  is  no  better  than  that  with  the  macadam  construction;  (2) 
that  on  any  well  drained  soil  there  is  no  tendency  of  the  stone  to 
work  down  or  of  the  soil  to  work  up;  (3)  that  tile  drainage  and 
macadam  construction  are  cheaper  than  the  telford  system;  and 
(4) .  that  since  the  introduction  of  the  machine  rock-breaker,  it  is 
cheaper  to  crush  the  stone  and  lay  the  macadam  foundation  than 
to  place  the  telford. 

The  view  taken  by  different  road  builders  in  this  matter  is  prob- 
ably largely  due  to  the  conditions  in  the  vicinity  in  which  they  have 
worked  and  to  the  skill  with  which  the  two  systems  have  been  applied 
in  work  which  has  come  under  their  observation.  The  foundation 
which  is  proper  in  a  given  case  is  determined  by  the  nature  and  con- 
dition of  the  soil  upon  which  it  is  constructed.  If  the  road-bed  is 
thoroughly  drained  and  is  composed  of  material  which  will  not  readily 
soften,  there  will  be  no  need  of  a  telford  foundation.  If,  on  the  other 
hand,  the  soil  is  retentive  of  moisture  and  can  not  be  thoroughly 
drained,  it  may  be  necessary  to  provide  a  foundation  which  will 
•prevent  the  soil  from  working  up  into  the  stone  and  the  road  metal 
from  working  down  into  the  soil. 

To  MacAdam  is  due  the  credit  of  discovering  the  supporting 
power  of  a  layer  of  comparatively  small  angular  fragments  of  stone. 

351.  Forms  of  the  Subgrade.  The  finished  surface  of  the  road 
should  have  sufficient  crown  to  shed  the  rain  water  into  the  side 
ditches.  There  are  in  common  use  two  methods  of  securing  this 
crown.  In  one  the  earth  surface  is  made  level,  and  the  slope  is  given 
by  a  greater  thickness  of  metaling  at  the  center  than  at  the  sides;  in 
the  other,  the  slope  or  camber  is  given  to  the  earth  bed,  and  the  metal 
has  a  uniform  thickness.  The  advocates  of  the  first  system  say  that 
there  is  more  wear  at  the  center  than  at  the  sides,  and  that  conse- 
quently the  metaling  should  be  thicker  at  the  center.  Those  in 
favor  of  the  uniform  thickness  say  that  as  the  pressure  on  the  earth 
is  practically  the  same  at  the  sides  as  at  the  center,  the  thickness 
should  be  uniform,  since  the  principal  object  of  the  layer  of  stone 
is  to  distribute  the  concentrated  pressure  of  the  wheel  over  a  greater 
surface  of  the  earth  bed.  Both  forms  of  construction  are  in  common 
use,  although  the  preference  seems  to  be  slightly  in  favor  of  making 
the  subgrade  parallel  to  the  finished  road-surface  and  the  stone  of 
uniform  thickness.  A  level  subgrade  is  slightly  cheaper  to  form. 

Fig.  46  shows  a  cross  section  of  the  celebrated  Shrewsbury 
and  Holyhead  road  in  the  west  of  England,  built  by  Telford 
in  1815.  The  construction  of  this  road,  which  formed  a  link  in 


ART.    2]  CONSTRUCTION 


the  direct  line  of  communication  between  England  and  Ireland, 
was  made  a  national  undertaking,  and  resulted  in  what  was  at  that 


FIG.  46.  —  TELFOKD'S  SHREWSBURY  AND  HOLYHEAD  ROAD. 

time  one  of  the  finest  pieces  of  road  construction  in  the  world.  Notice 
that  the  subgrade  is  flat. 

Fig.  47  shows  a  modern  telford  road  as  built  in  New  Jersey. 


7/7 


FIG.  47. — MODERN  TELFORD  ROAD  AS  BUILT  IN  NEW  JERSEY. 

Notice  that  the  base  of  the  foundation  is  parallel  to  the  -surface  of 
the  finished  road. 

Compare  the  above  with  Fig.  51-56,  pages  197-99. 

352.  WIDTH.     For  a  discussion  of  the  principles  governing  the 
the  width  of  the  improved  way,  and  also  whether  it  shall  be  in  the 
center  or  at  the  side  of  the  wheelway,  see  §  95-98. 

353.  Shoulders.     The  discussion  referred  to  above  deals  only 
with  the  width  of  the  paved  portion;  but  there  should  be  an  addi- 
tional width  of  earth  sufficient  to  keep  the  broken  stone  in  place, 
particularly  while  being  rolled.     This  strip  of  earth  is  usually  called 
a  shoulder,  but  sometimes  and  improperly  a  wing  (see  §  364).     The 
proper  width  of  the  shoulder  will  depend  upon  the  soil,  the  climate, 
and  the  amount  of  rolling  it  receives.     Usually  2  or  3  feet  is  suf- 
ficient, although  5  to  7  feet  is  frequently  provided — see  Fig.  47. 
The  Swiss  road  shown  in  Fig.  53,  page  198,  has  a  shoulder  of  only  18 
inches.    An  excessive  width  of  shoulder  adds  greatly  to  the  cost  of  the 
road  when  in  excavation  or  on  embankment.     The  surface  of  the 
shoulder  should  conform  to  the  general  curve  of  the  finished  road- 
way.    The  shoulder  serves  the  double  purpose  of  holding  the  broken 
stone  in  place  and  of  affording  room  for  vehicles  to  pass  each  other. 
To  improve   the  shoulders  for  the  second  purpose,  they  are  some- 
times covered  with  a  thin  coat  of  gravel  to  harden  the  surface. 
Sand  shoulders  are  speedily  hardened  by  the  infiltration  of  fine  stone 


192  WATER-BOUND   MACADAM   ROADS  [CHAP.   VI 

dust  and  dirt  washed  from  the  surface  of  the  road.  This  effect  is 
quite  noticeable  with  coarse  sand;  and  is  appreciable  even  with  fine 
sand. 

354.  CROWN.     The  center  of  the  road  should  be  higher  than  the 
sides,  so  that  the  water  from  rains  may  flow  rapidly  into  the  side 
ditches.     If  originally  too  flat,  the  road  is  soon  worn  hollow,  and 
the  middle  becomes  a  pool  if  on  level  ground,  or  a  water  course 
if  on  an  inclination.     In  the  former  case  the  middle  of  the  road  is 
sloppy;  and  in  the  latter,  the  fine  material  washes  away  and  leaves 
the  larger  stones  bare.    There  has  been  much  discussion  both  as  to 
the  proper  amount  of  crown  and  the  exact  form  of  the  transverse 
profile  of  the  roadway. 

355.  Form  of  the  Crown.     Some  claim  that  the  upper  surface 
should  be  curved,  and  others  that  it  should  be  two  inclined  planes 
meeting  at  the  center  of  the  road  and  having  their  angle  slightly 
rounded  off.    Both  forms  are  in  common  use;  the  first  is  the  more 
common,  but  apparently  the  latter  is  the  better. 

The  following  objections  are  urged  against  the  curved  profile: 
1.  The  greater  slope  near  the  side  causes  vehicles  to  seek  the  center, 
and  consequently  the  road  wears  unequally.  2.  Owing  to  the  excess 
of  travel  at  the  center,  the  road  soon  wears  hollow  and  holds  water, 
which  is  both  unsightly  and  a  damage  to  the  road.  3.  The  slope 
is  so  slight  near  the  center  that  a  small  settlement  of  the  subgrade 
causes  a  depression  of  the  surface,  which  holds  water. 

The  only  objection  to  a  surface  composed  of  two  planes  is  that 
the  flanks  wear  hollow  and  hold  water;  but  this  objection  has  less 
force  than  any  of  the  three  against  the  curved  profile. 

Regularity  and  evenness  of  crown  is  more  important  than  the 
mathematical  form  of  the  cross  section.  A  slight  depression  be- 
comes very  conspicuous  when  filled  with  water;  and  besides  the 
water  standing  upon  the  surface  softens  it  and  tends  to  increase 
the  depression.  With  a  little  care  in  filling  the  low  places  devel- 
oped during  the  rolling,  it  is  possible  to  build  a  broken-stone  road 
with  an  almost  mathematically  exact  crown. 

356.  Amount  of  Crown.     The  proper  amount  of  crown  depends 
chiefly  upon  the  method  of  making  repairs.     If  new  material  is 
added  only,  say,  each  second  or  third  time  the  surface  is  smoothed 
up,  then  the  crown  should  be  greater  to  compensate  for  future 
wear;    but  if  new  material  is  added  practically  continuously,  the 
crown  may  be  considerably  smaller.     The  rate  of  transverse  slope 
should  be  smaller  on  wide  than  on  narrow  streets,  to  prevent  the 


AUT.   2]  CONSTRUCTION  193 

water  from  unduly  washing  the  surface  near  the  sides.  There 
should  be  more  crown  on  steep  grades  than  on  flat  ones,  to  throw 
the  water  quickly  to  the  side  ditch  and  to  prevent  it  from  flowing 
down  the  grade  on  the  surface  of  the  road  and  washing  out  the 
binder. 

Sometimes  wide  boulevards,  with  curved  profile  and  maintained 
by  continuous  repairs,  have  a  crown  of  one  sixtieth  of  the  width,  or  a 
rise  of  0.4  inch  per  foot  from  side  to  center,  or  an  average  slope  of 
1  in  30.  The  French  roads,  which  have  a  curved  profile  and  are 
maintained  by  the  system  of  continuous  repairs,  have  a  crown  of  one 
fiftieth  of  their  width,  or  a  rise  from  side  to  center  of  0.5  inch  per 
foot  or  a  slope  of  1  in  25.  Many  of  the  better  cared  for  streets  and 
park  drives  have  a  crown  of  one  fortieth,  or  a  rise  from  side  to  center 
of  0.6  inch  per  foot  or  an  average  slope  of  1  in  20.  On  the  state-aid 
roads  in  Massachusetts  (narrow  roads  and  continuous  repairs),  the 
surface  consists  of  two  planes  meeting  in  the  center,  the  transverse 
slope  being  f  inch  to  a  foot  or  1  in  16.  Broken-stone  roads  made  of 
soft  stone  and  maintained  by  periodic  repairs  frequently  have  an 
original  crown  of  one  twelfth — an  average  slope  of  1  inch  to  1  foot 
or  1  in  12. 

357.  With  a  broken-stone  road,  the  method  of  making  repairs 
has  more  weight  in  determining  the  amount  of  the  crown  than  in  the 
case  of  either  an  earth  road  or  a  gravel  road.  The  earth  road  is  easily 
and  cheaply  maintained  by  what  may  be  called  the  system  of  con- 
tinuous repairs  with  the  road  drag,  which  restores  or  rather  main- 
tains the  crown.  With  a  gravel  road,  when  it  is  necessary  to  restore 
the  crown  by  adding  more  gravel,  it  is  usually  sufficient  to  put  on 
only  a  thin  layer  and  wait  a  comparatively  short  time  for  travel  to 
consolidate  it.  With  a  water-bound  macadam  road,  if  the  crown  or 
rather  the  surface  is  to  be  perpetually  maintained,  it  is  necessary  to 
keep  a  man  upon  a  short  stretch  of  the  road  practically  all  of  the 
time,  adding  thin  patches  of  stone  in  first  one  place  and  then  another, 
a  method  so  expensive  that  it  is  practiced  in  this  country  only  on 
park  drives,  boulevards,  etc.;  and  if  the  crown  is  to  be  restored 
periodically,  it  is  necessary  to  add  a  considerable  layer  of  stone  and 
consolidate  it  by  long  continued  and  expensive  rolling  and  sprinkling, 
and  on'  account  of  the  expense  of  this  operation  and  the  obstruction 
to  traffic  it  is  customary  to  lay  such  a  thickness  of  stone  and  to  give 
the  surface  such  a  crown  as  not  soon  to  require  a  repetition  of  the 
process.  Therefore  it  happens  that  broken-stone  roads  are  often 
built  with  a  crown  nearly,  if  not  quite,  equal  to  that  of  gcxxl 


194  WATER-BOUND   MACADAM   ROADS  [CHAP.    VI 

earth  roads,  and  with  more  perhaps  than  is  given  to  good  gravel 
roads. 

358.  There  is  a  slight  advantage  of  a  very  high  crown  for  a  broken- 
stone  road,  particularly  for  one  that  is  not  frequently  cleaned.     If 
the  crown  is  great,  the  rains  will  the  better  wash  the  surface.     Dirt 
upon  the  surface  is  not  only  unsightly,  but  is  also  detrimental  since 
it  holds  the  water  and  softens  the  surface.     Of  course  the  material 
washed  by  rains  into  the  gutter  must  eventually  be  removed;  but 
this  can  be  removed  more  cheaply  from  the  gutter  at  comparatively 
long  intervals,  than  from  the  surface  with  brooms  or  scrapers  at 
short  intervals.     The  practice  of  making  a  high  crown  is  somewhat 
common  in  villages  using  soft  road  metal  and  having  earth  gutters 
and  only  surface  drainage. 

This  advantage  of  a  high  crown  is  less  for  a  country  road  than 
for  a  village  street,  since  the  wind  usually  gets  a  better  sweep  at  the 
former  than  at  the  latter. 

359.  Super-elevation  on  Curves.     For  a  rule  for  the  super-eleva- 
tion of  the  road  surface  on  curves,  see  §  90. 

360.  THICKNESS.     The   object   of    placing   a    layer   c:   broken 
stone  upon  the  trackway  is  to  secure  (1)  a  smooth  hard  surface, 
(2)  a  water-tight  roof,  and  (3)  a  more  or  less  rigid  stratum  which 
will  distribute  the  concentrated  pressure  of  the  wheel  over  so  great 
an  area  of  the  subgrade  that  the  soil  can  support  the  load  without 
indentation.     The  smoothness  and  tightness  of  the  surface  depends 
upon  the  quantity  and  quality  of  the  binding  material  (§  383-87), 
and  the  rigidity  of  the  layer  depends  somewhat  upon  the  binder, 
but  chiefly  upon  the  thickness  of  the  stratum.     The  supporting 
power  of  the  subgrade  depends  upon  the  nature  of  the  soil  and  the 
drainage.     Therefore  the  minimum  thickness  of  broken  stone  depends 
upon  the  nature  of  the  soil,  the  drainage,  the  traffic,  and  the  binding 
material;  and  the  initial  thickness  depends  upon  the  amount  of  wear 
permitted  before  new  material  is  added.     If  repairs  are  continuous, 
the  initial  thickness  may  be  the  minimum;   but  if  repairs  are  made 
periodically,  the  initial  thickness  must  be  equal  to  the  minimum 
thickness  plus  the  amount  allowed  for  wear.     After  a  road  has  been 
worn  down  3  inches,  it  is  usually  so  uneven  as  to  require  re-surfacing; 
and  therefore  it  is  uneconomical  if  the  road  in  this  stage  is  much  or 
any  thicker  than  the  minimum  required  to  prevent  its  breaking 
through. 

There  has  been  much  discussion  and  there  is  a  great  difference 
of  opinion  as  to  the  proper  depth  of  a  broken-stone  road.  The 


ART.    2]  CONSTRUCTION  195 

depth  considered  necessary  by  the  most  extreme  advocates  of  thick 
roads  has  decreased  with  the  introduction  of  more  improved  methods 
of  construction  —  particularly  the  use  of  binder  and  a  steam  roller,  — 
and  as  the  advantage  of  thorough  underdrainage  has  been  better 
appreciated.  Early  in  the  last  century,  a  depth  of  18  to  24  inches 
was  frequently  considered  necessary  for  heavy  traffic,  but  later  it 
was  reduced  to  12  or  15  inches,  while  now  6  inches,  or  less,  is  usually 
considered  sufficient. 

361.  Theoretical  Thickness.  The  concentrated  load  of  a  wheel  is 
transmitted  through  the  broken  stone  to  the  earth  in  lines  diverging 
downward,  and  the  wheel  may  be  assumed  as  resting  upon  the  apex 
of  a  cone  whose  base  is  upon  the  earth  subgrade.  It  is  not  wise  to 
attempt  to  find  a  mathematical  relation  between  tht  load  on  the 
wheel  and  the  resulting  pressure  on  the  earth,  since  neither  the 
angle  of  the  cone  nor  the  distribution  of  the  pressure  on  the  base 
of  the  cone  are  known. 

The  Massachusetts  Highway  Commission  assumes  the  concen- 
trated load  to  be  uniformly  distributed  over  an  area  equal  to  the 
square  of  twice  the  thickness  of  the  layer  of  crushed  stone,  which  is 
equivalent  to  assuming  that  the  sides  of  the  cone  make  an  angle  of 
48  \  degrees  with  the  vertical  and  that  the  pressure  is  uniformly  dis- 
tributed over  the  base.  According  to  this  theory,  if  t  =  the  thick- 
ness of  the  stone  in  inches,  w  =  the  maximum  weight  in  pounds 
per  wheel,  and  p  =  the  supporting  power  of  the  soil  in  pounds 
per  square  inch,  then 


The  Commission  has  applied  this  formula  to  roads  already  con- 
structed to  determine  the  safe  bearing  power  of  the  soil,  and  con- 
cludes that  non-porous  soils,  drained  of  -  ground  water,  at  their 
worst  will  support  a  load  of  4  Ib.  per  square  inch,  and  that  sand  and 
gravel  will  safely  support  20  Ib.  per  square  inch.* 

Although  the  method  of  arriving  at  equation  (1)  is  not  correct, 
the  manner  of  deducing  the  supporting  power  of  the  soil  in  a  measure 
offsets  the  error,  and  consequently  the  formula  may  be  used  with 
some  confidence. 

362.  Actual  Thickness.  In  Massachusetts  the  thickness  for 
state-aid  roads  varies  from  4  to  16  inches,  the  standard  for  crushed 

*  Massachusetts  Highway  Commission,  Report  for  1901,  p.  15, 


196  WATER-BOUND    MACADAM    ROADS  [CHAP.    VI 

stone  with  macadam  foundation  on  well-drained  sand  or  gravel  being 
6  inches,  "  which  appears  to  be  ample  for  the  heaviest  traffic." 

In  New  Jersey,  on  state-aid  roads,  the  depth  of  stone  with  mac- 
adam foundation  varies  from  4  to  12  inches,  but  is  generally  6  inches; 
and  the  telford  roads  are  from  8  to  12  inches  thick,  usually  8  inches. 
Most  of  the  roads  have  a  macadam  foundation,  the  telford  being 
used  as  a  rule  only  where  field,  stones  suitable  for  a  telford  foundation 
are  found  alongside  of  the  road. 

363.  The  experience  at  Bridgeport,  Conn.,  has  been  frequently 
cited  to  prove  that  a  comparatively  thin  road  is  sufficient.  Some- 
thing like  60  miles  of  4-inch  macadam  roads  built  in  that  place  gave 
excellent  service  even  under  heavy  traffic.  The  conditions  were 
very  favorable  for  a  thin  road:  (1)  the  soil  was  sand  or  sandy  loam, 
and  had  fairly  good  natural  drainage;  (2)  the  subgrade  was  thor- 
oughly rolled  Vith  a  15-ton  roller;  (3)  the  broken  stone  was  trap, 
which  is  hard  and  durable;  (4)  the  binder  was  hard  and  durable, 
being  either  stone  dust  or  siliceous  sand,  and  was  free  from  clay  or 
loam;  (5)  the  binder  was  worked  in  until  the  voids  in  the  crushed 
trap  were  practically  filled,  the  effect  of  frost  being  thus  reduced  to  a 
minimum  and  the  soil  being  prevented  from  working  up  from  below; 
(6)  the  stone  was  thoroughly  consolidated  with  a  steam  roller  of 
adequate  weight;  and  (7)  the  roads  were  maintained  by  the  system 
of  continuous  repairs. 

The  experience  at  Bridgeport  has  been  repeated  at  several  other 
places;  but  such  experiences  should  be  regarded  as  the  exception, 
rather  than  the  rule,  since  4-inch  roads  are  adequate  only  under 
favorable  natural  conditions  and  with  the  most  painstaking  con- 
struction and  careful  maintenance.  The  fact  that  a  very  thin  road 
can  carry  the  traffic  does  not  prove  that  such  a  road  is  the  most 
economical,  for  the  increased  cost  of  maintenance  may  more  than 
counter-balance  the  decreased  cost  of  construction.  The  engineer 
should  always  attempt  to  construct  economically  and  adapt  his 
construction  to  fit  the  natural  conditions. 

364.  Wings.  In  the  preceding  discussion  of  the  thickness  of 
the  road  metal  it  has  been  assumed  that  the  depth  was  practically 
uniform;  but  some  engineers,  in  recognition  of  the  fact  that  there  is 
less  travel  nearer  the  sides  than  at  the  center,  make  the  thickness  of 
a  strip  on  each  side  considerably  less  than  that  at  the  center.  The 
thin  strips  on  the  sides  are  called  wings.  Fig.  48,  a  portion  of  the 
Swedesboro  road  in  Gloucester  County,  New  Jersey,  shows  a  cross 
section  of  this  form.  This  construction  is  somewhat  common  in 


ART.    2] 


CONSTRUCTION 


197 


New  Jersey,  both  with  telford  and  macadam  foundations,  and  has 
been  adopted  by  the  U.  S.  A.  engineers  for  macadam  roads  in  Porto 
Rico.  The  wings  are  usually  2  or  2|  feet  wide.  A  road  with  wings 


M---  7ft **?25ft^ 9ft 

jrMorzu&n  w/nqs-^^  \ 


—  7ft 


I 

Walk 


wmmmmm 

FIG.  48. — NEW  JERSEY  TELFORD  ROAD  WITH  MACADAM  WINGS. 


FIG.  49. — STANDARD  SECTION  FOR  NEW  YORK  STATE-AID  ROADS. 


is  simply  a  compromise  between  a  narrow  thick  road  and  a  wide 
thin  one. 

365.  EXAMPLES  OF  CROSS  SECTIONS.    Fig.  46,  page  191,  shows 
a  cross  section  of  a  telford  road  built  under  Telford's  direction  in 


FIG.  50. — GENERAL  SECTION  OF  FLUSHING  AND  JAMAICA  ROAD. 


K3/T-  •»*--*-  7/7  6in  -  -  -++*  --7ft  6117 »t<5  ft->] 


Fia.  51. — STANDARD  SECTION  IN  EXCAVATION  FOR  MASSACHUSETTS  STATE-AID  ROADS. 


1815.  Fig.  47,  page  191,  shows  a  New  Jersey  telford  road.  Fig. 
48  shows  a  telford  road  with  macadam  wings.  Fig.  49  shows  the 
standard  cross  section  for  state-aid  roads  in  the  State  of  New  York. 
Fig.  50  is  a  section  of  a  road  in  Flushing,  Long  Island,  near  New  York 


198 


WATER-BOUND  MACADAM  ROADS 


CHAP.  VI 


City.     Fig.  51  and  52,  are  the  standard  cross  sections  in  excavation 
and  on  embankment,  respectively,  for  state-aid  roads  in  Massachu- 


25  ft 


15 ft »K- 4ft - 


Y/////////////////////////////////, 

CROSS  SECTION  OF  ROAD 


-fr  — ^"-H 


LONGITUDINAL  SECTION 
FIG.  52. — STANDARD  SECTION  ON  EMBANKMENT  FOR  MASSACHUSETTS  STATE-AID  ROADS. 


FIG.  53. — CLASS-!!  ROAD,  CANTON  OF  BERN,  SWITZERLAND. 


8M%da  S 


FIG.  54.— CLASS-IH  ROAD,  CANTON  OF  BERN,  SWITZERLAND 


Fio.    55.— TYPICAL  ROAD  IN  DEPARTMENT  OF  BAS-RHIN,  FRANCE. 

setts.    Fig.  53  and  54  show  two  Swiss  roads.     Fig.  55  shows  a  typi- 
cal road  in  the    Department    of   Bas-Rhin,    France.     The    broken 


ART.    2]  CONSTRUCTION  199 

stone  is  6  inches  deep.     Fig.  56  is   a  typical  French  road   in   the 
Department  of  Seine-et-Oise. 


FIG.  56. — TPYICAL  ROAD  IN  DEPARTMENT  OF  SEINE-ET-OISE,  FRANCE. 

366.  PERMISSIBLE   GRADES.      For  a  general  discussion    of  the 
subject  of  maximum  and  minimum  grades,  see  §  79-86.     The  fol- 
lowing examples  of  maximum  grades  for  water-bound  macadam  roads 
are  instructive. 

In  France  the  standard  is:  on  national  roads,  not  exceeding 
3  per  cent;  departmental  roads,  not  exceeding  4  per  cent;  and 
subordinate  roads,  not  exceeding  6  per  cent.  On  the  great  Alpine 
road  over  the  Simplon  Pass,  built  under  the  direction  of  Napoleon 
Bonaparte,  the  grades  average  1  in  22  (4J%)  on  the  Italian  side, 
and  1  in  17  (5.9%)  on  the  Swiss  side,  and  in  only  one  case  become 
as  steep  as  1  in  13  (7.7%). 

In  Great  Britain,  the  celebrated  Holy  head  road,  built  by  Tel- 
ford  through  the  very  mountainous  district  of  North  Wales,  has  an 
ordinary  maximum  of  1  in  30  (3f  %),  with  one  piece  of  1  in  22  (4|%) 
and  a  very  short  piece  of  1  in  17  (5.9%),  on  both  of  which  pieces 
special  care  was  taken  to  make  the  surface  harder  and  smoother 
than  on  the  remainder  of  the  road. 

On  the  National  Pike  over  the  Alleghenies,  built  before  the  intro- 
duction of  the  railroad,  the  maximum  was  7  per  cent.  At  an  early 
day  the  New  York  law  limited  the  grades  of  turnpikes  (toll  roads)  to 
1  in  11  (9%). 

In  New  York  on  state-aid  roads  the  nominal  maximum  is  5  per 
cent,  but  grades  as  steep  as  6j  per  cent  have  been  found  necessary 
in  some  cases.  In  New  Jersey  are  a  number  of  state-aid  roads  having 
grades  of  7  and  8  per  cent,  and  one  has  10J  per  cent.  In  Massa- 
chusetts no  hard-and-fast  standard  has  been  adopted  for  the  state- 
aid  roads,  but  a  few  have  5  per  cent  grades  and  a  considerable  num- 
ber have  4  per  cent  grades.  It  is  said  that  on  some  important 
Massachusetts  roads  the  grade  can  not  at  reasonable  expense  be 
reduced  below  7  per  cent. 

367.  In  improving  city  streets  it  is  often  impossible  to  make  any 
radical  change  in  the  grade  owing  to  the  resulting  damage  to  abut- 


200  WATER-BOUND   MACADAM    ROADS  [CHAP.    VI 

ting  property,  and  it  is  almost  impossible  to  avoid  the  steep  grade 
by  a  change  of  location;  and  consequently  some  city  streets  have 
very  steep  grades  which  are  used  with  surprisingly  good  results. 
Newton,  Mass.,  has  a  number  of  water-bound  macadam  streets 
which  have  long  stretches  of  9  and  10  per  cent  grades,  and  has  one 
12  per  cent  grade  1,000  feet  long.  Waltham,  Mass.,  has  one  400- 
foot  stretch  of  water-bound  macadam  on  a  12  per  cent  grade,  and 
another  on  a  13  per  cent  grade.  In  the  Borough  of  Richmond 
(Staten  Island),  New  York  City,  are  several  pieces  of  10,  11,  and  12 
per  cent  grades,  and  100  feet  of  14  per  cent,  two  stretches  of  200  feet 
each  of  16  per  cent,  and  one  piece  200  feet  long  of  20  per  cent  grade. 
368.  PREPARING  THE  SUBGRADE.  The  broken  stone  is  designed 
to  take  the  wear  of  hoofs  and  wheels,  but  the  earth  foundation  must 
support  the  load ;  and  therefore  any  road  which  is  constructed  with- 
out giving  due '  attention  to  the  earth  road-bed  is  wrong  from  the 
start,  and  will  never  be  a  good  road  until  the  defect  is  remedied. 

For  instructions  concerning  the  construction  of  embankments 
and  excavations,  see  §  132-35.  In  building  an  embankment  upon 
which  broken  stone  is  to  be  laid,  every  reasonable  care  should  be 
taken  to  prevent  uneven  settlement.  It  is  sometimes  advisable 
to  delay  the  laying  of  madacam  for  at  least  a  year  in  order  to  give 
the  embankment  time  to  settle,  for  it  is  impossible  to  construct  an 
embankment  of  earth  more  than  a  few  feet  in  height  without  having 
subsequent  settlement.  If  this  settling  took  place  evenly  all  along 
the  embankment,  no  particular  harm  would  be  done  to  the  mac- 
adam laid  upon  it;  but  owing  to  the  difference  in  the  soils  composing 
embankments  and  also  in  the  way  the  earth  is  dumped,  there  is 
always  a  tendency  for  some  parts  to  settle  more  than  others. 

Sometimes  the  road  surface  is  placed  so  low  that  it  forms  a  gutter 
to  dram  the  adjacent  fields,  which  of  course  is  very  objectionable. 
Occasionally  the  earth  from  the  side  ditches  and  from  the  trench  in 
which  the  stone  is  placed,  is  deposited  at  the  side  of  the  right-of-way 
instead  of  being  used  to  raise  the  road  surface.  In  this  connection, 
see  §  139. 

369.  After  the  subgrade  has  been  brought  to  the  proper  form 
(§  351),  it  should  be  rolled  thoroughly— both  to  consolidate  it  and  to 
discover  soft  spots.  For  a  discussion  of  road  rollers,  see  §  378-79. 
Fig.  57  shows  the  method  of  smoothing  the  subgrade  with  a 
scraping  grader;  and  also  shows  the  rolling  of  the  shoulder.  Fig. 
58  shows  the  subgrade  after  the  rolling  is  completed. 

In  rolling;  quicksand  spots  are  sometimes  discovered,  in  which 


ART.    2] 


CONSTRUCTION 


201 


case  the  troublesome  material  should  be  excavated  and  suitable 
material  substituted.  If  the  road-bed  be  of  sand  or  of  material  of 
such  a  nature  as  to  push  along  in  a  wave  in  front  of  the  roller,  a 
thin  layer  of  broken  stone  or  gravel  strewn  over  the  surface  will 


FIG.  57. — SMOOTHING  SUBGRADE  AND  ROLLING  SHOULDER, 


FIG.  58. — SUBGRADE  ROLLED  AND  READY  FOR  STONE. 


enable  the  roller  to  consolidate  the  road-bed.  If  the  surface  is  clay 
that  sticks  to  the  roller,  sprinkle  a  thin  layer  of  sand  or  cinders 
over  the  surface.  If  the  clay  is  soft  and  forms  a  wave  in  front  of  the 
roller,  additional  rolling  is  a  detriment,  as  it  increases  the  plasticity 
of  the  clay. 

370.  SETTING  THE  TELFORD.  The  distinguishing  feature  of  a 
telford  road  is  its  paved  foundation.  After  the  road-bed  has  been 
brought  to  the  proper  form  and  been  rolled,  rough  stones  are  set 
upon  the  surface  to  form  a  pavement  5  to  8  inches  thick,  the  thick- 
ness depending  upon  that  to  be  given  to  the  finished  road  (§  360), 
the  general  practice  being  to  make  the  paved  foundation  about  two 
thirds  of  the  total  thickness  of  the  road.  The  practice  of  Telford 


202  WATER-BOUND    MACADAM    ROADS  [CHAP.    VI 

was  to  grade  the  road-bed  flat,  and  then  construct  his  pavement 
deeper  in  the  middle  than  at  the  sides,  using  for  a  roadway  16  feet 
wide,  stones  about  8  inches  deep  at  the  middle  and  5  inches  at  the 
sides.  This  practice  is  still  followed  by  some  engineers;  but  it  is  now 
more  common  and  usually  considered  preferable  to  make  the  surface 
of  the  road-bed  parallel  to  the  finished  surface,  and  the  pavement  of 
uniform  thickness.  Fig.  46,  page  191,  shows  a  telford  road  with  a 
level  subgrade;  and  Fig.  47,  page  191,  a  telford  road  with  the  sub- 
grade  parallel  to  the  finished  surface. 

The  size  of  the  stones  for  the  telford  pavement  is  of  no  great 
importance,  at  least  there  is  a  great  difference  in  the  practice  of  the 
best  road  builders.  The  width  of  the  stones  varies  from  3  to  10 
inches,  3  to  6  being  most  common;  and  the  length  varies  from  6 
to  20  inches,  8  to  12  being  most  common.  It  is  desirable  to  have 
the  width  on  any  particular  job  somewhat  nearly  uniform,  and  the 
stones  in  any  course  should  be  still  more  nearly  equal.  The  stones 
are  set  upon  their  widest  edge  with  their  greatest  length  across  the 
road,  the  -joints  being  broken  as  much  as  possible.  Each  stone 
should  stand  independently  of  its  neighbor,  i.  e.,  one  stone  should 
not  lean  against  another.  The  irregularities  of  the  upper  surface 
are  then  broken  off  with  a  hammer,  and  the  interstices  between 
the  stones  are  filled  with  spalls  lightly  driven  into  place  with  a  ham- 
mer or  a  crow-bar.  This  knocking  off  of  the  projecting  points  and 
the  driving  of  spalls  into  the  interstices  should  not  be  done  so  near 
the  face  of  the  pavement  as  to  dislocate  the  stones  last  set.  It  is 
frequently  specified  that  no  wedging  shall  be  done  within  10  or  15 
feet  of  the  front  edge  of  the  pavement.  After  the  projecting  points 
have  been  knocked  off  and  the  interstices  have  been  filled  with  stone 
chips  or  ordinary  crushed  stone,  the  pavement  is  usually  rolled.  It 
is  usually  specified  that  the  roller  shall  not  go  nearer  to  the  front  of 
the  pavement  than  25  to  30  feet. 

The  cardinal  requisite  of  a  telford  foundation  is  the  interlocking 
of  the  stone  closely  and  compactly  together  by  barring,  wedging, 
and  rolling  until  the  entire  structure  is  brought  in  action  to  resist 
disturbance  as  a  single  mass. 

371.  CRUSHING  THE  STONE.  The  introduction  of  a  machine 
for  breaking  the  material  greatly  cheapened  the  cost  of  broken- 
stone  roads.  The  rock  crusher  was  introduced  into  America  in 
1860,  before  which  time  the  stone  was  broken  by  hand  with  ham- 
mers on  the  side  of  the  road.  Coincident  with  the  introduction 
of  power  for  breaking  the  stone,  came  the  revolving  screen  which 


ART.    2] 


CONSTRUCTION 


203 


permitted  the  fragments  to  be  assorted  as  to  size — an  important 
feature,  as  will  soon  be  shown. 

372.  Forms  of  Crushers.  There  are  two  types  of  crushers  now 
in  common  use.  The  older  one,  often  called  the  Blake  after  the 
original  inventor,  consists  of  a  strong  iron  frame,  near  one  end  of 
which  is  a  movable  jaw.  By  means  of  a  toggle-joint  and  an  eccen- 
tric, this  jaw  is  moved  backward  and  forward  a  slight  distance.  As 
the  jaw  recedes  the  opening  increases  and  the  stone  descends;  as  the 
jaw  again  approaches  the  frame,  the  stone  is  crushed.  The  maxi- 
mum size  of  the  product  is  determined  by  the  distance  the  jaw  plates 


FIG.  59. — OSCILLATORY  STONE  CRUSHER. 

are  from  each  other  at  their  lower  edge.  This  machine  is  also  fre- 
quently called  the  oscillatory  breaker,  or  jaw  breaker.  Fig.  59  shows 
one  form  of  this  type.  The  size  of  the  product  is  regulated  by 
raising  or  lowering  the  wedge  10,  or  by  inserting  a  different  pair  of 
toggles,— 7. 

The  second  form  of  crusher,  called  the  Gates  after  the  original 
inventor,  consists  of  a  solid  conical  iron  shaft  which  is  supported 
within  a  heavy  iron  receptacle  shaped  somewhat  like  an  inverted 
bell.  By  means  of  an  eccentric  bearing  a  rocking  and  rotary  motion 
is  given  to  the  shaft,  so  that  each  point  of  its  surface  is  successively 
brought  near  to  and  removed  from  the  surface  of  the  bell,  which 
causes  the  stone  to  be  successively  crushed  as  it  descends.  Fig.  60 


204 


WATER-BOUND   MACADAM   ROADS 


[CHAP,  vi 


shows  one  form  of  this  type  of  crusher.  An  adjustment  permits  a 
variation  in  the  size  of  the  product.  This  form  is  often  called  the 
rotary  breaker  or  gyratory  breaker. 

It  is  not  wise  here  to  consider  the  relative  merits  of  the  different 
forms  and  sizes  of  stone  crushers,  the  power  required,  the  output, 


FIG.  60. — GYRATOKY  ROCK  CRUSHER. 

etc.,  since  the  construction  of  a  reasonably  good  macadam  road 
requires  a  large  equipment  of  machinery  and  an  experienced  con- 
tractor, and  since  the  equipment  varies  with  the  conditions. 

Fig.  61  gives  a  hint  as  to  the  arrangement  of  the  crusher,  the 
elevator,  the  screens,  and  the  storage  bins.  Fig.  62,  shows  a  real 
crushing  plant  at  Green  Lake,  Wis. 

373.  SIZES  OF  STONE.  The  size  of  stone  used  for  road  metal 
depends  upon  the  hardness  and  toughness  of  the  stone  and  upon 
the  weight  of  the  traffic.  The  harder  and  tougher  the  material, 
the  smaller  it  may  be  broken  without  danger  of  its  crushing  or  shat- 


ART.   2] 


CONSTRUCTION" 


205 


FIG.  61. — DIAGRAMMATIC  ARRANGEMENT  OF  STONE-CRUSHING  PLANT. 


? 


FIG.  62. — STONE-CRUSHING  PLANT. 


206  WATER-SOUND  MACADAM  ROADS  [CHAP,  vt 

tering  under  the  load  of  wheels  and  the  impact  of  hoofs;  and  the 
harder  and  tougher  a  stone,  the  smaller  it  must  be  broken  in  order 
that  it  may  compact  well  in  the  road.  The  stones  in  the  top  course 
should  be  larger  for  heavy  traffic  than  for  light  traffic,  to  prevent 
their  being  ground  to  powder.  Larger  stones  CPU  be  used  in  the 
bottom  layers  of  a  road  than  at  the  top. 

One  of  Mac  Adam's  rules  was  to  exclude  any  fragment  weigh- 
ing more  than  6  ounces.  A  1  J-inch  cube  of  compact  limestone  weighs 
about  6  ounces.  Another  of  MacAdam's  rules  was  to  exclude  any 
stone  that  could  not  readily  be  put  into  a  man's  mouth.  These  rules 
are  frequently  quoted,  even  now,  although  improvements  in  road 
machinery  have  made  them  inappropriate  with  present  methods. 

The  bottom  course  of  a  macadam  road  built  of  soft  stones  is 
often  composed  of  fragments  3  to  4  inches  in  greatest  dimensions; 
but  if  it  is  built  of  hard  tough  stone,  the  sizes  are  2  to  2|  inches. 
The  size  of  rock  in  the  lower  courses  is  not  so  important  as  that  for 
the  surface  course  (see  §  374).  The  top  course  of  hard  tough  stones 
is  usually  1  to  2  inches  for  heavy  traffic,  and  J  to  1  inch  for  light 
traffic. 

The  custom  is  to  lay  the  stone  in  courses  of  substantially  one  size, 
although  some  road  builders  prefer  to  have  the  sizes  mixed  when 
thrown  into  the  road.  The  only  advantage  of  the  latter  practice  is 
that  with  a  skilful  proportioning  of  the  sizes  less  rolling  is  required; 
but  it  is  objectionable  owing  to  the  difficulty  of  getting  the  several 
sizes  properly  proportioned  and  keeping  them  thoroughly  mixed. 
There  is  generally  too  much  fine  material  in  the  mixed  sizes,  which 
makes  the  road  wear  rapidly  and  unevenly. 

Connected  with  the  crusher  and  run  with  the  same  power  is 
generally  a  rotary  screen  having  meshes  of  three  sizes — usually 
about  i,  1J,  and  2J  inches. 

374.  For  economic  reasons  the  size  of  stone  in  the  several  courses 
and  their  thickness  should  be  adjusted  so  as  to  use,  if  possible,  all 
of  the  output  of  the  crusher.  The  output  of  the  various  sizes  varies 
considerably  with  the  character  of  the  stone.  With  a  hard  stone, 
half  or  more  of  the  product  of  the  crusher  will  not  pass  through  the 
|-inch  screen;  while  with  field  stones  one  half  may  pass  through 
such  a  screen.  The  last  gives  more  "  fines "  or  "  screenings " 
than  can  be  used  profitably  during  construction,  but  the  surplus  is 
very  useful  in  maintaining  the  surface.  With  some  rocks  it  is  diffi- 
cult to  get  enough  fine  material  for  use  in  the  original  construc- 
tion. 


ART.    2] 


CONSTRUCTION 


207 


375.  SPREADING  THE  STONE.  The  stone  is  usually  hauled 
from  the  crusher  to  the  road  in  wagons  or  trucks,  dumped  upon  the 
roadway,  and  spread  with  forks  or  rakes.  Dumping  in  place  is  objec- 
tionable, since  the  coarse  and  fine  fragments  become  separated  in 


FIG.  63. — AUTO  TRUCK  DUMPING  STONE. 


the  process,  producing  a  layer  of  unequal  density  and  an  irregular 
surface  after  rolling.      It   is  sometimes  specified  that  the   stone 


FIG.  64. — SPREADING  STONE  WITH  "RAKES.    FIG.  65. — SPREADING  STONE  WITH  SHOVELS. 

shall  be  dumped  upon  a  platform,  from  which  it  is  distributed 
with  shovels.  This  method  of  spreading  costs  4  to  6  cents  per 
cubic  yard — about  twice  that  by  dumping  and  raking  and  is 
appropriate  only  when  the  very  best  results  are  sought.  Wagons  and 


208 


WATER-BOUND    MACADAM    ROADS 


[CHAP.    VI 


trucks  are  upon  the  market  which  can  automatically  dump  and  dis- 
tribute the  stone  in  layers  of  uniform  thickness;   but  owing  to  their 


FIG.  66. — SHTJART  GRADER. 


cost  and  weight  they  are  not  in  very  general  use.  Fig.  63  shows  an 
auto  truck  dumping  the  stone  in  a  ridge  on  the  subgrade. 

The  stone  is  sometimes  spread  by  hand  with  rakes  and  shovels — 
see  Fig.  64  and  65.  Notice  the  template  in  Fig.  65  used  to  gage 
the  thickness  of  the  layer  of  stone. 

There  are  several  methods  of  spreading  the  stone  by  machinery. 
Some  contractors  use  the  Shuart  grader,  Fig.  66,  a  machine  that 


FIG.  67. — HABBOWING  STONE.' 


68. — LEVELING  STONE  WITH 
SCRAPING  GRADER. 


was  devised   for  use  in  leveling  ground  that  is  to  be   irrigated, 
Other  contractors  level  the  stone  with  a  harrow  as  shown  in  Fig.  67. 


ART.    2 1  CONSTRUCTION  209 

Still  other  contractors  use  a  scraping  grader  to  level  the  stone — 
Fig.  68.  Fig.  69  shows  the  bottom  course  of  stone  ready  for  rolling. 
The  stone  should  be  applied  in  uniform  layers,  the  thickness 
of  each  depending  upon  the  total  thickness  of  the  road.  Two 
methods  are  in  use  for  gaging  the  thickness  of  the  layers  of  stone. 
1.  On  the  finished  subgrade,  wood  cubes  of  a  depth  equal  to  the 
thickness  of  the  layer  are  set  at  frequent  intervals,  and  the  loose 


FIG.  69.— BOTTOM  COTTBSE  READY  FOB  ROLLING. 

stone  is  laid  even  with  the  tops  of  these  blocks.  This  method  is 
sometimes  described  as  building  by  blocks,  and  is  the  one  employed 
on  the  state-aid  roads  of  New  Jersey.  2.  The  soil  is  brought  to  an 
established  grade,  and  the  finished  road  is  required  to  be  brought 
to  another  established  grade,  in  which  case  neither  the  absolute 
thickness  nor  the  uniformity  of  the  several  courses  is  a  matter  of 
much  importance.  This  method  is  employed  on  the  state-aid  roads 
in  Massachusetts. 

376.  SHRINKAGE  IN  ROLLING.  Before  beginning  to  spread  the 
layers  of  stone,  it  is  necessary  to  determine  the  amount  the  crushed 
stone  will  shrink  in  rolling.  The  shrinkage  has  an  important  bearing 
upon  the  thickness  and  cost  of  the  finished  road;  but  great  errors 
are  sometimes  made  in  estimating  the  amount  of  shrinkage.  The 
following  examples  from  practice  show  the  actual  shrinkage. 


210  WATER-BOUND   MACADAM   ROADS  [CHAP.   VI 

In  one  case,*  with  trap  rock  If  to  2J  inches,  rolled  with  a  12|-ton 
steam  roller  upon  a  subgrade  so  hard  that  the  wagons  hauling  the 
stone  made  no  ruts,  5.67  inches  of  loose  stone  rolled  to  4  inches, 
and  7.38  inches  rolled  to  6  inches.  The  average  thickness  of  the 
loose  "stone  was  determined  by  dividing  the  quantity  of  stone  used 
by  the  area  covered.  The  first  is  a  shrinkage  of  29  per  cent  and 
the  second  of  19  per  cent.  The  difference  between  these  two  results 
is  probably  due  to  errors  of  observation,  to  variations  in  the  thick- 
ness of  the  finished  road,  and  to  the  fact  that  the  thicker  layers  did 
not  compact  as  solidly  as  the  thinner  ones.  The  stone  was  rolled 
dry  until  the  desired  thickness  was  reached,  when  the  binder  was 
added,  and  sprinkling  was  commenced. 

In  another  case,f  with  2-inch  trap  laid  on  the  compact  surface 
of  an  old  crushed-stone  road  and  rolled  with  a  12-ton  roller,  3.9 
inches  of  loose  stone  rolled  to  3  inches.  The  shrinkage  was  23  per 
cent.  The  thickness  was  determined  from  the  area  covered  and 
the  quantity  of  stone  used.  No  stone  could  have  been  forced  into 
the  subgrade,  but  there  was  some  uncertainty  as  to  the  average 
elevation  of  the  surface  of  the  old  street. 

It  has  been  determined  {  by  tests  over  several  miles  of  road 
where  the  output  of  the  crusher  was  carefully  measured  in  wagons 
and  also  when  rolled  in  place,  that  6  inches  of  loose  hard  limestone 
rolled  down  to  4f  inches,  which  is  a  shrinkage  of  20  per  cent. 

377.  It  is  probable  that  the  maximum  actual  shrinkage  in  rolling 
is  less  than  20  per  cent.  The  apparent  shrinkage  depends  upon  the 
nature  and  condition  of  the  subgrade,  i.  e.,  upon  the  amount  of  stone 
forced  into  the  earth. 

If  the  soil  is  clay,  the  sprinkling  required  to  work  the  binder 
into  the  interstices  may  soften  the  subgrade  so  that  considerable 
stone  will  be  forced  into  the  earth.  This  condition  is  indicated  by 
the  roller's  leaving  tracks  upon  the  surface;  and  when  this  occurs,  the 
work  should  be  stopped  until  the  subgrade  dries  out.  To  prevent 
the  crushed  stone  from  being  forced  into  the  clay  subgrade  during 
construction  or  after  completion — particularly  when  the  frost  is 
going  out, — a  layer  of  sand,  stone  screenings,  ashes,  or  the  like, 
is  sometimes  interposed.  The  English  engineers  often  use  "  hard 
core  "  (a  mixture  of  brick  rubbish,  old  plastering,  and  broken  stone) 
on  a  clay  soil,  to  prevent  the  mud's  working  into  the  metaling.  Any 

*  W.  C.  Foster  in  Trans.  Amer.  Soc.  of  Civil  Eng'rs,  Vol.  41,  p.  135-38. 

t  F.  G.  Cudworth  in  Trans.  Amer.  Soc.  of  Civil  Eng'rs,  Vol.  41,  p.  126-28. 

t  H.  P.  Gillette  in  Economics  of  Road  Construction,  p.  19-20.     New  York,  1901. 


ART.    2]  CONSTRUCTION  211 

material  not  affected  by  water  is  useful  for  this  purpose;  and  the 
finer  it  is  the  better,  since  the  smaller  will  be  the  apertures  in  it,  and 
the  more  certainly  will  it  prevent  the  soil  from  coming  up  through  it. 

If  the  soil  is  sandy,  a  thin  layer  of  coarse  gravel  or  broken  stone 
laid  upon  the  surface  and  then  rolled,  will  prevent  any  further  loss 
of  the  road  metal  in  the  subgrade.  If  the  soil  is  nearly  pure  sand, 
the  wetter  it  is  the  less  crushed  stone  will  be  forced  into  it;  and 
therefore  if  water  is  plentiful,  it  may  be  wise  to  keep  the  sand  satu- 
rated while  the  rolling  is  in  progress  to  prevent  the  loss  of  the  stone. 
The  Massachusetts  Highway  Commission  used  cotton  cheese-cloth 
on  a  soft  fine  sand  to  prevent  the  stone  from  sinking  into  the  sub- 
grade.  "  It  is  not  at  all  needful  that  the  partition  should  be  endur- 
ing, for  as  soon  as  the  stones  in  the  lower  layer  have  been  forced 
into  contact  and  have  become  bound  together,  there  is  no  further 
danger  of  the  mingling  of  the  stone  with  the  sand;  and  hence  the 
decay  of  the  fabric  is  a  matter  of  no  consequence.  The  cloth  was 
spread  in  strips  lengthwise  of  the  way;  and  the  stone  for  the  bottom 
layer  was  shoveled  from  the  sides  upon  it  with  no  unusual  care.  A 
section  through  such  a  road  showed  that  the  stones  did  not  tear 
through  the  cloth.  At  3  cents  per  square  yard  on  the  road,  the  cost 
of  the  cloth  may  be  less  than  one  third  that  due  to  the  loss  of  the 
broken  stone  which  would  occur  if  it  were  allowed  to  come  directly  in 
contact  with  the  sand.  Various  kinds  of  strong  paper  were  tried, 
but  found  worthless."  A  thick  coating  of  straw  has  been  used  to 
hold  up  the  macadam  on  a  sandy  soil. 

However,  if  the  sand  is  firm  enough  to  hold  up  the  stone  during 
the  rolling,  it  is  not  necessary  to  prevent  the  mixing  of  the  sand 
and  the  stone,  since  the  subgrade  may  be  left  a  little  high,  with 
the  expectation  of  forcing  the  stone  into  the  sand.  This  is  equiva- 
lent to  using  the  sand  of  the  subgrade  as  a  filler  or  binder  for  the 
lower  portion  of  the  broken  stone.  If  the  sand  is  dry  and  nearly 
pure,  it  can  be  thus  forced  nearly  to  the  top  of  a  4-inch  course  of 
coarse  broken  stone. 

378.  ROAD  ROLLERS.  The  roller  is  indispensable  for  the  eco- 
nomic construction  of  water-bound  macadam  roads.  Roads  can  be 
built  without  the  use  of  a  roller,  but  always  at  large  expense  to  the 
traffic  and  with  great  waste  of  the  road  metal;  and  such  roads  never 
have  as  smooth  a  surface  and  are  not  as  durable  as  if  a  roller  had  been 
employed  in  their  construction.  With  traffic-consolidated  roads, 
much  of  the  metal  is  worn  round  and  smooth  before  the  fragments 
become  firmly  fixed  in  place:  and  the  dirt  brought  upon  the  road  by 


212  WATER-BOUND  MACADAM   ROADS  [CHAP.   VI 

the  traffic  mixes  with  the  stone  and  prevents  it  from  ever  packing  as 
solidly  as  the  clean  stone  would,  and,  besides,  the  dirt  when  wet 
has  a  lubricating  effect  upon  the  stone  which  under  the  action  of 
traffic  causes  the  surface  to  break  up  readily.  Further,  during 
the  time  travel  is  consolidating  the  stone,  the  surface  is  not  even 
approximately  water-tight;  and  therefore  the  subgrade  is  softened  by 
rains,  and  the  stone  is  mixed  with  the  earth  below  and  virtually  lost. 


FIG.  70. — THREE-WHEEL  OR  MACADAM  ROLLER. 

Ordinarily,  it  is  true  economy  to  compact  the  road  by  the  use  of  a 
roller. 

Formerly  both  horse  and  steam  rollers  were  employed;  but  now 
only  the  latter  are  used.  There  are  two  type  forms  of  steam,  or 
rather  power,  road-rollers — see  Fig.  70  and  71.  The  first,  or  three- 
wheel  type,  is  the  form  employed  in  macadam  road  construction; 
and  the  second,  or  tandem  type,  is  the  form  used  in  rolling  asphalt 
pavements  and  other  bituminous  road  surfaces.  Both  types  are 
driven  by  steam  or  by  gasoline,  but  the  latter  is  rapidly  gaining  in 
favor.  There  are  a  variety  of  forms  of  each  type,  but  the  essential 
features  of  all  are  practically  the  same. 

The  total  weight  of  the  macadam  roller  varies  from  7  to  15  tons; 
and  the  pressure  under  the  drivers  varies  from  300  to  500  Ib.  per  linear 
inch.  The  total  weight  of  the  tandem  roller  varies  from  2J  to  10 


ART.    2]  CONSTRUCTION  213 

tons,  and  the  pressure  under  the  driving  drum  from  125  to  300  Ib. 
per  linear  inch  and  for  the  lightest  roller  the  compression  under  the 
steering  drum  is  usually  about  60  Ib.  per  linear  inch.  Of  the  tandem 


FIG.  71. — TANDEM  OK  ASPHALT  ROLLEE. 

type  the  5-  and  8-ton  roller  are  most  common,  and  give  a  pressure 
under  the  driving  drum  of  about  200  and  280  Ib.  per  inch  respectively. 

379.  The  weight  of  the  roller  should  be  proportional  to  the  hard- 
ness of  the  stone,  as  too  heavy  a  roller  crushes  the  material  instead  of 
compacting  it.     An  excessively  heavy  roller  will  sometimes  sink  into 
light  or  loose  soil,  and  force  it  ahead  in  a  wave  which  the  roller  can  not 
surmount.     This  may  sometimes  be  prevented  by  spreading  a  thin 
layer  of  sand  or  gravel  on  the  surface  being  rolled.     A  similar  dif- 
ficulty sometimes  occurs  with  a    heavy  roller   on  a  layer  of  loose 
stones.     If  the  front  wheels  or  rollers  of  the  machine  were  larger, 
this  difficulty  would  be  decreased.     In  localities  where  the  soil  is  of  a 
loose  sandy  nature,  a  roller  weighing  10  or  12  tons  is  usually  preferred; 
and  in  districts  where  the  soil  is  stiff  or  gravelly  clay,  a  weight  of  12 
or  15  tons  is  used.     In  localities  where  the  road  material  is  hard,  a 
15-ton  roller  is  necessary;  but  with  the  softer  stones  a  weight  of  10 
or  12  tons  is  sufficient. 

380.  ROLLING  THE  STONE.     Rolling  is  a  very  important  part  of 
the  construction  of  a  water-bound  macadam  road.     The  subgrade 
should  be  rolled  to  prevent  the  stones'  being  forced  into  the  earth. 
The  lower  course  of  the  stone  should  be  rolled  to  compact  it  so  that 
the  pieces  will  not  move  one  upon  the  other  under  the  traffic  and  the 
top  course  should  be  rolled  to  pack  or  bind  the  pieces  into  place. 


214  WATER-BOUND    MACADAM    ROADS  [CHAP.    VI 

Rolling  accompanied  by  sprinkling  (see  Fig.  72)  is  necessary  also 
to  work  the  binding  material  into  the  interstices  so  as  to  make  the 
surface  water-tight.  Roads  that  have  been  consolidated  by  traffic 
are  largely  held  together  by  mud,  and  after  long  use  are  fairly 
smooth  and  hard  in  dry  weather,  but  become  soft  and  muddy  dur- 
ing a  wet  time. 

The  stone  is  put  on  in  two  or  three  layers, — according  to  the  total 
thickness  of  the  finished  road, — and  each  course  is  thoroughly  rolled 


FIG.  72. — SPRINKLING  AND  ROLLING. 

before  the  next  is  added.  No  course  should  be  more  than  4  to  6 
inches  thick.  When  a  telford  foundation  is  used,  broken  stone  is 
spread  over  the  pavement  to  bring  the  top  surface  to  the  proper 
form  and  height,  after  which  it  is  rolled. 

381.  The  rolling  should  proceed  gradually  from  both  sides  toward 
the  center.  If  the  weight  of  the  roller  can  be  varied,  commence 
with  the  unballasted  roller,  and  increase  the  weight  as  the  stone 
becomes  consolidated.  If  the  surface  of  the  layer  shows  a  wavy 
motion  after  being  rolled  three  or  four  times,  the  subgrade  is  too  wet; 
and  time  should  be  given  it  to  dry  out.  Some  coarse  brittle  granitic 
rocks  begin  to  crawl  and  the  sharp  edges  to  break  off  after  the  roller 
has  passed  over  them  a  few  times;  but  a  light  sprinkling  of  sand  or 
stone  screenings  will  prevent  this,  and  facilitate  the  consolidation 
of  the  layer.  All  irregularities  of  the  surface  developed  by  the 
rolling  should  be  corrected  by  filling  the  depressions  with  stone  of  the 
size  used  in  the  layer. 

The  rolling  should  be  continued  until  the  stone  ceases  to.  creep 
in  front  of  the  roller,  and  until  the  macadam  is  firm  under  the  foot 
as  one  walks  over  it.  When  the  rolling  is  complete,  one  of  the 


ART.   2]  CONSTRUCTION  215 

larger  stones  of  the  course  can  be  crushed  under  the  roller  without 
indenting  the  surface  of  the  layer. 

When  the  first  course  has  been  consolidated,  a  second,  usually 
a  thinner  one  of  smaller  stones,  is  added ;  and  then  it  is  rolled  the  same 
as  the  first.  Finally  a  third  course  consisting  of  about  half  an  inch 
of  sand  or  fine  stone  and  stone  dust  is  added.  The  roller  is  then 
passed  over  this  layer,  with  the  result  that  the  bits  are  ground  to 
powder.  As  the  rolling  of  this  course  proceeds  it  is  sprinkled,  the 
aim  of  the  sprinkling  and  rolling  being  to  work  the  fine  material 
into  the  cavities  between  the  pieces  of  crushed  stone,  thus  binding 
the  whole  into  a  solid  ma*ss.  The  proper  binding  of  the  road  is  the 
most  important  part  of  the  construction,  and  will  be  more  fully 
considered  presently  (see  §  383). 

382.  Amount  of  Rolling.  The  total  amount  of  rolling  required 
varies  with  the  weight  of  the  roller,  the  hardness  and  the  size  of  the 
stone,  and  the  amount  of  binder  and  water  used.  Trap  rock  being 
very  hard  requires  two  or  three  times  as  much  rolling  as  most  other 
stone.  An  excess  of  binding  material  and  of  water  gives  a  compact 
surface  with  comparatively  little  rolling,  but  the  road  is  not  as  dur- 
able as  though  it  had  been  more  thoroughly  rolled. 

The  following  examples  are  representative  of  the  best  American 
practice. 

In  New  York  City,  5  inches  of  crushed  gneiss  on  telford  and 
5  inches  of  trap  on  the  gneiss,  bound  with  trap  screenings,  was  rolled 
with  a  15-ton  steam  roller  at  the  rate  of  40.6  square  yards  per  hour, 
or  10  cubic  yards  per  hour.  Although  it  is  common  to  give  the 
amount  of  rolling  in  terms  of  the  time  required,  the  statement  is 
somewhat  indefinite,  since  the  work  accomplished  varies  with  the 
speed  of  the  roller  and  also  with  the  length  of  run,  i.  e.,  with  the  time 
lost  in  starting  and  stopping.  The  usual  speed  of  steam  rollers  is  2  to 
2J  miles  per  hour.  The  above  work  is  equivalent  to  0.553  ton- 
miles  per  square  yard,  or  2.246  ton-miles  per  cubic  yard.  The  num- 
ber of  trips  was  130.* 

In  making  repairs,  a  6-inch  course  of  2-inch  trap  was  rolled  at 
the  rate  of  26.2  square  yards  per  hour,  or  4.4  cubic  yards  per  hour. 
The  work  amounted  to  about  0.859  ton-miles  per  square  yard,  or 
5.177  ton-miles  per  cubic  yard.  The  number  of  trips  over  the  sur- 
face was  201.  f 

An  area  of  22,000  square  yards  01  a  3-inch  course  of  2-inch  trap 

*  Trans.  Amer  Soc.  of  Civil  Engineers,  Vol.  8,  p.  105-6. 
jlbid.,  p.  107. 


216  WATER-BOUND   MACADAM   ROADS  [CHAP.    VI 

upon  an  old  broken-stone  road,  bound  with  trap-rock  screenings 
and  rolled  with  a  10-ton  steam  roller,  was  finished  at  an  average 
rate  of  47.15  square  yards  per  hour  of  rolling,  the  extremes  being  38.4 
and  61.1  square  yards  per  hour.  This  was  an  average  of  about  4.0 
cubic  yards  per  hour.  * 

A  6-inch  course  of  If-  to  2J-inch  trap  rock,  bound  with  lime- 
stone screenings,  was  rolled  with  a  12^-ton  steam  roller  at  an  average 
rate  of  31.4  square  yards  per  hour,  or  5.2  cubic  yards  per  hour.f 

The  Hudson  County  Boulevard  (Jersey  City,  N.  J.)  consists 
of  8  inches  of  telford,  2^  inches  of  2|-inch  stone,  1J  inches  of  1|- 
inch  stone,  and  then  \  to  1  inch  of  coarse  screenings — all  trap  rock. 
The  macadam  top  was  supposed  to  roll  down  to  4  inches,  i.  e.,  4| 
to  5  inches  of  loose  stone  was  supposed  to  roll  to  4  inches.  The 
rolling  was  distributed  about  as  follows:  On  the  telford,  10  to  12 
passages;  on  the  2^-inch  course,  8  to  10  passages;  on  the  IJ-inch 
course,  10  to  12;  and  on  the  screenings,  80  to  90, — making  a  total 
of  100  to  120  passages  of  the  roller  over  the  road. 

383.  BINDING   THE   ROAD.     The  interstices  between  the  frag- 
ments of  stone  should  be  filled  with  a  fine  material  which  will  act 
mechanically  to  keep  out  the  rain  water    and    thereby  keep  the 
subgrade  dry,  and  also  to  support  the  fragments  and  prevent  them 
from  being  broken,  and  which  will  bind  or  cement  the  fragments 
into  a  single  more  or  less  solid  mass.     The  proper  binding  of  the 
stone  is  the  most  important  part  of  the  construction  of  a  water- 
bound  macadam  road. 

384.  Nature  of  the  Binder.     The  binding  material  or  the  filler 
should  be  finely  divided  so  as  to  be  easily  worked  into  the  interstices, 
should  have  a  considerable  resistance  to  crushing  so  as  to  properly 
support  the  pieces  of  crushed  stone,  and  should  not  change  its  phys- 
ical condition  when  wet.    Various  materials  have  been  employed 
— clay,  loam,  shale,  sand,  and  limestone  and  trap-rock  screenings. 

Clay  and  loam  are  frequently  used.  Their  merit  is  that  they  are 
cheap,  are  easily  applied  and  have  a  high  cementing  power;  but  they 
are  easily  affected  by  water  and  frost,  and  when  wet  act  more  as  a 
lubricant  than  as  a  binder.  Clay  or  loam  binder  will  give  a  smooth 
road  without  much  rolling,  but  such  a  road  is  liable  to  be  very  dusty 
in  dry  weather,  and  muddy  in  wet  weather.  When  clay  or  loam  is 
employed  as  a  binder,  the  utmost  care  should  be  taken  that  no  more 
is  used  than  just  enough  to  fill  the  voids. 

*  Trans.  Amer.  Soc.  of  Civil  Engineers,  Vol.  41,  p.  127. 
.,  P.  138. 


ART.    2]  CONSTRUCTION  217 

Shale  and  slate  are  only  hard  and  compact  clay,  and  their  only 
merit  is  that  they  give  a  smooth  surface  with  but  little  rolling.  They 
are  speedily  reduced  to  dust,  and  then  have  all  the  disadvantages 
of  clay.  They  have  only  fair  cementing  power. 

Sand  is  often  used  as  a  filler,  and  if  composed  of  fine,  clean,  hard 
grains,  gives  fair  results;  but  sand  which  is  resistant  enough  for  a 
good  binding  material  usually  consists  of  silica  or  quartz,  neither 
of  which  has  a  high  cementing  power.  If  the  grains  are  coated  more 
or  less  with  iron  oxide,  or  if  accompanied  by  bits  of  ironstone  (clay 
cemented  with  iron  oxide),  sand  makes  an  excellent  binding  material, 
since  the  iron  possesses  considerable  cementing  power.  This  form  of 
binder  is  particularly  valuable  in  making  repairs  over  an  opening 
when  a  roller  is  not  available,  or  when  water  for  washing  in  the  binder 
is  scarce.  Low-grade  iron  ore  has  been  used  for  a  binder — either 
alone  or  mixed  with  stone  dust. 

Fine  screenings — the  finest  product  of  the  stone  crusher,  say, 
from  J  or  |  inch  to  dust — from  the  stone  used  in  the  body  of  the 
course  is  the  most  desirable  material  for  a  binder,  partly  because  it 
helps  to  utilize  the  entire  product  of  the  crusher,  partly  because  of 
its  high  crushing  strength,  and  partly  because  the  stone  is  usually 
selected  for  the  high  cementing  power  of  its  dust.  Limestone  has 
very  high  cementing  power,  but  is  soft  and  pliable.  Trap  has  .a 
fair  cementing  power,  and  is  hard  and  durable.  Limestone  screenings 
require  less  rolling,  but  the  trap  dust  makes  a  more  durable  road. 

Sometimes  the  detritus  removed  from  the  surface  of  a  stone  road 
during  maintenance  or  preparatory  to  making  repairs,  is  employed 
as  a  binder.  At  best,  such  material  is  very  poor  for  this  purpose. 
It  is  worn  out  and  has  performed  its  duty;  and,  besides,  it  is  composed 
largely  of  manure  and  vegetable  and  earthy  matter — all  of  which  are 
very  undesirable  in  a  binder.  Such  detritus  is  more  valuable  as  a 
fertilizer  than  as  a  road  material. 

385.  Applying  the  Binder.  There  is  a  difference  of  opinion 
among  competent  engineers  as  to  the  best  method  of  applying  the 
binding  material.  Some  apply  it  on  the  top  of  each  course,  and 
some  on  top  of  only  the  last  course.  In  the  first  case,  all  the  voids 
from  the  bottom  to  the  top  of  the  road  are  filled  with  fine  material; 
in  the  second  case,  the  binder  usually  fills  the  voids  of  the  top  course 
only.  Those  who  advocate  the  first  method  claim  that  the  whole 
mass  should  be  filled  to  prevent  the  stones  from  moving  under  the 
traffic,  and  also  to  prevent  the  soil  from  working  up  from  below; 
while  the  advocates  of  the  second  method  claim  (1)  that  filling  the 


218  WATER-BOUND   MACADAM   ROADS  [CHAP.    VI 

top  layer  is  sufficient  to  hold  the  stone  in  place  near  the  surface,  (2) 
that  the  stones  of  the  lower  courses  have  no  tendency  to  move,  (3) 
that  the  unfilled  voids  of  the  lower  course  promote  drainage,  and  (4) 
that  as  the  upper  layer  wears  away,  the  dust  will  wash  down  into  the 
lower  open  spaces  in  such  a  manner  as  always  to  keep  the  3  or  4 
inches  just  below  the  surface  properly  bound.  If  the  stone  is  hard, 
or  if  the  lower  courses  are  not  thoroughly  rolled,  applying  the  binding 
material  only  on  the  top  of  the  last  course  practically  fills  the  voids 
to  the  earth  foundation;  but  of  course  it  is  cheaper  to  apply  the  filler 
on  the  top  of  each  course  than  to  attempt  to  fill  all  of  the  voids  by 
applying  it  on  the  top  course  only.  If  the  stone  in  the  lower  courses 
is  soft,  or  if  the  top  of  the  next  to  the  last  course  is  thoroughly  rolled, 
applying  the  binder  on  the  top  fills  the  voids  in  the  top  course  only. 
It  is  sufficient  to  fill  the  voids  of  the  top  course. 

The  binder  is  applied  by  spreading  a  layer  of  "  fines  "  about  half 
an  inch  thick  over  the  partially  rolled  surface.  The  filler  should 
be  dumped  upon  a  board  platform,  and  not  directly  upon  the  road 
surface;  and  should  be  distributed  evenly  over  the  stone  with  a 
shovel.  Under  no  consideration  should  loam  or  vegetable  matter 
be  allowed  to  contaminate  the  stone  screenings.  After  the  binding 
material  has  been  evenly  distributed,  the  surface  is  then  sprinkled 
and  rolled.  The  sprinkler  should  have  many  fine  openings,  the 
object  being  to  give  a  gentle  shower  rather  than  a  violent  flooding. 
The  water  washes  the  fine  material  into  the  cavities  below,  and  the 
roller  crushes  the  small  fragments  and  makes  more  dust.  The 
rolling  also  aids  in  working  the  binder  into  the  mass;  in  fact,  the 
binder  can  be  worked  in  to  a  considerable  extent  by  dry  rolling, 
and  consequently  the  quantity  of  water  used  varies  widely  with 
the  method  of  doing  the  work,  but  is  usually  about  4  to  6  cubic 
feet  per  cubic  yard  of  stone.  Sometimes  men  with  heavy  brooms 
are  kept  upon  the  road  sweeping  the  binding  material  about  to  assist 
in  working  it  in,  and  also  to  secure  a  more  uniform  distribution  of  it. 
While  applying  the  screenings  care  should  be  taken  to  pick  off  any 
coarse  stone — particularly  flat  ones, — as  they  do  not  bind  well  and 
their  subsequent  loosening  causes  the  road  to  ravel  (§  397). 

As  the  rolling  and  sprinkling  proceed,  fine  material  should  be 
added  where  needed,  i.  e.,  as  open  spaces  appear.  All  the  filler 
should  not  be  put  on  in  the  beginning,  since  a  thin  layer  can  be 
worked  in  to  better  advantage  than  a  thick  one;  and,  besides,  it  is 
desirable  to  use  only  enough  to  fill  the  voids. 

Occasionally  the  surface  of  the  road  becomes  muddy  and  sticks 


ART.    2]  CONSTRUCTION  219 

to  the  roller.  This  can  be  remedied  in  either  of  two  ways:  viz., 
by  sprinkling  the  roller  and  keeping  it  constantly  wet,  or  by  keeping 
the  sprinkling  wagon  immediately  in  front  of  the  roller  and  having 
the  binder  always  fully  saturated.  The  rolling  is  continued  until 
the  water  is  forced  as  a  wave  in  front  of  the  roller  and  until  the  sur- 
face behind  the  roller  is  mottled  or  puddled  and  is  covered  with  a 
thin  paste.  The  binding,  or  the  puddling  of  the  surface,  can  not  be 
done  satisfactorily  when  the  surface  freezes  nightly. 

When  finished,  if  the  road  is  allowed  to  dry  and  is  then  swept 
clean,  the  surface  will  be  seen  to  have  the  appearance  of  a  rude 
mosaic,  the  flat  faces  of  the  fragments  of  stone  being  crowded  against 
one  another  and  the  interspaces  being  filled  with  the  binding  material 
— the  latter  occupying  about  half  of  the  area.  Such  a  surface  when 
dry  will  stand  considerable  sweeping  with  a  steel  broom  or  brush 
without  the  fragments  of  stone  being  loosened.  The  water  used  in 
construction  not  only  aids  in  working  the  binder  into  the  interstices, 
but  also  develops  the  cementing  power  of  the  rock  dust. 

386.  Usually  after  the  rolling  has  been  completed  a  thin  coating 
of  binding  material  is  sprinkled  over  the  surface.  Authorities 
differ  as  to  the  amount  of  fine  material  to  be  left  on  the  finished 
surface,  some  specifying  as  little  as  f  inch  and  some  as  much  as 
1  inch,  the  usual  quantity  being  f  to  J  inch.  If  only  enough  binding 
material  to  fill  the  interstices  between  the  coarser  fragments  is  left 
upon  the  road,  the  fine  material  will  be  blown  and  washed  away, 
and  soon  there  will  not  be  enough  to  level  up  between  the  large 
bits  and  to  hold  the  surface  stones  in  place,  when  the  wear  will 
come  directly  upon  the  stones.  On  the  other  hand,  if  any  con- 
siderable quantity  of  fine  material  is  left  upon  the  surface,  it  is 
speedily  ground  up,  and  becomes  offensive  dust  if  it  is  not  sprinkled, 
and  equally  objectionable  mud  if  it  is  sprinkled.  It  is  probably 
best  to  put  on  a  quantity  just  sufficient  to  give  a  thin  layer,  say, 
J  to  f  inch,  over  the  surface,  and  when  this  amount  is  blown  or 
washed  away  renew  it.  By  this  method,  the  wear  on  the  body  of 
the  road  will  be  prevented,  a  minimum  amount  of  sprinkling  will 
be  required,  and  there  will  be  as  little  dust  as  possible.  The  surface 
coat  is  also  serviceable  in  decreasing  the  tendency  of  the  binding 
material  to  dry  out  and  to  lose  part  at  least  of  its  cementing  power. 
Fine  material  over  and  above  that  required  to  fill  the  interstices  is 
useful  only  to  prevent  raveling  and  to  keep  the  wear  from  the  sur- 
face of  the  stone ;  and  therefore  sand  is  as  good  for  the  top  dressing 
as  stone  dust,  and  is  usually  much  cheaper.  It  is  desirable  that  this 


220  WATER-BOUND   MACADAM   ROADS  [CHAP.   VI 

coat  of  fine  material  shall  be  sprinkled  and  rolled  before  the  traffic 
is  admitted. 

The  road  is  now  finished;  and  after  it  has  dried  out  for  a  day  or 
two,  it  may  be  thrown  open  to  traffic. 

387.  Amount  of  Binder.     The  amount  of  binder  required  depends 
upon  the  hardness  of  the  stone  and  the  amount  of  rolling  preceding 
the  application  of  the  binder  (§  385).     The  voids  in  the  broken  stone 
can  be  reduced  by  rolling  to  20  or  25  per  cent,  say  22  per  cent,  of 
the  compacted  mass;  and  the  completed  road  will  contain  4  to  7  per 
cent,  say  5  per  cent,  of  voids;  and  therefore  enough  binder  must  be 
added  to  fill  about  17  (=22-5)  per  cent  of  voids.     The  binder 
itself  usually  contains  40  to  50  per  cent  of  voids,  and  therefore  the 
volume  of  filler  required  is  40  to  50  per  cent  more  than  the  voids  to  be 
filled,  i.  e.,  40  to  50  per  cent  more  than  17  per  cent  of  the  original 
volume  of  stone;  or,  in  other  words,  the  amount  of  filler  required  is 
25  to  35  per  cent  of  the  thickness  filled.     This  allows  a  little  for 
waste  and  for  the  thin  coating  spread  upon  the  finished  surface.     If 
the  binder  is  applied  before  the  rolling  has  progressed  very  far,  more 
fine  material  will  be  required,  since  some  of  it  will  work  in  between 
the  fragments  of  stone  and  prevent  them  from  coming  into  as  close 
contact  as  they  otherwise  would.     In  this  case,  part  of  the  surplus 
binder  will  be  flushed  to  the  surface  during  the  sprinkling  and  rolling, 
as  mortar  flushes  to  the  surface  in  tamping  concrete;  but  in  no  case 
does  all  the  surplus  thus  work  out,  and  consequently  the  road  is  not 
as  durable  as  though  only  enough  binder  had  been  used  to  fill  the 
voids;  and,  further,  the  binder  which  flushes  to  the  surface  must  be 
removed  as  mud.     An  excess  of  binder  is  often  used  to  reduce  the 
cost  of  construction  by  decreasing  the  amount  of  sprinkling  and  roll- 
ing required;  but  such  a  practice  adds  to  the  cost  of  maintenance, 
and  the  road  is  less  durable  and  more  dirty. 

388.  COST  OF  CONSTRUCTION.     The  cost  of  construction  of  a 
crushed-stone  road  varies  greatly  with  the  size  of  the  job,  the  con- 
ditions of  the  material  and  labor  markets,  the  specifications  under 
which  the  work  is  done,  etc.     It  is  unwise  to  give  here  any  details 
as  to  the  cost  of  the  several  parts  of  the  work;  and  only  a  few  data 
will  be  given  concerning  the  total  cost  of  a  road.     The  total  cost 
varies  with  the  amount  of  grading  and  drainage  required,  the  length 
improved  in  a  single  season,  the  length  of  railroad  and  wagon  haul, 
the  specifications,  the  labor  market,  etc.     The  following  are  a  few 
representative  examples  of  first-class  construction. 

The  values  here  given  are  somewhat  out  of  date;   but  present 


ART.   2]  CONSTRUCTION  221 

values  are  quite  abnormal  owing  to  the  Great  European  War.  The 
data  given  below  are  interesting  chiefly  as  showing  relative  cost  in 
different  localities  and  of  the  different  parts  of  the  work.  For  cur- 
rent prices  consult  the  construction  news  in  technical  journals. 

389.  New  Jersey.     In  northern  New  Jersey,  the  total  cost   of 
trap  macadam  roads  4  to  6  inches  deep,  where  the  rock  was  obtained 
near  the  road,  ranged  from  20  to  45  cents  per  square  yard;    and 
telford  roads  consisting  of  8  inches  of  telford  and  two  courses  of 
broken  stone  2|  and  1|  inches  thick  respectively,  cost  from  $1.02 
to  $1.29  per  square  yard.     In  the  southern  part  of  that  state,  where 
the  stone  is  transported  20  to  70  miles,  8-inch  trap  macadam  roads 
cost  from  23  to  70  cents  per  square  yard,  the  average  being  from  50  to 
60  cents  per  square  yard.* 

390.  Massachusetts.     The  average   cost  of  220  miles  of  state- 
aid  roads  in  Massachusetts  built  from  1894  to  1899,  f  reduced  to  the 

TABLE  27 
COST  OF  MASSACHUSETTS  STATE-AID  WATER-BOUND  MACADAM  ROADS 

Per  Cent 

Items  of  Expense.  of  Total 

Cost, 

Earthwork  at  32.1  cents  per  cubic  yard 16.4 

Rock  excavation  at  $1.80  per  cubic  yard 2.0 

Shaping  earth  subgrade  at  2.0  cents  per  cubic  yard 2.4 

Gravel  for  foundation  and  wings  at  55.8  cents  per  cubic  yard 3.5 

Telford  foundation  at  33.9  cents  per  square  yard 0.2 

(  $1.503  per  ton  for  local  stone  ) 
Broken  stone  at    «. „«-»*_         _  ,_  }• 45.3 


j  $1.503  per  ton  for  local  stone  \ 
(  $1.920  per  ton  for  trap  j 


Side  drains  at  34.5  cents  per  lineal  foot 2.7 

Rubble  masonry — dry,  at  $3.133  per  cubic  yard 2.6 

"         "           in  cement,  at  $5.770  per  cubic  yard 3.3 

Guard  rails  at  16  cents  per  lineal  foot 1.7 

Stone  boundary-posts  at  $1.417  each 0.6 

Paved  cobble  gutters  66.0  cents  per  square  yard 1.1 

Vitrified-clay  pipe-culverts — 12-inch,  at  65  cents  per  lineal  foot 1.2 

Land  damages,  catch  basins,  and  minor  items  of  construction 3.0 

Engineering  and  inspection 14.0 

Total 100.0 

equivalent  cost  of  a  "  standard  mile  "  (15  feet  wide),  was  $9,931.23 
per  mile  for  construction  and  engineering  expenses,  exclusive  of  cost 
of  administration  and  the  salaries  of  the  chief  engineer  and  two 

*  Compiled  from  the  Reports  of  the  State  Commission  of  Highways  of  New  Jersey,  1895- 
1900. 

t  Report  of  Massachusetts  Highway  Commission,  1900,  p.  150-57. 


222 


WATER-BOUND   MACADAM   ROADS 


[CHAP,  vi 


assistants.  The  maximum  average  for  the  roads  in  any  township 
was  $20,257.48  and  the  minimum  $4,871.30  per  "  standard  mile." 
The  above  gives  an  average  cost  of  $1.126  per  square  yard,  a  maximum 
of  $2.302,  and  a  minimum  of  $0.564. 

In  Massachusetts  in  1897,  52  miles  were  built  in  187  towns 
(townships),  the  average  cost  of  the  several  items  being  as  shown 
in  Table  27.*  An  examination  of  the  reports  for  other  years  indi- 
cates that  the  above  exhibit  is  fairly  representative,  except  that  the 
expenditure  for  stone  is  smaller  than  the  average.  In  the  state- 
aid  roads  built  from  1894  to  1899,  the  cost  of  the  broken  stone  was 
equal  to  55  per  cent  of  the  total  cost  of  the  road,  but  in  later 
years  the  amount  of  stone  used  was  decreased. 

391.  New  York.      In  the  State  of  New  York  in  1898,  22  miles  of 
state-aid  macadam  roads  were  built  in  six  sections,  with  an  average 
cost  of  84.0  cents  per  square  yard,  the  maximum  being  $1.085  and 
the  minimum  64.8  cents.     The  roads  consisted  of  4  inches  of  native 
stone,  and  2  inches  of  trap  rock  bound  with  limestone  screenings,  f 

392.  Michigan.     In  Michigan  the  average  cost  of  52  miles  of 
water-bound  macadam  roads  is  as  follows: 


ITEMS. 

AVERAGE. 

PER  MILE. 

PER  SQ.  YD. 

Broken  stone,  cubic  yards 

1653 

$    435.85 
2264.88 
71.80 
1621.91 

0.313 

$0.083 
0.523 
0.013 
0.307 

Grading,  shaping  and  draining  . 

Crushed  stone. 

Culverts.   . 

Surfacing,  including  loading  and  hauling. 

Total  

$4394.44 

$0.925 

393.  Missouri.     In  Missouri  the  average  cost  of  two-course  work 
is  as  stated  in  the  table  at  the  top  of  page  223.  § 

394.  SPECIFICATIONS.     The  American  Society  of  Municipal  Im- 
provements   publish    specifications    for    water-bound    broken-stone 
roads,  printed  copies  of  which  may  be  had  for  a  nominal  sum.     These 
specifications  are  changed  from  time  to  time  as  is  necessary  to  keep 
them  up  to  date. 

*  Report  of  Massachusetts  Highway  Commission.,  1898,  Appendix  C,  p.  74-75 
Report  of  New  York  State  Engineer  and  Surveyor,  1899,  p.  37, 
Engineering  and  Contracting, Vol.  41  (1914),  p   705 
76id.,  Vol.  38  (1912),  p.  14, 


ART.    3] 


MAINTENANCE 


223 


ITEMS. 

Per  Sq.  Yd.  of 
Road  Surface. 

Per  Cu.  Yd.  of 
Loose  Stone. 

Quarry  rent.                  

$0  013 

$0  05 

Quarrying      

0  100 

0  40 

Crushing   

0  075 

0  30 

Hauling,  1  mile  

0  075 

0  30 

Shaping  road-bed 

0  025 

0  10 

Spreading  material 

0  025 

0  10 

Rolling 

0  013 

0  05 

Sprinkling 

0  013 

0  05 

Superintendence.    ... 

0  013 

0  05 

Incidentals            

0  013 

0  05 

Total,  exclusive  of  interest  and  depreciation  on 
plant,  profits,  and  administration  

$0  .  364 

$1  45 

ART.  3.     MAINTENANCE 

395.  After  the  road  has  been  properly  rolled  and  the  surface 
has  been  made  compact  and  smooth,  it  is  very  desirable  that  it 
should  always  be  maintained  in  that  condition.  Many  seem  to 
believe  that  a  macadam  road  is  a  permanent  construction  which 
needs  no  attention  after  completion;  but  proper  maintenance  is  as 
important  as  good  construction. 

Formerly  much  attention  and  study  was  given  to  the  causes  of 
wear  of  water-bound  macadam  roads;  and  it  was  needed,  for  many 
such  roads  were  subjected  to  a  heavy  travel,  and  required  great  care 
to  keep  them  in  usable  condition.  To  maintain  the  roads  in  good 
condition  it  was  necessary  to  make  repairs  either  at  frequent  intervals 
or  continuously,  and  to  add  new  material  as  the  old  was  washed  off 
or  blown  away.  But  the  coming  of  the  automobile  made  neces- 
sary a  radical  change  in  the  methods  of  maintenance  and  con- 
struction of  broken-stone  roads.  With  horse-drawn  vehicles  the 
abrasion  of  the  horses'  feet  and  of  the  metal  tires  wore  off  dust  or 
binder  to  replace  that  washed  off  and  blown  away;  but  the  fast- 
moving  low-hung  body  of  the  automobile  threw  more  of  the  road  dust 
into  the  air  than  horse-drawn  vehicles,  and  hence  more  of  the  binder 
was  blown  away,  and  besides  the  automobile  made  no  dust  to  replace 
that  blown  away.  Further,  the  action  of  the  wheels  of  the  auto- 
mobile, particularly  in  starting  and  stopping  and  in  rounding  curves, 
tends  to  dislodge  the  stones. 

Therefore  the  introduction  of  the  motor-driven  vehicles  radically 
changed  the  method  of  maintenance  of  the  broken-stone  roads. 
Instead  of  trying  to  maintain  a  water-bound  macadam  road  having 


224  WATER-BOUND   MACADAM   ROADS  [CHAP.    VI 

any  considerable  amount  of  motor-driven  traffic,  by  adding  new 
material  to  replace  that  washed  off  and  blown  away,  it  became  the 
practice  to  give  the  surface  of  such  roads  a  coating  of  bituminous 
material,  such  as  tar  or  asphalt,  which  prevented  the  formation  of 
dust  and  also  protected  the  surface  from  wear.  This  method  of 
treatment  is  discussed  in  Chapter  IX. 

396.  Since  the  introduction  of  motor-driven  vehicles,  the  only 
water-bound  macadam  roads  not  protected  by  a  bituminous  coating 
are  those  that  carry  only  a  small  amount  of  travel,  particularly  a 
small  number  of  motor  vehicles;  and  hence  the  maintenance  of  such 
roads  is  not  of  great  importance.     Therefore  the  maintenance  will 
be  considered  only  briefly. 

397.  RAVELING.     One  of  the  chief  evils  to  be  contended  with  in 
the  maintenance  of  a  crushed-stone  road  is  the  tendency  to  ravel, 
i.  e.,  for  one  stone  after  another  to  work  loose  on  the  surface.     This 
occurs  only  after  a  long  dry  spell  or  in  a  road  originally  deficient  in 
binding  power,  and  is  more  likely  to  occur  on  lightly  traveled  roads 
than   on  those  having  heavy  traffic.     Raveling  may   take   place 
where  the  wind  sweeps  away  the  binding  material  from  the  surface, 
or  on  a  steep  grade  where  the  water  has  washed  the  fine  material 
away  from  between  the  fragments;  and  is  chiefly  due  to  the  picking 
of  the  horses'  shoes,  which  in  a  measure  is  counteracted  by  the  rolling- 
action  of  the  wheels. 

Two  expedients  are  employed  to  prevent  raveling.  1.  Sprink- 
ling the  road  with  water  effectually  stops  raveling,  and  causes  the 
surface  to  solidify  again.  This  is  the  most  common  remedy  on  city 
streets  and  suburban  roads — where  water  is  usually  convenient  and 
plentiful.  For  a  further  discussion  of  Sprinkling,  see  §  401.  2. 
A  thin  coating  of  coarse  sand  is  very  effective  in  preventing  raveling. 
Ordinarily  on  country  roads  a  layer  half  an  inch  thick  is  sufficient. 
Unless  the  season  is  very  dry  or  the  road  is  unusually  exposed  to  the 
wind,  a  single  application  will  be  enough  for  one  season. 

398.  RUTS.     Next  after  raveling,  the  tendency  to  form  ruts  is 
the  most  serious  evil  to  be  contended  against  in  the  maintenance  of 
crushed-stone  roads.     Ruts  are  due  either  (1)  to  a  greater  wheel 
load  than  the  road  is  capable  of  standing,  or  (2)  to  the  use  of  an 
inferior  binding  material,  as  loam,  or  (3)  to  tracking.     Ruts  are  most 
likely  to  occur  in  the  spring  or  during  a  wet  time,  when  the  road- 
bed is  soft,  and  are  more  common  on  country  roads  than  on  city 
streets,  since  in  the  latter  the  frequent  changes  in  direction  to  avoid 
other  vehicles  produce  a  more  uniform  wear  over  the  whole  surface  of 


ART.   3]  MAINTENANCE  225 

the  road.  However,  a  street-car  track  in  a  broken-stone  road  pre- 
vents the  distribution  of  traffic  uniformly  over  the  entire  surface  and 
greatly  increases  the  tendency  to  form  ruts. 

After  ruts  appear  the  only  remedy  is  to  fill  them  either  with 
new  material  or  by  picking  down  the  sides  of  the  ruts  and  raking 
the  loosened  material  into  the  depression.  Usually  the  latter  course 
is  the  wiser,  particularly  on  a  new  road.  Frequently  the  tendency 
to  form  a  rut  may  be  effectually  arrested  by  sweeping  into  it  the  loose 
detritus  from  the  adjacent  parts  of  the  road.  If  the  road  surface 
is  compact  and  hard,  it  may  be  necessary  to  loosen  the  bottom  and 
sides  of  the  rut  before  adding  new  material,  so  that  the  new  will 
thoroughly  unite  with  the  old.  The  new  material  should  be  of  the 
same  character  as  the  old,  as  otherwise  the  surface  will  wear  unequally 
and  become  rough. 

399.  PATCHING.  Formerly  much  attention  was  given  to  the 
patching  of  macadam  roads  to  keep  the  surface  free  from  ruts  and 
depressions  (see  pages  253-57  of  the  former  editions  of  this  treatise) ; 
but  the  only  water-bound  macadam  surfaces  now  in  use  are  those 
having  a  comparatively  small  amount  of  travel,  and  therefore  the 
repair  of  such  roads  is  a  comparatively  simple  matter. 

When  new  stone  is  added,  the  old  surface  should  be  loosened  to 
insure  that  the  new  stone  will  unite  with  the  old.  The  patch  should 
be  rounded  rather  than  square  cornered.  Care  should  be  taken  to 
leave  no  place  where  water  may  lodge.  When  new,  the  patch  should 
be  a  little  higher  than  the  adjoining  surface.  The  stone  employed 
in  patching  should  be  a  little  smaller  than  that  used  in  the  original 
construction.  It  is  better  to  lay  the  stone  in  two  thin  courses  than 
in  a  single  thick  one,  and  allow  the  first  to  become  consolidated 
before  the  second  is  added.  Ordinarily,  in  applying  patches  uTthin 
coats  over  small  areas  it  is  unnecessary  to  use  binding  material,  since 
the  road  usually  has  enough  detritus  to  fill  the  interstices  of  the  new 
stone.  If  laid  in  damp  weather,  when  the  surface  of  the  road  is  soft, 
there  is  usually  no  difficulty  in  getting  a  layer  one-stone  thick  to 
consolidate  without  any  binding  material.  If  the  patch  is  small 
and  thin,  it  will  usually  not  be  necessary  to  tamp  or  roll  it. 

When  the  surface  of  the  road  has  become  uneven  and  rough, 
and  when  the  broken  stone  is  thick  enough  not  to  require  much  new 
material,  the  top  of  the  road  is  loosened,  re-graded,  and  re-rolled. 
The  loosening  is  usually  done  by  running  over  the  road,  one  or  more 
times,  with  a  steam  stone-road  roller  having  spikes  in  the  rear  wheels, 
or  by  breaking  up  the  surface  with  a  scarafier, — a  cross  between  a  plow 


226  WATER-BOUND   MACADAM    ROADS  [CHAP.  VI 

and  a  harrow.  After  the  crust  is  broken  up,  the  surface  is  leveled 
off  by  the  use  of  a  harrow  and  hand  shovels  and  rakes;  and  then  it  is 
sprinkled  and  rolled  as  in  the  original  construction.  Usually  no  new 
binding  material  is  required,  the  detritus  from  the  old  road  being 
sufficient. 

400.  ROLLING.     In  the  spring  after  the  frost  goes  out,  the  road 
bed  is  soft  and  porous;   and  a  thorough  rolling  with  a  steam  roller 
at  this  time,  before  the  subgrade  is  dry,  is  one  of  the  best  and  cheap- 
est methods  of  keeping  a  macadam  road  in  good  condition.     Just 
before  this  rolling  is  the  time  to  add  a  little  fresh  surface  material, 
here  and  there,  as  may  be  needed  to  fill  up  slight  depressions. 

401.  SPRINKLING.    Moisture  is  necessary  to  preserve  the  cement- 
ing power  of  the  binding  material,  and  also  to  prevent  an  excessive 
removal  of  dust  by  the  wind;  and  therefore  sprinkling  to  the  extent 
required  to   prevent  these  injuries  is  an   advantage.     The  water 
should  be  applied  in  a  fine  spray,  and  not  be  allowed  to  run  in  streams 
on  the  road;  that  is,  several  light  sprinklings  are  better  than  a  single 
flooding.     If  sprinkled  too  heavily  or  too  often,  the  road  is  softened 
and  breaks  up  easily. 

Sprinkling  is  usually  employed  on  park  drives  and  city  streets, 
where  it  is  generally  conceded  to.  be  true  economy,  without  taking 
into  consideration  the  prevention  of  dust;  but  it  never  was  much 
used  on  rural  roads  on  account  of  the  expense,  and  the  introduction 
of  bituminous  coatings  as  dust  preventatives  has  entirely  done 
away  with  sprinkling  as  a  road  preservative. 

402.  For  a   discussion  of  sprinkling  with  water,   oil,   etc.,  see 
§  325-31  under  Gravel  Roads. 

For  a  discussion  of  bituminous  coatings  for  macadam  roads, 
see  Art.  1  and  2  of  Chapter  IX. 

403.  COST  OF  MAINTENANCE.     The   introduction    of  automo- 
biles greatly  increased  the  cost  of  maintaining  water-bound   mac- 
adam roads  (see  §  395).     Data  on  the  cost  of  maintaining  water- 
bound  roads  before  the  advent  of  motor-driven  vehicles  are  now  of 
little  or  no  value ;  and  there  are  almost  no  data  on  the  cost  of  main- 
tenance of  such  roads  where  the  amount  and  character  of  the  travel  is 
known. 


CHAPTER  VII 
PORTLAND-CEMENT  CONCRETE  ROADS 

406.  DEFINITIONS.     The  wearing  coat  of  a  concrete  road  is  a 
layer  of  portland-cement  concrete.     The  word  concrete  ordinarily 
means  a  mass  of  pebbles  or  broken  stone  bound  together  into  a  solid 
mass  by  hydraulic  cement ;  and  the  word  cement  in  engineering  liter- 
ature ordinarily  means  hydraulic   cement,  and   hi   recent  years  it 
usually  means  portland  cement.     Until  recently  the  type  of  road 
considered  in  this  chapter  was  called  a  concrete  road.     However, 
the  preceding  meanings  do  not  now  apply  in  discussions  concerning 
roads  and  pavements.     Recently  a  form  of  construction  has  been 
introduced  in  which  the  pebbles  or  fragments  of  broken  stone  are 
held  together  by  bituminous  cement  instead  of  hydraulic  cement; 
and  therefore  to  prevent  the  possibility  of  misunderstanding  it  is 
wise  to  employ  the  terms  portland-cement  concrete  road  or  simply 
Portland-concrete   road,   and  bituminous-cement  concrete  road  or 
simply  bituminous-concrete  road,  to  designate  these  two  types. 

407.  HISTORY.     Except  three  comparatively  small  sections  con- 
structed earlier,  concrete  was  not  used  for  road  surfaces  until  the 
early  years  of  this  century.     In  1909  there  were  in  this  country  less 
than  a  half  million  square  yards;  but  in  1916  there  were  laid  in  this 
country  24  million  square  yards  of  such  road  surfaces,  an  increase  of 
forty-eight  times  in  7  years.     At  present  about  50  per  cent  of  such 
surfaces  are  on  rural  roads,  and  about  25  per  cent  each  on  streets  and 
alleys. 

ART.  1.    THE  MATERIALS 

408.  The  concrete  is  composed  of  portland  cement,  sand  or  stone 
screenings,  and  pebbles  or  broken  stone.     The  sand  or  stone  screenings 
are  often  called  the  fine  aggregate,  and  the  pebbles  or  the  crushed 
stone  the  coarse  aggregate. 

409.  CEMENT.     For  a  discussion  of  the  characteristics  of  the 

227 


228  PORTLAND-CEMENT    CONCRETE    ROADS  [CHAP    VII 

different  types  of  hydraulic  cement  and  of  the  methods  of  testing  the 
same,  see  pages  54-83  of  the  tenth  edition  of  the  author's  Treatise  on 
Masonry  Construction.*  The  only  form  of  hydraulic  cement  used 
in  concrete  roads  is  portland  cement. 

It  is  almost  universally  specified  that  the  cement  shall  meet  the 
requirements  of  the  latest  standard  specifications  of  the  American 
Society  for  Testing  Materials,  which  also  have  been  adopted  by  all 
of  the  national  engineering  associations.  Printed  copies  of  these 
specifications  can  be  had  of  the  above  Society  for  a  nominal  sum; 
and  they  have  been  republished  by  various  organizations. 

410.  FINE    AGGREGATE.     For   an   extended   discussion   of   the 
characteristics  and  requirements  of  sand  and  screenings  for  making 
concrete  and  the  method  of  testing  each,  and  also  for  a  discussion 
of  the  relative  merits  of  sand  and  screenings,  see  pages  85-97  of 
the  tenth  edition  of  the  author's  Treatise  on  Masonry  Construc- 
tion.* 

411.  Sand  vs.  Screenings.     The  finer  particles  screened  out  of 
the  broken  stone  are  sometimes  used  in  concrete  instead  of  sand; 
and  there  is  much  discussion  as  to  the  relative  merits  of  sand  and 
screenings.     Experiments  show  that  if  all  conditions  are  the  same, 
the  screenings  make  slightly  stronger  mortar;  but  in  practice  it  has 
been  found  nearly  impossible  to  exclude  dust  from  the  stone  screen- 
ings, particularly  if  the  stone  is  soft  (as  is  most  limestones)  or  if  the 
screenings  get  wet  before  being  screened,  and  hence  in  practice  mor- 
tar or  concrete  made  from  screenings  is  not  usually  as  strong  as  that 
made  of  sand  of  the  same  degree  of  fineness.     For  these  reasons 
many  engineers  prohibit  the  use  of  stone  screenings. 

412.  The  specifications  for  the  fine  aggregate  recommended  by 
the  1916  National  Conference  on  Concrete  Road  Building  are  as 
follows: 

"The  fine  aggregate  shall  be  natural  sand  or  screenings  from  hard,  tough, 
durable  crushed  rock  or  gravel;  and  shall  consist  of  quartzite  grains  or  other 
equally  hard  material  graded  from  fine  to  coarse  with  the  coarse  particles  pre- 
dominating. When  dry  it  shall  pass  a  screen  having  four  meshes  per  linear  inch, 
and  not  more  than  25  per  cent  shall  pass  a  sieve  having  fifty  meshes  per 
linear  inch,  and  not  more  than  5  per  cent  shall  pass  a  sieve  having  one  hundred 
meshes  per  linear  inch.  It  shall  not  contain  vegetable  or  other  deleterious  mat- 
ter, nor  more  than  3  per  cent  by  weight  of  clay  or  loam.  Routine  field  tests  shall 
be  made  on  the  fine  aggregate  as  delivered.  If  there  is  more  than  5  per  cent  of 


*  A  Treatise  on  Masonry  Construction,  by  Ira  O.  Baker.     745  pages,  6X9  inches.       John 
Wiley  &  Sons,  New  York  City.     Tenth  Edition,  1909. 


ART.    1]  MATERIALS  229 

clay  or  loam  by  volume  settled  after  one  hour's  shaking  in  an  excess  of  water, 
the  material  represented  by  the  sample  shall  be  held  pending  laboratory  tests. 

"The  fine  aggregate  shall  be  of  such  quality  that  mortar  composed  of  one 
part  portland  cement  and  three  parts  fine  aggregate,  by  weight,  when  made  into 
briquettes,  shall  show  a  tensile  strength  (at  seven  and  twenty-eight  days) 
equal  to  or  greater  than  the  strength  of  briquettes  composed  of  one  part  by  weight 
of  the  same  cement  and  three  parts  standard  Ottawa  sand.  The  percentage  of 
water  used  in  making  the  briquettes  of  cement  and  fine  aggregate  shall  be  such 
as  to  produce  a  mortar  of  the  same  consistency  as  that  of  the  Ottawa  sand  bri- 
quettes of  standard  consistency." 

413.  COARSE  AGGREGATE.     For  a  discussion  of  the  requisites 
of  gravel  and  broken  stone  for  concrete,  see  pages  97-103  of  the 
tenth  edition  of  the  author's  Treatise  on  Masonry  Construction. 

414.  Gravel  vs.  Broken  Stone.     There  is  difference  of  opinion 
as  to  the  relative  merits  of  gravel  and  broken  stone  for  concrete. 
The  elements  to  be  considered  are  the  strength,  density,  and  cost  of 
the  concrete. 

Gravel  concrete  has  only  about  80  to  90  per  cent  of  the  strength 
of  broken-stone  concrete. 

Experience  shows  that  gravel  concrete  is  more  easily  compacted, 
and  has  fewer  cavities  than  broken-stone  concrete;  and  hence  gravel 
concrete  is  denser  and  more  waterproof. 

As  a  rule,  the  first  cost  of  gravel  is  less  than  that  of  broken  stone, 
and  the  former  is  considerably  easier  to  handle. 

415.  However,  since  gravel  is  liable  to  contain  so  much  clay  or 
loam  as  to  materially  reduce  the  strength  of  the  concrete,  some 
engineers  for  this  reason  alone  prefer  broken  stone  to  gravel.     Even 
though  only  portions  of  the  gravel  are  naturally  dirty,  or  even  though 
only  portions  of  it  are  likely  to  contain  an  undue  amount  of  the  strip- 
ping, some  engineers,  owing  to  the  greater  care  required  in  inspection 
and  to  the  uncertainty  of  eliminating  all  dirty  gravel,  prefer  broken 
stone  to  gravel. 

416.  The  Specifications  for  the  coarse  aggregate  recommended  by 
the  1916  National  Conference  on  Concrete  Road  Building  are  as 
follows : 

"The  coarse  aggregate  shall  consist  of  clean,  hard,  tough,  durable  crushed 
rock  or  pebbles  graded  hi  size,  free  from  vegetable  or  other  deleterious  matter; 
and  shall  contain  no  soft,  flat  or  elongated  particles.  The  size  of  the  coarse 
aggregate  shall  be  such  as  to  pass  a  l£-inch  round  opening,  and  shall  range  from 
1£  inches  down;  and  not  more  than  5  per  cent  shall  pass  a  screen  having  four 
meshes  per  linear  inch,  and  no  intermediate  sizes  shall  be  removed. 

"Crusher-run  stone,  bank-run  gravel,  or  artificially  prepared  mixtures  of 
fine  and  coarse  aggregate  shall  not  be  used." 


230  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

417.  THEORY   OF    PROPORTIONS.      The  whole   theory  of  the 
proper  proportions  for  concrete  is  comprised  in  two  laws  as  follows : 

1.  For  the  same  fine  aggregate  and  the  same  coarse  material,  the 
strongest  concrete  is  that  containing  the  greatest  per  cent  of  cement 
in  a  unit  of  volume. 

2.  For  the  same  per  cent  of  cement  and  the  same  aggregates,  the 
strongest  concrete  is  made  with  that  combination  of  fine  and  coarse 
aggregate  which  gives  a  concrete  of  the  greatest  density. 

The  first  law  is  almost  self-evident,  and  concerns  the  relative 
richness  of  the  concrete.  Experiments  have  shown  this  law  to  be 
almost  mathematically  exact. 

The  second  law  is  very  important,  and  concerns  the  qualities  of 
the  fine  and  coarse  aggregates.  The  second  law  is  equivalent  to  say- 
ing that  the  cement  should  fill  the  voids  of  the  sand,  and  that  the 
resulting  mortar  should  fill  the  voids  of  the  coarse  aggregate.  If  the 
cement  does  not  fill  the  voids  of  the  sand,  or  if  the  mortar  does  not 
fill  the  voids  of  the  coarse  aggregate,  the  concrete  will  obviously  be 
less  dense  than  though  the  voids  were  just  filled.  If  the  paste  is  more 
than  enough  to  fill  the  voids  in  the  sand,  or  if  the  mortar  is  more  than 
enough  to  fill  the  voids  in  the  coarse  aggregate,  the  concrete  will  be 
less  dense  than  though  the  voids  were  just  filled;  since  both  the  paste 
and  the  cement  mortar  have  a  less  density  than  ordinary  concrete; 
and  hence  the  strength  due  to  the  increased  amount  of  cement  may 
be  neutralized  by  the  decrease  in  density,  but  the  possibilities  of 
this  depend  upon  the  plasticity  of  the  mortar,  the  amount  of  tamp- 
ing, the  character  of  the  sand  and  the  stone,  and  the  gradation  of 
the  sizes.  Experiments  and  experience  have  shown  this  law  to  be 
almost  mathematically  exact. 

418.  METHODS  OF  PROPORTIONING.     There  are  four  methods  in 
more  or  less  general  use  for  proportioning  concrete,  which  may  be 
briefly  designated    as   follows:     (1)   by  arbitrary   assignment;    (2) 
by  voids;    (3)  by  trial;   and  (4)  by  sieve-analysis.     These  methods 
will  be  considered  separately  in  the  above  order. 

419.  Proportioning  by  Arbitrary  Assignment.     This  is  the  least 
scientific  method  of  proportioning,  it  virtually  assumes  the  relations 
as  a  matter  of  judgment,  without  much,  if  any,  consideration  of  the 
character  of  the  aggregate;    that  is,  the  proportions  are  assigned 
without  any  reference  to  the  fineness  or  coarseness  of  the  sand  and  the 
stone,  or  to  the  gradation  of  the  sizes  of  each. 

420.  Proportioning  by  Voids.     To  determine  the  best  propor- 
tions for  any  particular  sand  and  aggregate,  find  the  per  cent  of  voids 


ART.    1]  MATERIALS  231 

in  the  sand  and  in  the  stone,  and  use  enough  cement  paste  to  fill  the 
voids  in  the  sand  and  enough  mortar  to  fill  the  voids  in  the  coarse 
aggregate.  Owing  to  the  errors  in  using  this  method,  and  particu- 
larly in  getting  the  cement  paste  to  fill  the  voids  of  the  sand  and  the 
mortar  to  fill  the  voids  of  the  stone,  this  method  is  not  very  prac- 
tical. 

421.  Proportioning  by  Trial.     The  second  law  in  §  417  leads  to  a 
simple  method  of  finding  the  best  relation  of  the  sand  and  the  stone. 
According  to  that  law  that  combination  of  sizes  of  sand  and  stone 
which  with  a  constant  quantity  of  cement  gives  the  least  volume  of 
concrete  is  the  best. 

To  apply  this  method  procure  a  vessel  of  uniform  cross  section, 
say  a  cylinder,  10  or  12  inches  in  diameter  and  12  or  14  inches  deep, 
its  strength  being  such  that  its  volume  will  not  be  changed  in  tamping 
it  full  of  concrete.  Weigh  out  a  unit  of  cement,  and  any  number  of 
units  of  sand,  say  two,  and  also  weigh  out  any  number  of  units  of 
broken  stone,  say  five,  taking  care  that  the  quantities  are  such  that 
when  the  ingredients  are  thoroughly  mixed  and  placed  in  the  cylinder, 
the  mixture  will  fill  it  only  partly  full, — say  three  quarters  full. 
Make  a  concrete  of  any  desired  consistency  by  mixing  the  cement, 
sand  and  stone  with  water  on  a  sheet  of  steel ;  tamp  the  concrete  into 
the  cylinder  leaving  the  upper  surface  smooth  and  horizontal,  and 
then  measure  the  depth  of  the  concrete  from  the  top  of  the  cylinder. 
Next  empty  the  concrete  from  the  cylinder,  clean  it  and  the  tools; 
and  then  make  another  batch  with  different  proportions  of  sand  and 
stone,  keeping  the  quantity  of  cement  and  the  plasticity  of  the  con- 
crete the  same  as  before.  If  this  batch  when  tamped  into  the  cylin- 
der gives  a  less  volume  of  concrete,  this  proportion  is  better  than  the 
first.  Continue  the  trials  until  the  proportion  has  been  found  which 
will  give  the  least  depth  in  the  cylinder. 

422.  Proportioning  by  Sieve  Analysis.*    "  Sieve   analysis  con- 
sists in  separating  the  particles  or  grains  of  a  sample  of  any  material- 
such  as  broken  stone,  gravel,  sand  or 'cement — into  the  various  sized 
particles  of  which  it  is  composed,  so  that  the  material  may  be  repre- 
sented by  a  curve  each  of  whose  ordinates  is  the  percentage  of  the 
weight  of  the  total  sample  which  passes  a  sieve  having  holes  of  a 
diameter  represented  by  the  distance  of  this  ordinate  from  the  origin 


*  This  method  of  proportioning  the  sizes  of  the  sand  and  stone  in  concrete  was  devised  by 
Wm.  B.  Fuller,  and  is  described  by  him  in  detail  on  pages  183-215  of  Taylor  and  Thomson's 
Concrete  Plain  and  Reinforced  (1909  edition),  from  which  these  extracts  are  taken  by  permis- 
sion of  the  authors, 


232 


PORTLAND-CEMENT    CONCRETE    ROADS  [CHAP.    VII 


in  the  diagram."  The  line  DBKLA,  Fig.  73,  is  a  typical  sieve- 
analysis  curve  for  crusher-run  micaceous-quartz  stone;  and  the  line 
OF  represents  a  fine  sand. 

"  The  objects  of  sieve-analysis  as  applied  to  concrete  aggregates 
are:  (1)  to  show  graphically  the  sizes  and  relative  sizes  of  the  par- 
ticles; (2)  to  indicate  what  sized  particles  are  needed  to  make  the 
aggregate  more  nearly  perfect,  and  so  to  enable  the  engineer  to  im- 
prove it  by  the  addition  or  substitution  of  another  material;  and  (3) 
to  afford  means  for  determining  the  best  proportions  of  different 
aggregates." 

"  The  experience  which  the  writer  [Fuller]  has  had  and  the  various 
experiments  which  he  has  made  indicate  that  concrete  which  works 


F        E 


0.25  0.50  075  1.00  1£5  t.50  1.75 

Diameter  of  Parftcfe3  in  Inches 

FIG,  73, — SIEVE-ANALYSIS  CURVE. 


the  smoothest  in  placing  and  gives  the  highest  breaking  strength  for 
a  given  percentage  of  cement,  is  made  from  an  aggregate  whose 
sieve  analysis,  taken  after  mixing  the  sand  and  the  stone,  forms  a 
curve  approaching  a  parabola  having  its  beginning  at  the  zero  of  co- 
ordinates and  passing  through  the  intersection  of  the  curve  of  the 
coarsest  stone  with  the  100  per  cent  line,  that  is,  passing  through  the 
upper  end  of  the  coarsest  stone  curve."  In  Fig.  73,  the  parabola 
OCPA  represents  a  theoretically  perfect  combination  of  sizes  of 
sand  and  stone  all  of  whose  pieces  pass  a  If -inch  screen.  This  curve 
shows,  for  example,  that  for  the  best  combination  of  the  above 
materials  93  per  cent  of  the  mixture  should  pass  the  li-inch  sieve,  76 


ART.    1]  MATERIALS  233 

per  cent  should  pass  the  1-inch  sieve,  54  per  cent  the  ^-inch,  and  so 
on." 

"  Where,  as  in  Fig.  73,  the  materials  to  be  mixed  are  represented 
by  only  two  curves  no  combination  of  which  will  make  a  curve  as 
close  to  a  parabola  as  is  desirable,  there  is  another  limiting  condition 
which  was  brought  out  by  the  experiments,  viz.,  that  for  the  best 
results  the  combined  curve  shall  intersect  the  parabola  on  the  40 
per  cent  line,  at  C,  and  that  the  finer  material  shall  be  assumed  to 
include  the  cement." 

423.  The  curve  DBKLA,  Fig.  73,  may  be  transformed   so  that 
it  will  pass  through  C,  by  changing  the  distances  from  the  top  of  the 

TPC*       f\C\ 

diagram  to  the  line  DBKLA  in  the  proportion  -==  =  —  =  61  per  cent, 

tin      Uo 

which  shows  that  61  per  cent  of  the  dry  materials  should  be  broken 
stone.  In  a  similar  manner  the  line  OF  is  re-plotted  in  the  position 
OJ.  The  line  OJCGA  is  assumed  to  represent  the  best  possible 
combination  of  sizes  of  this  sand  and  stone.  For  example,  with  the 
best  possible  combination  of  sizes  of  this  stone  and  sand,  89  per  cent 
would  pass  the  1  J-inch  sieve,  67  per  cent  would  pass  the  1-inch  sieve, 
46  per  cent  the  J-inch  sieve,  and  so  on. 

"  The  proportion  of  cement  to  be  used  to  give  the  required 
strength  of  concrete  must  always  be  assumed;  and  in  this  example 
it  will  be  assumed  that  the  cement  is  to  constitute  one  eighth  of  the 
dry  materials  (measured  before  the  sand  and  stone  are  mixed  together), 
which  will  make  the  cement  one  ninth  or  11  per  cent  of  the  total  dry 
materials.  Since  the  diagram  shows  that  the  sand  and  cement  are 
to  constitute  39  per  cent  of  the  dry  materials,  the  sand  must  then  be 
39- 11  =28  per  cent." 

"  The  proportions  of  concrete  for  1  part  cement  to  8  parts  of 
sand  and  stone,  measured  separately,  then  are:  11  per  cent  cement, 
28  per  cent  sand,  and  61  per  cent  broken  stone,  or  1  :  2.5  :  5.5  by 
weight.  If  the  proportions  are  required  by  volume  and  the  relative 
weights  of  the  sand  and  the  stone  differ  from  their  relative  volumes, 
the  proportions  should  be  corrected  accordingly." 

424.  "  An  important  feature  of  the  sieve-analysis  curves  is  that 
they  show  how  the  materials  may  be  improved  by  adding  or  sub- 
tracting some  particular  size.     For  example,   if  the  stone  repre- 
sented by  the  curve  DBKLA  in  Fig.  73  had  contained  more  pieces 
0.5  and  1.0  inch  in  diameter,  its  curve  would  have  more  nearly  ap- 
proached the  parabola  in  the  region  SG.     If  a  stone  giving  the  line 
DRHA  were  used,  the  ratio  for  transforming  the   line  to  make  it 


234  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

EC     60 
pass  through  C  would  be  7^  =  —  =  66  per  cent,  which  shows  that  with 


the  assortment  of  sizes  of  broken  stone  represented  by  this  line  the 
best  concrete  is  made  by  using  66  per  cent  of  broken  stone.  For  a 
1  :  8  mixture  as  before,  the  proportions  would  be  11  :  23  :  66,  or 
1:2:  6,  —  a  cheaper,  stronger,  and  denser  concrete  than  that  made 
with  the  stone  represented  by  the  line  DBKLA." 

For  further  details  concerning  this  method  and  for  later  slight 
modifications  in  the  form  of  the  ideal  sieve-analysis  curve,  see  Taylor 
and  Thompson's  book  mentioned  above  or  the  author's  Treatise  on 
Masonry  Construction.* 

"  This  method  affords  a  means  of  determining  the  best  propor- 
tions in  which  to  mix  the  fine  and  coarse  aggregate,  and  also  shows 
how  the  aggregate  may  be  improved  by  adding  or  subtracting  some 
particular  size.  Sieve  analyses  can  be  made  from  time  to  time  as 
the  work  progresses  to  see  whether  or  not  the  sizes  of  the  aggregate 
have  changed;  and  if  the  sizes  have  changed,  the  proportions  can  be 
varied  to  secure  the  most  economical  and  the  densest  concrete. 
In  a  work  of  any  magnitude  the  greater  labor  required  in  determining 
the  proportions  by  sieve-analysis  curves  is  likely  to  be  justified  by 
the  better  quality,  or  the  less  cost,  of  the  concrete;  and  the  extra 
labor  required  to  make  sieve  analyses  during  the  progress  of  the  work 
will  be  worth  all  it  costs  because  of  the  better  control  of  the  propor- 
tions of  the  concrete. 

:<  To  secure  the  maximum  benefit  of  this  method  of  proportion- 
ing, it  is  necessary  to  screen  the  aggregate  to  several  sizes  and  then 
combine  them  in  the  proportions  indicated  by  the  sieve-analysis 
curve.  As  to  whether  or  not  the  increased  cost  of  screening  and 
proportioning  would  be  justified  by  the  saving  of  cement,  depends 
upon  the  magnitude  of  the  work  and  other  conditions.  The  following 
example  illustrates  the  possibilities: 

'The  ordinary  mixture  for  water-tight  concrete  is  about 
1  :  2J  :  4J,  which  requires  1.37  barrels  of  cement  per  cubic  yard  of 
concrete.  By  carefully  grading  the  materials  by  the  methods  of 
sieve  analysis  the  writer  [Fuller]  has  obtained  water-tight  work  with 
a  mixture  of  about  1:3:7,  which  requires  only  1.01  barrels  of 
cement  per  cubic  yard  of  concrete.  This  saving  of  0.36  barrel  is 
equivalent,  with  portland  cement  at  $1.60  per  barrel,  to  $0.58  cubic 


*  For  an  explanation  of  the  advantages  of  plotting  sieve-analysis  curves  on  logarithmic 

11'1  of  an  ingenious  use  °f  the 


ART.    1]  MATERIALS  235 

yard  of  concrete.  The  added  cost  of  labor  for  proportioning  and 
mixing  the  concrete  because  of  the  use  of  five  grades  of  aggregate 
instead  of  two,  was  about  $0.15  per  cubic  yard,  thus  effecting  a  net 
saving  of  $0.43  per  cubic  yard." 

425.  DATA  FOR  ESTIMATES.     Portland  Cement.     Portland  ce- 
ment, which  is  now  practically  the  only  cement  used  in  engineering 
construction,  is  usually  shipped  in  cloth  bags  containing  94  Ib.  each. 
The  cement  is  usually  sold  by  the  barrel,  which  is  considered  as  four 
bags.     In  computations  involving  the  proportions  for  concrete,  a 
bag  is  usually  considered  as  containing  one  cubic  foot  of  packed 
cement. 

Until  the  disturbance  of  prices  by  the  Great  European  War  the 
price  of  portland  cement  at  any  place  east  of  Omaha,  Nebraska, 
was  from  $1.50  to  $1.75  per  barrel  in  cloth  bags,  the  bags  being 
charged  at  10  cents  each  and  undamaged  bags  being  returnable  at 
that  price. 

426.  Sand  and  Gravel.     Before  the  Great  European  War  the 
price  of  washed,  screened,  and  graded  sand  at  the  pit  was  about  25 
cents  per  ton;  and  of  washed,  screened,  and  graded  gravel,  about  35 
cents  per  ton.     The  freight  was  about  47  to  48  cents  per  ton  per  100 
miles  in  each  case. 

427.  Broken  Stone.     Before  the  disturbances  of  prices  by  the 
Great   European   War,  the   price   of    crushed   limestone,    screened 
and  graded,  f.o.b.  the  quarry,  was  about  55  cents  per  ton;  and  the 
freight  was  about  47  to  48  cents  per  ton  per  100  miles.     Limestone, 
graded  and  loaded  into  a  freight  car  weighs  about  2500  Ib.  per  cubic 
yard. 

The  cost  of  crushed  trap  f.o.b.  the  quarries  in  New  Jersey  for 
several  years  previous  to  1900,  was  40  to  50  cents  per  ton  (about  50 
to  62  cents  per  cubic  yard) ;  but  in  that  year  the  price  was  increased 
nearly  50  per  cent.  In  Massachusetts,  the  cost  of  broken  trap  on 
cars  at  the  end  of  the  railroad  transportation,  varies  from  $1.10  to 
$1.60  per  ton  (about  $1.32  to  $1.93  per  cubic  yard).  In  Boston,  the 
cost  of  crushed  granite  delivered  on  the  streets  is  $1.65  to  $1.90  per 
ton.  In  Montreal,  syenite  delivered  on  the  streets  costs  an  average 
of  $1.15  to  $1.20  per  ton. 

428.  Ingredients  for  a  Cubic  Yard.     Table  28,  page  236,  shows 
the  quantities  of  cement,  sand,  and  stone  required  for  a  cubic  yard 
of  concrete  of  different  proportions,  using  three  grades  of  broken 
stone  or  gravel.     The  concrete  was  mixed  wet  but  not  soupy;   and 
was  also  mixed  very  thoroughly.     If  it  had  been  mixed  drier  or  less 


236 


PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.    VII 


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ART.    1]  MATERIALS  237 

thoroughly,  it  would  have  been  less  dense,  and  consequently  less 
quantities  of  materials  would  have  been  required  to  make  a  yard. 

Data  like  that  in  Table  28  are  affected  by  the  fineness  of  the 
cement,  the  fineness  and  dampness  of  the  sand,  the  kind  and  coarse- 
ness of  the  stone,  the  proportions  of  the  several  sizes  of  sand  grains 
and  stone  fragments,  the  thoroughness  of  the  mixing,  the  amount  of 
tamping,  etc.;  and  different  experimenters  have  obtained  widely 
different  results.  Most  experimenters  obtain  a  less  quantity  because 
the  concrete  is  mixed  drier  and  entrains  more  air,  and  hence  is  less 
dense. 

429.  Fuller's  Rule.  The  following  rule,  devised  by  Wm.  B. 
Fuller,*  gives  the  quantities  of  cement,  sand,  and  stone  required  to 
make  a  cubic  yard  of  concrete;  and  is  fairly  representative  of  all 
classes  of  materials.  This  rule  is  valuable  because  it  is  so  simple 
that  it  can  be  carried  in  the  memory. 

c  =  number  of  parts  of  cement. 

s  =  number  of  parts  of  sand. 

g  =  number  of  parts  of  gravel  or  broken  stone. 

C  =  number  of  barrels  of  packed  portland  cement  required  for  1 

cubic  yard  of  concrete. 
S  =  number  of  cubic  yards  of  loose  sand  required  for  1  cubic  yard 

of  concrete. 
G  =  number  of  cubic  yards  of  loose  gravel  or  broken  stone  required 

for  1  cubic  yard  of  concrete. 

11 

~  c  +  s  +  g' 


"  If  the  coarse  material  is  broken  stone  screened  to  uniform 
size,  it  will  contain  less  solid  matter  in  a  given  volume  than  average 
stone,  and  hence  about  5  per  cent  should  be  added  to  quantities  of  all 
three  ingredients  as  computed  by  the  above  rule.  On  the  other  hand, 
if  the  coarse  material  is  well  graded  in  size,  about  5  per  cent  may  be 
deducted  from  all  of  the  quantities." 

*  Taylor  and  Thompson's  Concrete  Plain  and  Reinforced,  Second  Edition,  p.  16. 


238  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.    VII 

The  preceding  formulas  are  sometimes  modified  by  changing  the 
constants  11  and  3.8.  For  example,  one  engineer  substitutes  9.5 
for  the  11,  and  4  for  the  3.8. 


ART.  2.     THE  CONSTRUCTION 

431.  DRAINAGE.  The  drainage  of  the  road-bed  of  a  concrete 
road  or  pavement  is  of  vital  importance.  If  the  subgrade  is  not  well 
drained,  there  is  danger  that  after  the  concrete  is  laid,  the  drying  of 
the  soil  under  the  edges  of  the  concrete  may  permit  the  pavement  to 
settle  and  thus  cause  longitudinal  cracks  on  the  surface.  Further, 
if  the  subgrade  is  not  well  drained,  there  is  a  possibility  that  the  frost 
may  lift  the  edges  of  the  concrete  roadway  and  cause  a  longitudinal 
crack, — at  least  on  the  lower  side.  With  some  forms  of  roads  a  crack 
in  the  surface  will  heal  under  travel;  but  a  crack  in  a  concrete  pave- 
ment not  only  can  not  heal  under  travel,  but  will  continually  enlarge. 
There  is  no  part  of  the  work  of  the  construction  of  a  concrete  road  or 
pavement  that  is  more  worthy  of  intelligent  care  and  painstaking 
labor  than  the  preparation  of  the  subgrade;  and  the  slight  addi- 
tional cost  necessary  to  insure  good  results  is  abundantly  justifiable. 

If  the  soil  is  sandy,  there  is  a  probability  that  the  natural  under- 
drainage  is  sufficient  for  the  purpose. 

If  the  soil  is  only  moderately  retentive,  a  shallow  longitudinal 
ditch  should  be  constructed  just  outside  of  the  edge  of  the  concrete 
slab.  The  ditch  should  extend  about  8  or  10  inches  below  the  sur- 
face of  the  road-bed,  i.  e.,  below  the  bottom  of  the  concrete  slab;  and 
should  be  filled  with  coarse  gravel  or  broken  stone.  From  this  longi- 
tudinal ditch  short  transverse  trenches  should  be  dug  across  the 
shoulder  to  the  ditch  at  the  side  of  the  roadway.  These  transverse 
trenches  should  have  a  grade  sufficient  to  permit  them  to  carry  the 
water  promptly  and  completely  to  the  side  ditch.  In  particularly 
retentive  soil,  these  transverse  trenches  should  not  be  placed  more 
than  50  feet  apart.  On  the  level  stretches,  these  transverse  trenches 
should  be  practically  at  right  angles  to  the  direction  of  the  road; 
but  if  the  road  is  on  a  grade,  they  should  make  an  acute  angle  with 
the  roadway,  the  amount  of  this  angle  depending  upon  the  grade  of 
the  road.  These  lateral  ditches  should  be  filled  level  full  with  broken 
stone  or  coarse  gravel  to  a  point  at  least  a  little  beyond  the  outer 
edges  of  the  shoulders  and  preferably  nearly  to  the  bank  of  the  ditch 
at  the  side  of  the  roadway. 

If  the  soil  is  so  retentive  that  the  underground  water  level  is  likely 


ART.    2] 


THE    CONSTRUCTION 


239 


to  rise  within  a  foot  or  so  of  the  surface,  then  a  farm  tile  should  be  laid 
on  one  or  both  sides  of  the  paved  portion.  For  a  discussion  of  the 
purposes  of  underdrainage  and  the  method  and  cost  of  laying  the 
tile,  see  §  114-24  of  Chapter  III, — Earth  Roads.  For  a  discussion  of 
the  subject  of  Side  Ditches,  see  §  125-28. 

432.  V-Drains.     Some  engineers  employ  what  is  usually   called 
a  V-drain,  Fig.  74.     The  subgrade  is  excavated  to  a  depth  below 


Concrete  I/fearing  Cog f 


FIG.  74. — THE  V-DRAIN. 

the  base  of  the  concrete  slab  of  12  to  18  inches  at  the  center  and  4  to 
6  inches  at  the  side,  and  this  trench  is  filled  with  loose  stone. 
The  sizes  of  the  stones  are  usually  limited  as  follows:  the  largest 
the  equivalent  of  a  6-inch  cube,  and  the  smallest  a  2-inch  fragment. 
The  largest  stones  are  placed  in  the  bottom  of  the  trench,  and  no 
3-inch  stone  is  allowed  within  2*  inches  of  the  upper  surface  of  the 
stone  in  the  trench.  After  the  trench  is  filled  with  stones,  it  is  usually 
rolled  with  a  self-propelling  roller,  and  any  depressions  that  appear 
are  filled  with  stones. 

This  form  of  drain  ordinarily  costs  50  to  60  cents  per  linear  foot, 
and  is  of  doubtful  economy,  unless  where  bowlders  or  loose  stones 
abound  on  or  near  the  road,  or  where  the  road  is  on  very  wet  and 
retentive  soil. 

433.  PREPARING  THE  SUBGRADE.     The  fundamental   require- 
ment is  that  the  subgrade  shall  at  all  times  be  of  uniform  density, 
so  that  it  will  not  settle  unevenly  and  cause  cracks  in  the  concrete 
surface. 

434.  Before  the  grading  is  begun,  or  at  least  before  it  is  com- 
pleted, the  curbs  (Chapter  XIV)  are  built,  or  the  side  forms  (§  448) 
are  set,  to  serve  as  guides  in  bringing  the  subgrade  to  the  proper 
surface. 

435.  On  Virgin  Soil.    All  brush,  trees,  stumps,  and  large  roots 
for  a  width  of  25  feet  on  each  side  of  the  center  line  of  the  proposed 
roadway,  should  be  cut  off  and  be  removed  from  the  limits  of  the 
right-of-way.*     The  road  should  be  grubbed  for  the  full  width  of  the 

*  If  the  pavement  is  to  be  more  than  15  feet  wide,  this  quantity  is  to  be  proportionally 
greater. 


240  PORTLAND-CEMENT    CONCRETE    ROADS  [CHAP.    VII 

excavation,  and  no  stumps  or  large  roots  should  be  left  within  these 
limits  except  when  a  fill  of  more  than  5  feet  is  called  for  on  the  plans, 
in  which  case  all  stumps  should  be  cut  off  to  within  12  inches  of  the 
original  ground  level.  Stumps  on  the  cleared  portion  not  within  the 
grubbed  limits,  should  be  cut  off  to  within  12  inches  of  the  original 
ground  level.  Stumps  on  the  cleared  portion  not  within  the  grubbed 
limits,  should  be  cut  off  not  more  than  2  feet  from  the  ground. 

All  soft  or  spongy  spots  and  all  vegetable  matter  should  be 
removed,  and  the  space  be  re-filled  with  suitable  material.  It  is  not 
wise  to  put  any  dependence  upon  the  ability  of  the  concrete  slab  to 
bridge  soft  spots.  After  the  subgrade  has  been  brought  to  the  proper 
surface,  it  should  be  rolled  with  a  3-wheel  10-ton  self-propelling  roller 
to  disclose  any  soft  spots.  Any  depressions  discovered  in  rolling 
should  be  filled  and  be  re-rolled  until  no  depressions  appear.  The 
rolling  should  be  continued  until  the  roller  leaves  no  material  track 
or  depression.  However,  clean  dry  sand  can  not  be  consolidated  by 
rolling;  and  some  plastic  clay  can  be  rolled  too  much. 

After  the  grading  is  completed,  if  the  natural  lay  of  the  ground 
is  such  that  there  are  sumps  or  pockets  which  hold  water,  or  if  such 
sumps  or  pockets  are  made  during -the  progress  of  the  work,  they 
should  be  thoroughly  drained. 

436.  On  Earth  Road.    If  the  concrete  road  or  pavement  is  to  be 
constructed  upon  virgin  soil,  that  is,  if  it  is  not  to  be  constructed 
on  an  old  road-bed,  the  precautions  described  above  are  sufficient  to 
secure  a  reasonably  good  foundation.     But  if  the  pavement  is  to  be 
constructed  upon  an  old  road-bed  of  any  kind,  great  care  must  be 
taken  in  preparing  the  subgrade.     The  old  road-bed  is  likely  to  be 
more  compact  in  the  center  than  at  the  sides;    and  consequently 
there  is  danger  that  the  pavement  will  settle  more  at  the  sides  than 
at  the  center,  and  therefore  will  crack  longitudinally.     Further,  it  is 
likely  that  the  traveled  way  of  the  old  road  will  not  at  all  places  be 
central  under  the  new  pavement,  and  consequently  the  latter  will 
settle  unevenly  and  crack. 

When  the  subgrade  is  an  old  roadway,  since  the  roller  is  likely  to 
balance  upon  the  more  compact  central  core  and  therefore  not 
consolidate  the  soil  at  the  side  of  the  old  roadway,  it  is  not  sufficient 
to  roll  the  subgrade  longitudinally.  In  extreme  cases  it  may  be 
necessary  to  plow  the  old  road-bed  and  then  harrow  it,  and  finally 
consolidate  the  entire  new  road-bed  with  the  roller. 

437.  On  Gravel  or  Macadam  Road.    Not  infrequently  a  concrete 
slab  is  constructed  on  an  old  gravel  or  macadam  road.    There  is 


ART.    2]  THE   CONSTRUCTION  241 

considerable  difference  of  opinion  as  to  the  wisdom  and  the  method 
of  utilizing  the  old  road.  The  objections  and  difficulties  are  that  the 
old  road  usually  has  too  much  crown;  the  surface  is  full  of  holes;  the 
old  road  often  has  an  undesirable  profile;  and  the  old  road  is  narrower 
than  the  new  one,  and  consequently  the  sides  are  likely  to  settle  and 
crack  the  concrete  slab  longitudinally.  On  the  other  hand,  the  old 
road  is  already  in  place,  is  usually  well  consolidated,  and  a  careful 
investigation  should  be  made  to  determine  whether  it  can  be  econom- 
ically used. 

The  first  thing  is  to  establish  a  grade  line  for  the  subgrade.  This 
may  require  the  cross  sectioning  of  the  old  road  at  frequent  intervals, — 
perhaps  each  50  or  100  feet,  depending  upon  the  condition  of  the  sur- 
face and  the  regularity  of  its  profile.  After  having  established  a 
grade  line,  it  is  usually  necessary  to  scarify  the  old  road  in  places  to 
remove  the  high  spots.  The  low  places  should  be  filled  with  gravel 
or  stone  to  avoid  an  excessive  depth  of  concrete.  The  subgrade  is 
then  rolled;  and  if  necessary,  the  low  places  are  again  filled  and 
again  rolled.  The  permissible  degree  of  variation  of  the  subgrade 
from  the  proposed  contour  depends  uporf  the  relative  cost  of  the 
labor  required  in  finishing  and  of  the  concrete  required  to  fill  the  low 
places. 

It  is  particularly  important  that  the  side  forms  for  the  concrete 
(see  §  448)  should  be  set  up,  or  that  the  curbs  should  be  built,  before 
the  old  road  is  scarified,  so  they  may  carry  the  templet  used  as  a 
guide  in  re-shaping  the  old  road. 

If  the  old  road  is  narrower  than  the  new,  or  not  central  under  it, 
great  care  must  be  taken  in  consolidating  the  soil  at  the  edge  of  the 
old  road.  These  edges  should  be  covered  with  gravel  or  broken  stone 
and  be  rolled  until  they  are  firm  and  solid. 

438.  Cross  Section  of  Subgrade.     The  cross  section  of  the  sub- 
grade  is  made  either  flat  or  crowned  to  conform  to  the  finished  sur- 
face of  the  concrete.     The  former  seems  to  be  the  better  and  more 
common.     It  is  claimed  that  experience  shows  that  a  pavement  hav- 
ing a  flat  subgrade  is  less  likely  to  crack  longitudinally.     No  entirely 
satisfactory  reason  for  this  has  been  given;  but  possibly  it  is  due  to 
the  greater  tendency  of  the  two  portions  to  separate  when  the  sub- 
grade  is  crowned. 

439.  ONE  VS.  TWO-COURSE   PAVEMENTS.     Usually    the   con- 
crete is  laid  in  one  course  at  a  single  operation ;  but  sometimes,  when 
material  suitable  for  the  wearing  surface  is  expensive,  and  when  local 
but  less  suitable  material  is  much  cheaper,  the  concrete  is  laid  in  two 


242  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.    VII 

courses,  the  lower  one  of  the  poorer  and  cheaper  material.  The 
lower  course  is  usually  also  a  little  leaner  and  the  top  course  a  little 
richer  than  for  a  one-course  pavement.  At  times  such  construction 
may  be  economical. 

On  the  other  hand,  the  two-course  work  is  objectionable  for  the 
following  reasons:  1.  It  is  difficult  to  keep  the  two  classes  of  material 
separate.  This  is  more  serious  on  a  narrow  rural  road  than  on  a  wide 
city  pavement.  2.  There  is  a  possibility  of  not  securing  a  good  union 
between  the  two  courses.  3.  If  the  lower  course  is  leaner,  it  will 
absorb  water  from  the  subgrade  and  expand,  while  the  top  course 
is  likely  to  dry  out  and  contract;  and  consequently  the  two  are  likely 
to  separate.  4.  The  bottom  course  absorbs  water  and  expands,  and 
is  likely  to  produce  a  longitudinal  crack. 

Good  two-course  pavements  have  been  laid;  but  they  require 
more  labor  and  greater  care  in  construction.  Statistics  show  that  a 
little  less  than  30  per  cent  of  all  concrete  roads  and  pavements  are  two- 
course  work. 

All  the  discussions  that  follow  relate  to  one-course  work. 

440.  If  reinforcement  fe  to  be  used  (§  469),  it  is  necessary  to  lay 
the  concrete  in  two  courses  and  place  the  reinforcing  net  between 
them;    but  the  same  mixture  is  ordinarily  used  for  both  courses, 
and  hence  such  construction  is  not  usually  classed  as  two-course 
work. 

441.  CROSS  SECTION  OF  PAVEMENT.    The  cross  section  of  the 
concrete  slab  depends  upon  its  width  and  thickness,  and  upon  the 
crown  of  its  upper  surface. 

Fig.  75  shows  the  cross  sections  recommended  by  the  1916 
National  Conference  on  Concrete  Road  Construction,  for  cuts 
and  for  fills.  For  the  recommendations  of  a  committee  of  the 
American  Society  of  Civil  Engineers  concerning  crown,  see  Table  16, 
page  65. 

442.  Crown    of    Surface.     In    determining    the    proper    crown 
for  a  concrete  road  surface,  a  distinction  should  be  made  between  a 
country  road  and  a  city  street.     The  former  need  be  crowned  only 
enough  to  afford  lateral  drainage,  particularly  after  the   middle   is 
worn  down  somewhat;    while  on  a  city  street  with  side  curbs    the 
crown  should  be  enough  to  prevent  an  undue  portion  of  the  pave- 
ment from  being  covered  with  water  during  a  rain. 

The  only  advantage  in  crowning  a  road  surface  is  to  secure  surface 
drainage,  and  with  perfect  work  a  very  small  crown  would  suffice 
An  excessive  crown  drives  travel  to  the  middle  of  the  road  and  con- 


ART.    2] 


THE    CONSTRUCTION 


243 


sequently  does  not  distribute  the  wear  uniformly  over  the  pavement; 
therefore  the  less  the  crown  the  better,  provided  good  surface  drain- 
age is  secured.  Some  crown  is  necessary  on  account  of  (1)  inev- 
itable imperfections  in  finishing  the  surface,  (2)  the  accumulation  of 
leaves,  twigs,  straws,  etc.,  on  the  surface,  and  (3)  the  wear  of  the 
pavement. 

443.  The  crown  used  in  practice  for  concrete  roads  without 
curbs,  varies  from  -£$  to  ^w  of  the  width,  without  much  tendency 


ofC/rc/e 


SECTION  ON  LEVEL  o/=?  /N  CUT 

*tV'  Denotes  Mdfh  of  Pbremenf 
FIG.  75. — CROSS  SECTIONS  FOR  CONCRETE  ROADS. 


to  group  withir  any  smaller  limits.  For  concrete  pavements  with 
curbs  the  crown  employed  in  practice  varies  from  -gV  to  y^  of  the 
width,  the  intermediate  values  being  used  about  equally.  The 
National  Conference  on  Concrete  Road  Building  recommends  a 
crown  of  T^-Q. 

444.  Super-elevation  on  Curves.    For  a  discussion  of  this  subject, 
see  §  90. 

445.  MAXIMUM  GRADE.     For  a  discussion  of  the  general  sub- 
jects of  maximum  grades,  see  §  79-85;  and  for  the  recommendations 
of  a  committee  of  the  American  Society  of  Civil  Engineers  concerning 
maximum  grades,  see  Table    15,  page   57.     The  above  committee 
recommends  8  per  cent  as  the  permissible  maximum;  but  numerous 
examples  could  be  cited  of  concrete  surfaces  on  steeper  grades.     For 
example,  Sioux  City,  Iowa,  lays  concrete  on  grades  up  to  16  per  cent, 
and  Mankato,  Minn.,  on  14.3  per  cent. 


244  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.    VII 

446.  WIDTH.     For  a  general  discussion  of  the  width  of  the  im- 
proved portion  of  a  road,  see  §  95-6.     Both  the  1914  and  the  1916 
National  Conference  on  Concrete  Road  Building  recommended  a 
width  of  concrete  of  10  feet  for  a  single-track  road,  and  18  feet  for  a 
double-track  road;  and  that  for 'roads  18  feet  or  more  in  width,  it  is 
not  necessary  to  harden  the  shoulders  by  applying  gravel  or  broken 
stone.     For  a  consideration  of  extra  width  on  curves,  see  §  97. 

447.  THICKNESS  OF  CONCRETE.     The  proper  thickness  depends 
to  some  degree  upon  the  climate,  the  width  of  the  slab,  the  character 
of  the  soil,  the  thoroughness  of  both  the  surface  and  the  under- 
drainage,  the  degree  of  consolidation  of  the  subgrade,  the  propor- 
tions and  quality  of  the  concrete,  the  character  of  the  workmanship 
of  every  part,  the  care  employed  in  curing  the  concrete,  and  the 
amount  and  character  of  the  traffic.     These  factors  are  not  of  equal 
importance,  and  the  importance  of  any  one  item  may  vary  consid- 
erably with  local  conditions. 

It  is  not  possible  to  compute  the  thickness  required  in  any  par- 
ticular case;  and  all  that  can  be  done  toward  determining  a  working 
rule  for  the  thickness  of  a  concrete  road  slab,  is  to  accumulate  data 
concerning  practice.  In  studying  the  result^  of  experience,  care 
should  be  taken  to  consider  each  of  the  factors  mentioned  above. 

As  a  rule  engineers  make  the  thickness  more  at  the  middle  than 
at  the  sides;  but  some  engineers  prefer  a  uniform  thickness,  on  the 
theory  that  traffic  is  likely  to  be  distributed  over  the  entire  surface. 
Apparently  the  latter  overlook  the  heaving  effect  of  frost,  and  also 
the  effect  of  wear,  which  is  more  at  the  middle  than  at  the  sides. 

In  California,  part  of  which  state  is  classed  as  semi-arid  and  part 
as  arid,  the  State  Highway  Commission  has  laid  many  miles  of  con- 
crete roads  16  and  18  feet  wide  and  only  4  inches  thick,  which  have 
given  entire  satisfaction. 

In  Oregon,  a  state  that  is  chiefly  semi-arid,  many  miles  have  been 
built  15  feet  wide  with  a  thickness  of  5J  inches  at  the  sides  and  6£ 
inches  at  the  crown,  which  competent  authority  pronounces  as  being 
satisfactory.  In  1914  three  miles  of  16-foot  roadway  were  built  in 
which  the  thickness  at  the  sides  was  only  4  inches  and  that  at  the 
crown  only  5  inches,  and  in  1917  this  road  was  in  excellent  con- 
dition. 

In  the  Mississippi  Valley,  the  general  practice  seems  to  be  to 
make  the  thickness  6  inches  at  the  side  and  7  or  8  inches  (usually  the 
latter)  at  the  crown. 

448.  SIDE  FORMS.     The  concrete  is  laid  between  side  forms  or 


ART.    2]  THE   CONSTRUCTION  245 

curbs.  The  former  is  the  custom  for  rural  roads,  and  the  latter 
for  city  pavements.  The  forms  for  curbs  or  combined  curbs  and 
gutters  will  be  considered  in  Chapter  XIV — Curbs  and  Gutters. 
For  rural  roads,  planks  or  steel  channels  are  set  at  the  edges  of  the 
concrete  slab  to  retain  the  concrete,  the  width  of  the  form  boards 
determining  the  thickness  of  the  slab.  The  forms  are  usually  set 
before  the  subgrade  is  brought  to  its  final  surface.  The  tops  of  the 
forms  are  used  as  guides  in  finishing  the  surface  of  the  subgrade,  and 
later  are  used  to  guide  the  template  in  striking  off  the  the  top  surface 
of  the  concrete. 

The  forms  may  consist  of  2-  or  2^-inch  plank  or  of  steel  channels. 
The  plank  forms  should  be  securely  held  in  place  by  means  of  stakes 
on  the  outside  driven  either  to  such  a  depth  that  their  tops  are  below 
the  upper  edge  of  the  forms  or  at  such  a  distance  outside  of  the  forms 
as  not  to  interfere  with  the  operation  of  the  template.  The  planks 
should  have  a  continuous  bearing  on  the  subgrade,  as  otherwise 
they  will  sag  when  the  concrete  is  struck  off.  Adjoining  ends  of 
the  several  planks  should  be  fastened  together  so  as  to  keep  them  in 
line.  Fig.  76,  page  247,  shows  plank  forms  in  reasonably  good  posi- 
tion. Fig.  77,  page  247,  shows  plank  forms  poorly  constructed  and 
poorly  set  up.  In  Fig.  77  notice  the  space  under  the  bottom  of  the 
side  form ;  and  apparently  the  forms  are  not  in  line  either  in  the  fore- 
ground or  the  back-ground. 

The  steel  channels  are  generally  more  economical  than  plank; 
and  are  easier  set,  and  keep  their  place  better.  They  are  provided 
with  telescoping  joints,  which  keep  the  different  sections  in  line; 
and  are  hung  upon  steel  stakes  previously  driven  to  the  right  depth, 
which  make  it  easy  to  place  the  forms  at  the  desired  grade. 

The  steel  side-forms  may  be  flat  and  leave  a  square  corner  on  the 
upper  edge  of  the  concrete  slab,  or  they  may  have  a  projection  that 
will  leave  a  beveled  edge.  However,  if  the  edge  of  the  concrete  slab 
is  to  be  beveled  or  rounded,  it  is  better  to  secure  this  by  the  use  of  a 
finishing  tool  than  by  a  bevel-edge  form. 

449.  THE  CONCRETE.  Proportions.  The  best  proportions  for 
any  particular  materials  can  be  determined  only  by  one  or  the  other 
methods  explained  in  §421  and  §422;  that  is,  either  by  finding  by 
trial  the  proportions  that  give  maximum  density  and  hence  maximum 
strength,  or  by  sieve-analysis  curves. 

The  proportions  originally  used  are  1  :  2  :  3,  1  :  2  :  5,  1  :  3  :  6, 
or  1  :  2J  :  7.  The  efficiency  of  these  proportions  can  not  be  known 
unless  the  gradation  of  the  aggregates  is  known. 


246  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

450.  The   National  Conference  on  Concrete  Road  Building  rec- 
ommends (for  the  fine  and  coarse  aggregates  specified  in  §  412  and 
§  416,  respectively)  1:2:3.     The  Ohio  Highway  Department  uses 
1:2:4;    and   the   Pennsylvania   Highway   Department  1  :  1J  :  3 
or  1:2:3.     The  U.  S.  Office  of  Public  Roads  uses  1  :  l\  :  3  for 
gravel  or  1  :  1  \  :  3  for  broken  stone. 

451.  Mixing.     The  ingredients  should  be  mixed,  preferably  in  a 
batch  mixer,  until  every  fragment  of  the  coarse  aggregate  is  covered 
with  mortar  and  until  the  concrete  is  of  uniform  consistency  and 
color.     Ordinarily  this  can  not  be  accomplished  unless  all  of  the 
materials  are  in  the  drum  for  at  least  1  minute  and  the  drum  is  run 
at  a  rate  of  not  less  than  12  or  16  revolutions  per  minute.     The  time 
element  is  as  important  as  the  number  of  revolutions,  since  time  is 
necessary  to  allow  the  water  to  diffuse  through  the  mass.     Some 
engineers  specify  that  the  concrete  shall  be  turned  for  If  minutes. 
Experiments  show  the  following  relation  between  crushing  strength 
and  the  time  of  mixing.* 

Crushing  strength  when  mixed  f  minute  or  9  revolutions  =  1400  Ib.  per  sq.  in. 
Crushing  strength  when  mixed  1  minute  or  17  revolutions  =  1587  Ib.  per  sq.  in. 
Crushing  strength  when  mixed  1£  minute  or  26  revolutions  =  1924  Ib.  per  sq.  in. 

The  drum  must  be  completely  emptied  before  another  batch  is 
added. 

452.  Fig.  76  shows    a    typical    batch    concrete-mixer.     In   this 
case  the  concrete  is  delivered  with  a  bottom-dump  bucket.     The 
bucket  should  close  tightly  so  as  not  to  leak.     Sometimes  the  con- 
crete  is   delivered  with  a  tilting  bucket.     Again,  the   concrete   is 
delivered   through  a  revolving  spout  having  spiral  blades  on  its 
inside;   and  sometimes  the  mixing  of  the  concrete  due  to  its  travel 
through  the  tube  is  offered  as  an  excuse  for  decreasing  the  amount  of 
mixing  in  the  drum,  but  this  should  not  be  allowed  as  in  its  trip 
through  the  revolving  spout  the  concrete  is  mixed  but  a  little.     Some- 
times the  concrete  is  delivered  by  sliding  it  down  a  trough;  but  this 
is  undesirable  as  the  tendency  is  to  mix  the  concrete  too  wet,  so  it 
will  slide  freely.     Fig.  77  shows  a   concrete   mixer  delivering  the 
wearing  coat  of  a  two-course  concrete  road  through  a  gravity  spout. 
Apparently  the  mixture  is  much  too  wet. 

The  concrete  is  sometimes  transported  in  wheel-barrows  or  bug- 
gies; but  this  is  objectionable  as  in  the  transportation,  particularly  if 

*  Engineering  News,  Vol.  75  (1916),  p.  768. 


ART.  2] 


THE   CONSTRUCTION 


247 


the  distance  is  more  than  100  feet  or  if  the  concrete  is  quite  wet,  the 
coarse  aggregate  settles  to  the  bottom,  and  the  thoroughness  of  the( 
mixing  is  destroyed. 

453.  Batch  mixers  are  made  in  various  capacities  from  6  to  60 
cubic  feet  of  concrete;  but  only  those  having  a  capacity  from  6  to  30 
cubic  feet,  a  1-  to  5-bag  mixer,  are  used  in  rural-road  or  city-pave- 
ment work.  A  one-bag  or  two-bag  mixer  is  the  most  common,  as  it  is 
practically  impossible  in  road  work  to  get  the  material  to  a  larger 
mixer  fast  enough  to  keep  it  going  economically. 

The  latest  models  of  batch  mixers  have  a  loading  skip,  a  device  for 
regulating  the  time  of  mix,  and  an  automatic  water  tank  (Fig.  76). 


FIG.  76. — CONCRETING  CREW  AT  WORK. 


FIG.  77. — DELIVERING  THE  TOPPING  OF  A 
TWO-COURSE  CONCRETE  ROAD. 


454.  With  continuous  mixers  it  is  difficult  to  control  accurately 
the  proportions  and  the  thoroughness  of  the  mixing.     Some  con- 
tinuous mixers  have  devices  for  automatically  measuring  the  several 
ingredients,  and  give  fairly  satisfactory  results  when  intelligently 
operated  and  kept  in  good  order;  but  ordinarily  specifications  do  not 
permit  the  use  of  a  continuous  mixer. 

455.  Whatever  the  type  of  concrete  mixer,  it  is  wise  to  check 
the  proportions  occasionally,  at  last  once  each  day,  by  noting  the 
quantities  of  the  several  ingredients  used  in  a  certain  area  of  pave- 
ment.    Since  the  volume  of  concrete  produced  from  stated  quanti- 
ties of  the  ingredients  can  not  be  computed  accurately,  this  method 
is  a  better  check  upon  uniformity  than  upon  the  amount  of  the 
ingredients  used;    but  after  a  little  experience  with  the  particular 
materials   and   consistency,   a  real  check  can  be  obtained   of   the 
amount  of  the  ingredients  used. 


248 


PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.  VII 


FIG.  78.— Box  WHEED-BARROW. 


Fig.  78  shows  a  form  of  wheel-barrow  used  on  the  Allentown 
and  Eastern  concrete  road  in  Pennsylvania,  which  secures  greater 
accuracy  in  measuring  the  ingredients  than  the  usual  wheel-barrow 

having  a  pressed  steel  bowl  with 
four  curved  edges.  It  can  be 
seen  at  a  glance  whether  the  bar- 
row shown  in  Fig.  78  is  full;  while 
with  the  form  having  four  curved 
edges,  it  is  difficult  to  determine 
whether  the  barrow  contains  the 
right  quantity. 

456.  Hand  Mixing.  Only  a 
few  years  ago  concrete  for  roads 
and  pavements  was  usually  mixed 
by  hand;  but  now  it  is  practi- 
cally all  mixed  by  machine.  The 
output  per  man  with  a  machine 
is  usually  about  50  per  cent  more 

than  by  hand;  and  the  total  cost  is  considerably  less  by  machine 
than  by  hand,  the  exact  difference  depending  upon  the  thoroughness 
of  mixing  in  both  cases,  upon  the  size  and  type  of  the  machine,  and 
upon  the  amount  of  work  per  year  done  with  it. 

457.  Re-tempering.    The  re-tenipering  of  concrete,  i.  e.,  the  re- 
mixing of  concrete  that  has  partially  set,  should  not  be  permitted 
under  any  circumstances. 

458.  Consistency.     The  materials  should  be  mixed  with  sufficient 
water  to  produce  a  concrete  which  will  hold  its  shape  when  struck  off 
with  the  template.    The  consist- 
ency should  not  be  such  as  to 

cause  a  separation  of  the  mortar 
from  the  coarse  aggregate  in 
handling.  The  tendency  is  to 
use  an  excess  of  water,  which 
facilitates  the  handling  of  the 
concrete,  but  also  tends  to  sepa- 
rate the  ingredients  and  greatly 
weakens  the  concrete. 

Fig.  79  shows  the  proper  con- 
sistency.    With  less  water  the   concrete   will  not  flow;    and  with 
more   water   there   will  be  segregation,  the  mortar  flowing  to  the 
bottom  of  the  pile.     The  consistency  of  concrete  is  not  a  sure  indi- 


FIG.  79. — PROPER  CONSISTENCY  OF  CONCRETE. 


ART.    2]  THE   CONSTRUCTION  249 

cation  of  its  quality,  since  a  given  consistency  may  be  produced  by 
using  more  cement  and  less  mixing,  or  by  more  mixing  and  less 
cement,  or  by  adding  hydrated  lime;  but  nevertheless  the  above 
test  is  a  reasonably  good  one. 

459.  Placing.     If  the  subgrade  has  been  rutted  up  in  hauling  over 
it,  the  surface  should  be  restored;  and  then  it  should  be  thoroughly 
sprinkled,  but  there  should  be  no  pools  of  water  when  the  concrete 
is  placed.     The  thorough  sprinkling  of   the  subgrade  adds  materi- 
ally to  the  wearing  quality  of  the  concrete,  by  keeping  it  from  dry- 
ing out  too  soon. 

Immediately  after  being  mixed,  the  concrete  should  be  deposited 
to  the  required  depth  and  width.  The  section  should  be  completed 
to  a  transverse  joint  without  the  use  of  intermediate  forms  or  bulk- 
heads, or  a  transverse  joint  may  be  placed  at  the  point  of  stopping 
the  work.  In  case  the  mixer  breaks  down,  the  concrete  should  be 
mixed  by  hand  to  complete  the  section. 

Where  reinforcement  is  used  it  should  be  embedded  in  the  lower 
course  of  concrete  before  the  concrete  has  begun  to  harden;  and  the 
concrete  above  the  reinforcement  should  be  placed  within  20  minutes 
after  the  placing  of  the  concrete  below. 

460.  The  placing  of  concrete  for  roads  when  the  temperature  is 
near  freezing  is  not  advisable;   but  if  such  work  is  practically  un- 
avoidable, the  water  and  the  aggregates  should  be  heated  before 
mixing,  and  the  fresh   concrete  should  be  protected  from  freezing 
for   at  last    10  days,    if   the    temperature   remains   near   freezing 
(see  §  464).     Concrete  should  never  be  deposited  on  a  frozen  sub- 
grade. 

461.  Striking.     As  soon  as  placed  the  concrete  should  be  struck 
off  to  the  established  crown  and  grade  by  means  of  a  template  resting 
on  the  side  forms  and  moving  with  a  combined  longitudinal  and 
transverse  motion.     The  concrete  should  have  originally  been  de- 
posited so  high  that  a  little  concrete  will  accumulate  in  front  of  the 
template ;  but  this  accumulation  should  not  become  excessive,  and  it 
should  be  kept  nearly  uniform  along  the  template  by  removing  the 
excess  with  a  shovel  and  throwing  it  ahead  or  where  needed  along 
the  template.     As  the  template  approaches  a  transverse  joint  most 
of  the  excess  should  be  removed. 

The  template  or  strike  board  for  a  10-  or  12-foot  road  consists  of 
a  2-inch  plank  6  or  8  inches  wide  cut  on  the  under  side  to  fit  a  crown 
slightly  in  excess  of  that  of  the  finished  surface,  and  having  handles 
and  a  shoe  to  run  on  the  side  forms;  and  for  a  14-  to  20-foot  road, 


250 


PORTLAND-CEMENT   CONCRETE   ROADS 


[CHAP.  VII 


two  2-  by  10-inch  planks  spiked  together.     When  the  pavement  is 
over  20  feet  wide  a  trussed  template  must  be  employed.* 

462.  A  variety  of  devices  have  been  invented  to  facilitate  the 
striking  of  the  concrete  and  at  the  same  time^to  consolidate  the 


FIG.  80. — HAND-TAMPING  TEMPLATES  FOE  CONCRETE  ROADS. 


FIG.  81. — BAKER  FINISHING  MACHINE. 

concrete  by  tamping  the  surface.  One  such  method  consists  in  first 
using  a  template  that  leaves  the  concrete  a  little  high,  particularly 
in  the  center;  and  then  following  with  a  heavy  template  having  a 

*  For  detailed  drawings  of  two  templates  running  on  rollers  and  having  levers  for  raising, 
and  also  being  adjustable  for  different  widths  of  pavements,  see  Proc.  1916  National  Confer- 
ence on  Concrete  Road  Construction,  p.  203  and  204. 


ART.    2J 


THE    CONSTRUCTION 


251 


handle  at  each  end  by  which  it  is  lifted  and  dropped  at  close  inter- 
vals—see Fig.  80.  Another  device  consists  of  a  self-propelling  tem- 
plate and  tamping  machine  running  upon  the  side  forms.  The 
front  end  of  the  machine  strikes  the  concrete  a  little  high  and  the 
rear  end  tamps  it  to  the  right  height  by  an  up  and  down  motion 
of  a  steel  plate  extending  across  the  pavement.  Fig.  81  shows  this 
machine. 

463.  Finishing.  After  the  concrete  is  brought  to  the  estab- 
lished grade  and  crown  with  the  template,  the  surface  is  smoothed  or 
finished,  usually  with  a  wood  float,  the  operator  working  upon  a 
suitable  bridge— see  Fig.  82.  The  wood  float  is  better  than  a 


FIG.  82. — BRIDGE  UPON  WHICH  FINISHER  WORKS. 


metal  trowel,  since  the  latter  gives  a  polished  surface  and  also  tends 
to  work  a  film  of  neat  cement  to  the  surface. 

The  time  of  finishing  has  a  marked  effect  upon  the  wearing  quality 
of  the  pavement.  The  tendency  is  to  finish  the  concrete  too  soon 
after  placing.  The  proper  time  depends  upon  the  weather  conditions 
and  the  wetness  of  the  concrete  when  placed.  The  concrete  should  not 
be  finished  until  it  is  nearly  ready  to  take  the  initial  set;  and  when  in 
this  condition  the  surface  will  contain  practically  no  free  water,  and 
will  be  of  such  a  consistency  that  the  wood  float  will  leave  distinct 
marks  on  the  surface.  All  foreign  substances,  such  as  sticks,  coal, 
lumps  of  clay,  etc.,  on  the  surface  should  be  removed  before  finishing. 

The  surface  is  sometimes  stippled  with  a  broom  or  stiff  brush  to 


252  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.  VII 

prevent  it  from  becoming  slippery,  but  this  is  unnecessary;  and  the 
making  of  grooves  in  the  surface  is  very  objectionable  even  on  grades, 
since  the  grooves  weaken  the  slab  and  greatly  increase  the  wear. 

Several  devices  have  been  invented  to  facilitate  the  finishing 
of  the  surface.  One  method  is  to  draw  an  8-  to  12-inch  rubber  belt 
or  a  rubber-faced  canvas  belt,  or  a  plain  canvas  belt  back  and  forth 
across  the  pavement  after  it  has  been  struck  off  with  the  template, 
and  to  move  it  along  the  pavement  as  the  surface  is  finished.  For 
the  best  results  the  surface  should  be  given  a  second  floating  just  as 
the  cement  takes  its  initial  set.  When  armored  contraction  joints 
(§465)  are  used,  it  is  necessary  to  do  a  small  amount  of  hand  floating 
near  the  joint.  The  belt  finish  gives  a  good  surface  at  small  cost.* 

The  most  recent  method  of  finishing  the  surface  is  to  roll  it. 
The  rolling  may  be  done  in  either  of  two  ways:  1.  With  a  roller 
about  8  inches  in  diameter  and  about  6  feet  long,  made  of  light  sheet 
steel  and  weighing  about  70  lb.,  attached  to  a  long  pole,  the  operator 
standing  at  the  side  of  the  road  and  rolling  the  concrete  back  and 
forth  across  the  road.  2.  With  a  roller  a  little  longer  than  the  con- 
crete road  is  wide,  having  a  round  steel  axle  projecting  at  each  end, 
the  roller  being  operated  by  a  man  on  each  side  of  the  roadway  by 
means  of  a  suitable  handle  with  a  hole  in  its  lower  end  through  which 
the  axle  passes.  Although  first  used  in  1917,  the  method  of  finishing 
by  rolling  seems  to  secure  greater  strength  and  density  than  hand 
floating;  and  is  rapidly  being  adopted. 

Another  method  is  to  draw  a  rubber  garden  hose  in  the  form  of  the 
letter  U  along  the  pavement.  Still  another  method  is  to  draw  a 
1-inch  plank  endwise  back  and  forth  over  the  concrete  by  means  of  a 
rope  attached  to  each  end.  A  long-handled  float  has  been  used,  but 
opinions  differ  as  to  its  efficiency. 

464.  Curing  and  Protecting.  The  green  concrete  may  be  seriously 
damaged  by  the  too  rapid  drying  out  of  the  surface  in  hot  or  windy 
weather,  or  by  exposure  to  low  temperature,  or  by  being  opened  to 
travel  too  soon. 

If  the  concrete  dries  out  too  rapidly,  the  surface  becomes  friable 
and  chalky,  and  is  covered  with  shrinkage  cracks,  which  are  a  source 
of  weakness.  Therefore  in  hot  or  windy  weather  it  is  usually  neces- 
sary to  cover  the  concrete  with  canvas  for  at  least  half  a  day  after 
it  is  floated.  This  canvas  is  made  either  in  6-foot  strips  2  or  3 
feet  longer  than  the  pavement  is  wide,  or  in  long  strips  a  little 

*  Engineering  News,  Vol.  77  (1917),  p.  197-8;    or  Illinois  Highways,  Dec.,  1916,  p.  155-56. 


ART.   2] 


THE   CONSTRUCTION 


253 


FIG.  83. — CONCRETE  ROAD  COVERED  WITH 
CANVAS. 


wider  than   the  pavement,  mounted  on  rollers.     In  either  case,  if 
the  concrete  has  not  set,  the  canvas  should  be  supported  on  frames 
so  it  will  not  touch  the  concrete. 
Fig.  83  shows   a   concrete   road 
protected     by     canvas.     Notice 
that  the  earth  at  the  sides  of 
the    concrete    has    been    plowed 
preparatory  to  covering  the  con- 
crete with  it. 

When  the  concrete  has  hard- 
ened sufficiently,  the  canvas  is 
removed,  and  the  pavement  is 
covered  with  at  least  2  inches  of 
earth  which  should  ordinarily  be 
kept  wet  for  10  to  15  days. 
Shavings  or  straw  are  sometimes 
used  to  cover  a  new  concrete 
pavement ;  but  they  are  liable  to 

be  washed  into  piles  in  sprinkling  or  to  be  blown  off,  in  either  case 
leaving  exposed  patches 

If  there  is  danger  from  frost,  the  ingredients  should  be  heated 
before  the  concrete  is  mixed  (see  §  451);  and  after  being  placed  the 
green  concrete  should  be  protected  by  canvas  or  building  paper. 
The  former  is  the  easier  handled,  and  is  usually  more  economical. 
In  extreme  cases  steam  may  be  blown  under  the  canvas  or  building 
paper.  To  protect  the  concrete  after  the  first  night,  a  layer  of  straw 
with  a  little  earth  on  it  has  been  used. 

Great  care  should  be  exercised  in  opening  the  pavement  to  travel. 
The  length  of  time  necessary  to  keep  the  pavement  closed  will  depend 
entirely  upon  weather  conditions.  During  warm  weather  the  pave- 
ment should  be  kept  closed  to  travel  for  at  least  fourteen  days,  and 
preferably  for  three  weeks.  When  the  conditions  are  such  that  the 
temperature  of  concrete  is  less  than  50°  when  placed,  hardening  takes 
place  very  slowly.  When  a  concrete  pavement  has  been  laid  in  the 
late  fall,  it  is  sometimes  difficult  to  determine  when  it  will  be  safe  to 
open  the  road.  In  rare  cases  it  may  be  necessary,  owing  to  local 
conditions,  to  open  the  road  or  street  before  it  is  absolutely  safe. 
Under  such  conditions  if  about  2  inches  of  straw  is  placed  on  the 
pavement  and  this  is  covered  with  a  few  inches  of  earth,  the  pave- 
ment will  be  protected  sufficiently  against  abrasion  to  allow  the  open- 
ing of  the  road  sooner  than  could  be  safely  done  without  such  pro- 


254  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

tection.  This  cover  will,  however,  not  minimize  the  danger  of  dam- 
age to  the  pavement  by  heavy  loads,  which  will  tend  to  crack  a 
pavement  that  has  not  developed  its  full  strength. 

465.  CONTRACTION  JOINTS.    Concrete  expands  and  contracts 
with  changes  of  temperature  and  moisture.     Since  the  pavement  is 
ordinarily  laid  in  warm  weather,  the  contraction  is  likely  to  be  greater 
than  the  expansion;    and  besides  the  concrete  can  resist  the  com- 
pression due  to  expansion  better  than  the  tension  due  to  contraction. 
To  prevent  the  formation  -of  unsightly  and  irregular  cracks  due  to 
contraction,  it  is  customary  to  provide  contraction  joints  at  regular 
intervals.     If   the    pavement   has    curbs,    longitudinal    contraction 
joints  also  are  provided  at  each  curb.     Concrete  roads  and  pave- 
ments are  usually  provided  with  transverse  contraction  joints  from 
25  to  75  feet  apart,  usually  about  50  feet. 

466.  Contraction  joints  are  made  in  any  of  three  ways:  (1)  by 
inserting  a  wood  strip  or  steel  plate,  and  removing  it  after  the  con- 


ji  > 
FIG.  84. — ASSEMBLING  AKMORED  JOINT. 


crete  is  in  place,  and  then  pouring  in  an  elastic  mastic  of  tar  or 
asphalt;  (2)  by  inserting  during  construction  a  sheet  of  mastic  pre- 
pared for  the  purpose;  or  (3)  by  inserting  one  or  more  thicknesses 
of  tar  paper  or  asphalt  felt. 

The  longitudinal  contraction  joints  are  made  by  placing  next 
to  the  curb  a  layer  of  bituminous  mastic  from  \  to  1  inch  thick,  de- 
pending upon  the  width  of  the  pavement. 

The  transverse  joints  are  sometimes  protected  by  placing  a  soft- 
steel  i-inch  plate  on  each  side  of  a  J-inch  sheet  of  mastic.  The  plates 
are  provided  with  projections  which  securely  tie  or  bind  them  to  the 
concrete.  Fig.  84  shows  the  two  metal  plates  and  the  intervening 
sheet  of  mastic  or  tar  paper  being  clamped  together  preparatory 


ART.   2] 


THE   CONSTRUCTION 


255 


to  being  set  into  the  pavement.  Fig.  85  shows  the  joint  being 
installed  in  position  in  the  concrete.  Notice  that  the  plates  are 
suspended  from  a  temporary  bar  which  rests  on  the  side  forms. 


FIG.  85. — INSTALLING  ARMORED  JOINT. 

Fig.  77,  page  247,  shows  an  armored  joint  almost  covered  with 
concrete;  but  notice  that  there  is  no  temporary  supporting  bar  as  in 
Fig.  77.  These  protected  joints  are  expensive,  complicated  to 
install,  and  do  not  wear  down  with  the  concrete;  and  consequently 
are  falling  into  disrepute. 

The  most  popular  joint  filler  for  transverse  joints  is  one  or  more 
thicknesses  of  3-ply  tar  paper,  which  when  new  projects  slightly 
above  the  surface  of  the  pavement.  Fig.  86  shows  the  method 


FIG.  86.— FINISHING  TAR-PAPER  JOINT. 


FIG.  87. — TRIMMING  TAR-PAPER  JOINT. 


of  finishing  the  concrete  next  to  one  of  these  joints;  and  Fig.  87 
shows  the  way  of  trimming  off  the  surplus  tar  paper.  Some 
engineers  use  two  thicknesses  in  hot  weather,  and  three  in  cool 


256  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

weather.  A  single  thickness  of  3-ply  tar  paper  has  been  entirely 
satisfactory.  To  facilitate  the  insertion  of  the  tar  paper,  it  is  tied 
with  a  small  string  to  a  steel  plate  or  1-inch  plank  thinned  a  little  at 
its  lower  edge  and  cut  to  the  proper  width  and  crown,  and  the  plank 
and  paper  are  set  into  place  and  concrete  is  deposited  on  both  sides 
of  the  plank;  and  then  the  strings  are  cut,  the  plank  and  strings 
removed,  and  the  space  occupied  by  the  plank  is  tamped  full  of  con- 
crete. 

It  is  important  that  the  face  of  the  joint  be  truly  vertical,  or 
there  is  danger  of  one  slab's  sliding  up  and  over  the  next  one 

A  transverse  joint  is  likely  to  be  a  source  of  weakness  owing  to  the 
tendency  to  force  an  excess  of  cement,  and  consequently  a  deficiency 
of  stone,  next  to  the  joint;  and  also  owing  to  the  tendency  of  the  con- 
crete to  chip  out  next  to  the  joint.  These  tendencies  arc  reduced 
somewhat  by  the  use  of  a  split  float  (Fig.  86),  which  will  finish 
both  sides  of  the  joint  at  the  same  time. 

467.  Sometimes  the  contraction  joints  are  placed  at  an  angle 
with  the  length  of  the  road,  which  is  an  advantage  if  there  is  either 
an  elevation  or  depression  at  the  joint;  but  if  the  joints  are  properly 
made  and  maintained,  the  extra  length  is  only  a  needless  expense, 
and  besides  it  is  very  difficult  to  strike  the  surface  adjacent  to  the 
diagonal  joint. 

468.  A  number  of  attempts  nave  been  made  to  determine  mathe- 
matically  the    proper    distance   between    contraction   joints;     but 
there  is  so  much  uncertainty  in  each  of  the  several  factors  of  the  prob- 
lem that  any  such   computation  is  practically  worthless.     There 
seems  to  a  growing  tendency  to  narrow  the  thickness  of  the  joint 
filler  and  to  lengthen  the  distance  between  the  joints.     Some  rural 
roads  have  been  built  without  any  contraction  joints,  on  the  theory 
that  when  cracks  form  they  can  be  filled  with  pitch.     In  filling  a 
crack  the  mastic  is  piled  up  over  the  crack  a  little  to  protect  the  edge 
of  the  concrete.     Of  course,  all  contraction  cracks  as  well  as  all  others 
should  be  kept  full  of  bituminous  filler  as  a  part  of  the  maintenance 
of  the  road. 

469.  REINFORCEMENT.     Some   engineers   claim    that    concrete  - 
pavements  should  be  reinforced  to  prevent  cracks  due  (1)  to  changes 
of  temperature  and  moisture,  (2)  to  improper  drainage  and  defective 
foundation,    (3)   to  insufficient  thickness  of  concrete,   and   (4)   to 
defective    construction.     The    most    simple    and    most    economical 
method  of  eliminating  each  of  the  three  last  classes  of  cracks  is  to 
remove  the  cause.     The  use  of  reinforcement  simply  distributes  the 


ART.    2]  THE    CONSTRUCTION  257 

cracks  due  to  changes  of  temperature  and  moisture,  thus  substi- 
tuting many  minute  cracks  for  a  few  large  ones.  The  large  cracks 
can  be  protected  by  filling  with  tar  or  asphalt,  while  the  small  ones 
can  not  be  protected,  or  rather  will  not  be  protected,  and  hence  will 
be  a  cause  of  deterioration  of  the  pavement. 

It  is  impossible  to  compute  with  any  degree  of  accuracy  the 
amount  of  reinforcement  required  to  prevent  temperature  cracks, 
and  much  more  so  to  determine  the  amount  required  to  prevent 
cracks  due  to  the  other  causes  mentioned  in  the  preceding  para- 
graph. To  be  most  efficient  in  preventing  cracking  due  to  some  of 
the  causes,  the  reinforcement  should  be  near  the  top  of  the  slab,  and 
for  others  near  the  bottom.  When  reinforcement  is  used,  it  is  gen- 
erally placed  2  inches  from  the  top;  but  when  so  placed  it  is  not 
very  effective.  With  the  same  depth  of  embedment  the  reinforce- 
ment in  a  thin  broad  slab  is  much  less  effective  than  that  in  a 
narrow  deep  beam.  Further,  the  reinforcement  is  expensive,  and  is 
troublesome  to  install.  The  reinforcement  usually  adds  15  to  20 
cents  per  square  yard  to  the  cost  of  the  pavement.  When  reinforce- 
ment is  used,  the  concrete  must  be  laid  in  two  courses,  which  further 
adds  to  the  expense,  and  also  there  is  danger  that  the  two  courses 
may  not  thoroughly  unite.  Those  who  professedly  use  reinforce- 
ment primarily  to  prevent  temperature  cracks,  usually  recommend 
that  contraction  joints  be  constructed  75  feet  apart;  but  many 
concrete  roads  have  been  reasonably  satisfactory  without  either 
reinforcement  or  contraction  joints. 

However,  reinforcement  does  serve  to  keep  the  parts  of  the  slab 
from  separating  after  cracks  have  formed. 

It  is  probably  unwise  to  reinforce  a  concrete  pavement,  except 
perhaps  where  the  slab  rests  upon  spongy  soil  which  it  is  not  prac- 
ticable to  replace,  or  where  it  is  impossible  to  obtain  adequate  drain- 
age. Reinforcement  is  more  common  for  wide  city  pavements  than 
for  narrow  country  roads.  Only  5  per  cent  of  all  concrete  roads  and 
pavements  laid  in  this  country  have  been  reinforced. 

470.  SHOULDERS.  The  shoulders  should  be  partially  con- 
structed when  the  subgrade  is  prepared;  and  after  the  concrete  is 
completed  and  cured,  the  shoulder  should  be  finished.  It  is  usual 
to  reinforce  the  earth  shoulder  by  adding  broken  stone  or  coarse 
gravel,  making  it  4  to  6  inches  thick  next  to  the  paved  roadway  and 
feathering  out  to  nothing  at  a  distance  of  3  to  5  feet  out.  The 
shoulders  should  be  thoroughly  consolidated  by  rolling,  and  should 
be  finished  flush  with  the  pavement  but  not  any  above  it. 


258 


PORTLAND-CEMENT   CONCRETE    ROADS  [CHAP.    VII 


Some  competent  engineers  do  not  strengthen  the  earth  shoulders 
of  double  track  roads;  but  this  seems  of  doubtful  widsom,  because 
there  is  always  more  or  less  turning  off  from  the  pavement,  even  on 
double-track  roads,  —  if  for  no  other  reason,  than  that  the  slow-moving 
vehicle  may  give  the  right-of-way  to  the  fast-moving  one,  as  is 
required  by  law  in  some  states. 

471.  CURBS.  A  concrete  road  ordinarily  does  not  have  a  curb, 
since  it  is  expected  that  vehicles  will  turn  off  upon  the  shoulders. 
When  a  road  is  in  a  cut  or  upon  a  hillside,  it  may  be  necessary  to 
provide  a  gutter  for  drainage.  If  the  road  surface  is  water-bound 
gravel  or  water-bound  macadam,  the  gutter  must  be  outside  of  the 


WinnetkaJ//. 


V® 


/<4 

j 


IL_ 

>  \v*  :Va4;;;?.VMr*  Fabnc^b^\  f/M  fe£±.$- 


Indianapofo,Ir)d  ff/mber/y, 

FIG.  88.  —  INTEGRAL  CURBS  FOR  CONCRETE  PAVEMENTS. 


paved  way,  since  the  flowing  water  would  destroy  the  road  surface; 
but  if  the  road  is  a  concrete  one,  flowing  water  will  not  damage  it, 
and  consequently  instead  of  building  a  gutter  outside  of  the  paved 
way,  it  is  considerably  cheaper  to  build  a  curb  against  or  on  the  edge 
of  the  concrete  slab,  and  allow  the  water  to  flow  down  the  edge  of  the 
concrete  slab. 

The  curb  is  most  cheaply  constructed  if  it  is  cast  at  the  same  time 
as  the  slab,  and  hence  is  called  an  integral  curb.  The  integral  curb 
is  usually  cheaper  to  build  than  a  durable  gutter,  and  in  a  cut  its  use 
saves  considerable  excavation. 

The  integral  curb  has  been  used  for  concrete  driveways  for  a 
number  of  years,  but  was  first  used  for  public  roads  about  1914. 

Various  forms  of  integral  curbs  have  been  used.  The  simplest  is 
made  by  shaping  the  end  of  the  strike  board  to  form  a  low  curb,  but 


ART.   2] 


THE   CONSTRUCTION 


259 


this  form  is  used  only  on  park  drives  where  a  prominent  curb  is  not 
desired  but  where  a  waterway  is  necessary.  The  cost  of  such  curbs 
is  nearly  negligible. 

Fig.  88  shows  four  types  of  integral  curbs  that  have  been  used.* 
These  curbs  are  constructed  by  making  a  form  board  for  the  edge 
of  the  roadway  slab  and  the  back  of  the  curb,  and  forming  the  face  of 
the  curb  by  clamping  with  carpenter's  screw  clamps  a  form  board 
against  spacing  diaphragms.  The  curbs  shown  in  Fig.  88  are  much 
cheaper  to  construct  than  the  type  of  combined  concrete  curb  and 
gutter  used  with  other  forms  of  pavements — see  Chapter  XIV. 

Fig.  89  shows  the  from  used  in  integral  curb  construction 
in  Milwaukee,  Wisconsin,  f  As  soon  as  the  pavement  is  struck 
off,  the  form  is  set  in 
place  and  weighted  with 
bags  of  sand  to  prevent 
it  from  rising  when  the 
concrete  is  deposited  in 
it. 

The  concrete  founda- 
tion for  brick  pave- 
ments for  rural  roads 
were  formerly  built  with 
integral  curbs  similar  to 
the  last  two  shown  in 
Fig.  88  except  that  the 
corner  at  both  the  bottom  and  the  top  of  the  inside  face  was  made 
square  rather  than  rounded.  Strictly  speaking  the  projection  on  the 
concrete  foundation  is  not  a  curb,  since  it  does  not  project  above 
the  pavement.  It  is  usually  referred  to  as  an  integral  curb;  but  a 
more  appropriate  name  is  concrete  edging,  which  is  occasionally  used. 
At  present  brick  pavements  for  rural  roads  are  usually  built  mono- 
lithic (§  982), which  does  away  with  the  need  of  any  curb  or  edging. 

472.  COST  OF  CONCRETE  ROAD.  The  cost  of  a  concrete  road- 
slab  varies  with  the  specifications  and  the  local  conditions,  and  hence 
no  record  of  cost  will  apply  strictly  in  all  cases;  but  cost  data  are 
useful  for  comparison  and  as  a  guide  in  preparing  estimates. 

Some  of  the  following  cost  data  are  a  little  out  of  date;  but  prices 
for  1917  are  abnormal  owing  to  the  disturbance,  due  to  the  Great 
European  War,  and  besides  any  cost  data  presented  in  a  book  of  this 


Fio.  89. — FORM  FOR  INTEGRAL  CUBE. 


*  Engineering  Record,  Vol.  71  (1915),  p.  111. 

t  Engineering  and  Contracting,  Vol.  45  (1916),  p.  544. 


260  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.    VII 

character  soon  becomes  out  of  date.  The  costs  given  below  are  full, 
and  are  believed  to  be  accurate  and  representative.  They  are  inter- 
esting chiefly  as  showing  relative  values  in  different  localities,  and  of 
the  different  parts  of  the  work.  For  current  prices  for  concrete 
roads,  consult  the  construction  news  in  technical  journals. 

473.  Cost   of  Materials.    For  data   on   the   cost    of  portland 
cement,   see   §425;   for  the  same  for  sand  and  gravel,  see  §426; 
and  for  broken  stone,  see  §  427.     For  more  recent  prices,  see  the 
market  reports  in  current  technical  journals.     For  information  con- 
cerning the  amount  of  the  several  ingredients  required  for  a  cubic 
yard  of  concrete,  see  §  428-29. 

474.  Cost  of  Labor.     The  work  of  the  two  following  examples 
of  hand  and  machine  mixing  was  done  under  substantially  the  same 
conditions  and  hence  the  results  are    fairly  comparable.     Notice 
that  the  cost  with  hand  mixing  is  about  a  half  more  than  with  ma- 
chine mixing.     Further,  it  is  probable  that  no  practicable  amount  of 
hand  mixing  will  give  as  good  concrete  as  ordinary  machine  mixing; 
or  in  other  words,  if  the  hand  mixing  had  been  as  thorough  as  the 
machine  mixing,  the  difference  in  cost  would  probably  have  been  still 
greater.     Hand  mixing  has  practically  been  abandoned  in  concrete 
road  and  pavement  work. 

475.  Hand  Mixing.     The  construction  was  two-course  work — a 
5-inch  1:3:5  base  and  a  2-inch  1  :  2  wearing  coat;  but  the  cost  is 
not  given  separately  for  the  base  and  the  top.     There  were  steel- 
protected  contraction  joints  every  25  feet.     The  pavement  was  30 
feet  wide.* 

ORGANIZATION.  COST. 

CTS.  PER  SQ.  YD. 

1  foreman  @  40ff  per  hour 2 . 10 

1  finisher  @  40^ 

1  finisher's  helper  @  20 ^f 

Total  for  finishing 2.88 

1  form  setter  @  25  £ 

1  form  setter's  helper  @  20ff 

Total  for  setting  forms 1.76 

8  mixers  @  20^ 

1  cement  man  @  20  ^f 

1  man  on  sand  @  20  £ 

4  men  on  broken  stone  @  20  j£ 

2  spreaders  @  2Q£ 

Total  for  mixing  and  spreading 12 . 24 

1  watchman Q  50 


Total  for  mixing  and  laying 19 .. 

*  Engineering  and  Contracting,  Vol.  38  (1912),  p.  710. 


ART.    2]  THE   CONSTRUCTION  261 

476.  Machine  Mixing.  The  width  of  pavement  was  30  feet. 
The  construction  was  two-course  work, — a  5-inch  1:3:5  base  and 
a  11-inch  1:1:1  wearing  coat.* 

ORGANIZATION.  COST. 

CTS.  PER  SQ.  TD. 

BASE: 

9  men  on  broken  stone  @  22%£ 2 . 18 

3  men  on  sand  @  22^ , 0.73 

1  man  at  skip  @  22^. 0.24 

1  man  wheeling  cement  @  22%£ 0 . 24 

1  man  leveling  concrete  @  25£ 0 . 27 

1  helper  leveling  concrete  @  22^ 0 . 24 

1  tamper  @  22^ 0.24 

1  engineer  @  25£ 0.27 

1  fireman  @  25£.  .  . 0.27 

1  bucket  operator  @  15£ 0.16 

1  water  boy  @  5£ 0.05 

1  sack  boy  @  5£ 0.05 

1. foreman  @  45ff 0.48 

Total  for  base 5.43 

WEARING  COAT: 

4  men  on  granite  chips  @  22^ .• 0.48 

4  men  on  sand  @  22^ 0.48 

2  men  at  skip  @  22^ 0.24 

2  men  wheeling  cement  @  11\i 0 . 24 

2  rough  spreaders  @  22^ 0.24 

1  fine  spreader  and  tamper  @  25ff 0.13 

1  fireman  @  25^f 0.13 

1  engine  runner  @  25^ 0 . 13 

1  bucket  operator  .@  15^ 0.08 

1  sack  boy  @  5?f 0.02 

1  water  boy  @  5^ 0.02 

1  foreman  @  45?f 0 . 24 

1  finisher  @  25£ 0.61 

1  finisher's  helper  @  22^ 0.55 

Total  for  wearing  coat 3 . 61 

SETTING  FORMS: 

1  man  @  22^ 0.42 

MISCELLANEOUS: 

1  man  trimming  grade  @  22|ff 0 . 43 

2  men  cleaning  up  sand  and  stone 0 . 36 

moving  machine 1 . 30 

Total  for  miscellaneous  labor .  .  2 . 09 


Grand  total 13.50 

*  Engineering  and  Contracting  Vol.  38  (1912),  p.  710. 


262 


PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 


477.  Relative  Cost  of  Labor  and  Materials.  The  following  data 
are  the  averages  for  seven  Michigan  State-aid  roads,  eight  Wayne 
Co.  (Mich.)  roads,  and  all  the  concrete  roads  built  by  the  Illinois 
Highway  Commission  in  1912-13. 

MATERIALS  :  aggregates 27 . 7  per  cent 

cement 21.6 

expansion  joints  and  supplies 6.4 

Total  materials 55 . 7       ' ' 

LABOR..  44.3       " 


Total 100.0 

478.  Total  Cost.  The  three  examples  in  Table  29  are  believed 
to  be  fairly  representative. 

The  values  given  in  Table  30,  page  263,  are  the  average  of  the 
contractor's  costs  exclusive  of  over-head  expenses  and  profits,  but 
inclusive  of  the  preliminary  shaping  of  the  surface  and  the  finishing 
of  the  earth  shoulders;  and  include  representative  states. 

For  more  recent  data,  see  the  bidding  prices  in  the  construction 
news  of  current  technical  journals. 

TABLE  29 
COST  OF  ONE-COURSE  CONCRETE  SLAB  FOR  ROADS  IN  ILLINOIS  * 

DATA  AND  DIMENSIONS 


ITEMS. 

'     McLEAN. 

CARLJNYILLE. 

SPRINGFIELD. 

Area  of  pavement,  sq.  yd  
Thickness  of  pavement,  inches  

5000 
6 

7  111 
6.5 

5594 

7 

Width  of  pavement,  feet  
Length  of  haul,  miles 

45 
0  12 

16      ; 

1  5 

18 
12 

Cost  of  cement  per  bbl  
Barrels  of  cement  used  per  sq.  yd  
Cost  of  gravel  per   cu.  yd.  f.o.b.  desti- 
nation   
Labor,  price  per  hour  

$1.06 
0.29 

$1.25 
0.22 

0.98 
0.33 

$1.25 
0.225 

1.025 
0.29 

$1.25 
0  25 

Teams,  pi  ice  per  hour  

0.50 

0.50 

0.50 

COST  OF  LABOR  AND  SUPPLIES 


ITEMS. 

TOTALS. 

Per 

Sq.yd. 

TOTALS. 

Per 

Sq.  yd. 

TOTALS. 

Per 

Sq.  yd. 

Superintendence  
Shaping  subgrade  
Loading  and  hauling  sand  and  stone  .  .  . 
Mixing  and  placing  concrete  
Watchman  and  miscellaneous  labor.  .  .  . 
Cost  of  sand  and  stone 

$140.00 
307.11 
267.34 
414.63 
110.26 
1017.63 
1  547.15 
48.67 
30.75 
35.00 
45.18 

$0.028 
.061 
.053 
.083 
.022 
.204 
.309 
.010 
.006 
.007 
.010 

$    157.50 
108.70 
795.05 
700.58 
131.46 
741.00 
2307.90 
112.40 
25.00 
31.75 

100.66 
591.73 

$0  .  0220 
.0153 
.1120 
.0986 
.0184 
.1050 
.3246 
.0156 
.0034 
.0047 

.oiio 

.0840 

$    202.00 
343.44 
603.50 
644.25 
383.75 
1  622.01 
1  551.17 
206.74 
119.19 
18.33 

$0.0361 
.0415 
.1078 
.1150 
.0686 
.2897 
.2772 
.0369 
.0213 
.0033 

Cost  of  cement  
Expansion  joints  
Coal,  oil,  and  miscellaneous  supplies.  .  . 
Forms  and  other  lumber  
Filling  curb  expansion  joints  

Reinforcing  steel  
Excavation  .  . 

2li'  .38 

'  '  !  0378 

Trimming  shoulders  

Total 

$3964.02 

$0.793 

$5803.07 

$0.8176 

$5794.76 

$1.0352 

*  Report  111.  Highway  Com.,  1912,  p.  240,  241,  and  247,  respectively. 


ART.    2]  THE    CONSTRUCTION  263 

TABLE  30 
AVERAGE  COST  PER  SQUARE  YARD  OF  CONCRETE  ROAD  SLABS  IN  1915  * 

Connecticut $1 . 13           Missouri $1 .09 

Illinois 1 . 03           New  Jersey 1 . 23 

Indiana 0 . 98          New  York Q  98 

Iowa 1 . 19          Ohio. • i  .02 

Kansas 1 .28          Pennsylvania 1 .01 

Maryland £ 1 . 08           Texas 1.15 

Massachusetts .     0.95          West  Virginia 1 .03 

Michigan 1 . 10          Wisconsin 1 . 02 

Minnesota 1.11 

479.  CHARACTERISTICS  OF  A  CONCRETE  ROAD.    The  character- 
istics of  a  well-built  concrete  road  are:    1.  It  is  reasonable  in  first 
cost  in  proportion  to  its  probable  durability.     2.  It  has  a  low  tractive 
resistance,  but  gives  a  good  foothold  for  horses  and  automobiles.     3. 
It  is  free  from  dust.     4.  It  is  easily  maintained.     5.  It  is  reasonably 
durable  when  properly  maintained.     6.  Its  only  fault  is  that  the 
color  is  somewhat  trying  to  the  eyes  of  the  user;  although  the  light 
color  is  some  advantage  at  night. 

480.  CONCRETE  STREET  PAVEMENTS.    The  preceding  discussion 
has  reference  primarily  to  strips  of  concrete  10  to  20  feet  wide  for 
rural  roads,  since  concrete  is  in  more  common  use  for  rural  roads 
than  for  city  streets.     The  same  methods  without  material  change 
can  be  employed  for  pavements  up  to  30  feet  wide;   but  for  wider 
pavements  it  is  necessary  to  modify  the  plan  in  one  of  two  ways  as 
follows:    (1)  Make  a  longitudinal  joint  in  the  middle  of  the  street 
and  lay  half  of  the  street  at  a  time;  or  (2)  insert  screeds  transversely 
to  the  street,  and  strike  the  concrete  with  a  straight  edge  held  parallel 
to  the  curb. 

On  a  wide  street  the  finishing  is  done  with  a  belt  or  a  board  laid 
flatwise  and  reaching  half-way  across  the  pavement,  the  end  at 
the  curb  being  handled  by  a  man  and  the  end  at  the  center  being 
guided  by  a  small  rope  in  the  hands  of  a  man  standing  on  the  remote 
curb.  The  former  method  is  considered  the  better,  since  a  longi- 
tudinal joint  is  undesirable.  Such  joints  are  sometimes  protected 
with  steel  plates,  and  should  always  be  covered  with  tar;  but  even 
then  a  rut  is  likely  to  form  along  them. 

Concrete  pavements  having  separate  curbs  require  longitudinal 
contraction  joints  at  each  edge  (§  465-68);  but  the  integral  curb 
(§  471)  eliminates  this  complication. 

*  Data  collected  by  the  Portland  Cement  Association. 


264 


PORTLAND-CEMENT    CONCRETE    ROADS  [CHAP.    VII 


481.  SPECIFICATIONS.  Complete  specifications  for  concrete 
roads  are  printed  by  the  several  State  Highway  Departments,  of 
which  copies  may  doubtless  be  had  by  citizens  of  the  respective 
states  upon  request.  The  Portland  Cement  Association  publishes  for 
gratuitous  distribution  the  specifications  adopted  by  the  American 
Concrete  Institute  and  recommended  by  the  1916  National  Con- 
ference on  Concrete  Road  Building,  copies  of  which  may  be  had 
gratuiously  of  the  Secretary,  Portland  Cement  Association,  111  W. 
Washington  St.,  Chicago. 


ART.  3.    MAINTENANCE 
482.  CHARACTER  OF  WORK  REQUIRED.    The  work  required  to 

maintain  the  concrete  slab  is:  (1)  keep  the  joints  and  cracks  filled 
with  bituminous  cement;  (2)  fill  with  bituminous  cement  any  small 
pits  that  appear;  (3)  clean  out  any  holes  left  where  pebbles  have 


Fio.  90.— FILLING  A  DIAGONAL  CRACK. 

been  dislodged  or  where  a  friable  fragment  has  disintegrated,  and  fill 
them  with  bituminous  concrete;  and  (4)  repair  any  places  where  the 
concrete  is  otherwise  defective. 

The  bituminous  material  may  be  either  a  heavy  grade  of  refined 
tar  or  a  corresponding  grade  of  asphalt,    After  the  bituminous 


ART.   3]  MAINTENANCE  265 

material  has  been  applied,  its  surface  should  be  sprinkled  with  coarse 
sand  or  fine  chips  of  a  hard  stone. 

Fig.  90  shows  the  method  of  repairing  a  transverse  contraction 
joint. 

Before  being  filled,  all  cracks  and  joints  should  be  swept  clean 
with  rattan  or  steel  brooms.  The  old  tar  need  not  be  removed ;  but 
any  matted  earth  or  other  foreign  material  not  removed  by  the  first 
sweeping  should  be  loosened  and  removed  with  a  steel  hook.  It  is 
usually  necessary  to  cover  all  joints  and  fill  all  cracks  twice  each  year. 

The  pits  and  the  cup-like  holes  from  which  pebbles,  sticks,  etc., 
have  been  dislodged,  unless  early  filled,  will  enlarge  rapidly  under 
travel.  Where  defects  of  any  considerable  size  are  to  be  repaired, 
the  edges  should  be  made  vertical  with  a  chisel  and  the  depth  of  the 
hole  increased  to  1  inch,  if  it  is  not  already  that  deep.  The  hole 
should  be  thoroughly  cleaned  and  painted  with  hot  tar;  and  then  be 
filled  with  bituminous  concrete  and  sprinkled  with  coarse  sand  or  fine 
hard  stone  chips. 

When  it  is  necessary  to  repair  any  considerable  defective  portion 
of  the  concrete,  the  place  to  be  patched  should  be  trimmed  and 
cleaned  as  described  above,  and  painted  with  neat  cement  mortar; 
and  then  the  hole  should  be  tamped  solidly  full  of  good  portland- 
cement  concrete.  The  patch  should  be  kept  damp  and  protected 
from  travel  until  the  cement  has  fully  set. 

483.  The   above   method   of  maintenance   will   probably  serve 
indefinitely,  and  will  preserve  the  surface  except  for  the  natural 
wearing  away  of  the  concrete  by  travel. 

484.  Bituminous   Surface.      Many  attempts   have   been  made 
to  cover  the  surface  of  a  concrete  road  with  a  bituminous  coating. 
This  phase  of  the  subject  is  considered  in  Art.  2  of  Chapter  VIII. 

485.  COST  OF  MAINTENANCE.     The  cost  of  maintenance  con- 
sists of  the  annual  expense  for  repairs  and  the  annual  contribution  to 
a  fund  for  rebuilding  the  slab  when  it  is  worn  out.   The  introduction 
of  concrete  roads  is  so  recent,  particularly  in  proportion  to  their' life, 
that  no  reliable  data  have  been  accumulated  as  to  the  second  item 
of  the  cost  of  maintenance.     There  are  reasonably  accurate  data  for 
the  annual  cost  of  repairs,  but  as  a  rule  there  is  no  information  as  to 
amount  of  or  character  of  the  travel  on  the  road;  and  therefore  it  is 
not  possible  to  make  any  accurate  comparisons  between  such  data. 
Further,  no  standard  has  been  established  as  to  what  constitutes 
good  maintenance;    and  no  system  of  doing  the  work  has  been 
fully  tested. 


266  PORTLAND-CEMENT   CONCRETE   ROADS  [CHAP.   VII 

Apparently  the  most  complete  data  are  those  obtained  in  1915 
by  the  Illinois  Highway  Department.*  The  average  annual  cost  for 
repairs  on  75  miles  of  concrete  rural  roads  in  comparatively  short 
sections,  was  0.4  cent  per  square  yard  for  supervision,  labor,  equip- 
ment and  materials.  Most  of  the  roads  were  built  in  1914  or  1915. 
Two  methods  of  maintenance  were  tried, — (1)  by  the  use  of  a  one- 
horse  wagon  drawing  a  portable  heating-kettle,  and  (2)  by  an  auto- 
mobile truck  carrying  a  heating  tank.  By  the  first  method  the  total 
cost  was  0.32  cent  per  square  yard  per  treatment,  and  by  the  second 
0.22  cent.  In  this  work  it  was  found  that  joints,  even  though  pro- 
tected by  steel  plates,  required  about  the  same  attention  as  ordinary 
cracks. 

In  Connecticut  the  average  cost  of  repairs  to  the  concrete  slab 
was  0.4  cent  per  square  yard  per  annum  and  to  drainage  0.3  cent 
per  square  yard,  or  a  total  of  0.7  cent. 

*  Illinois  Highways,  August,  1915,  p.  118-22;   or  Engineering  and  Contracting,  Vol.  47  (1917) 
p.  14. 


CHAPTER  VIII 
BITUMINOUS  ROAD  MATERIALS 

486.  DEFINITIONS.     Bitumen.      A  mixture  of  native  or  pyro- 
genous  hydrocarbons  and  their  non-metallic  derivatives.     It  may  be 
a  gas,  liquid,  or  solid;  and  if  solid,  is  soluble  in  carbon  disulphide. 

487.  Bituminous   Material.     Any   material    containing   bitumen 
or  constituting  a  source  of  bitumen.     Bituminous  coal,  peat,  etc., 
are  called  pyro-bitumens  because  a  bitumen  may  be  produced  from 
them  by  distillation.     The  ordinary  bituminous  materials  used  in 
roads  and  pavements  are  asphalt,  petroleum,  and  tar.* 

488.  Cut-back   Product.    A   petroleum   or   tar   residuum   which 
has  been  fluxed  with  distillate. 

489.  Flux.    Fluid  oil  or  tar  which  is  incorporated  with  asphalt, 
petroleum,  or  tar  residuum  for  the  purpose  of  reducing  their  con- 
sistency. 

ART.  1.    ASPHALT 

490.  DEFINITIONS.     Asphalt.    Solid  or  semi-solid  native  bitumen 
or  solid  or  semi-solid  bitumen  obtained  by  refining  petroleum,  which 
consists  of  a  mixture  of  hydrocarbons  and  which  melts  upon  the 
application    of   heat.     Asphalt   is    usually   found    associated    with 
various   mineral    and    organic    substances.     Different   varieties    of 
asphalt  are  called  albertite,  grahamite,  gilsonite,  turrellite  uintatite, 
wurtzelite,  etc. 

491.  Crude  Asphalt.    A  native  mixture  of  bitumen,  sand,  clay, 
water,  organic  matter,  etc. 

492.  Refined  Asphalt.    A  native  mixture  after  it  has  been  freed 
wholly  or  in  part  from  water  and  organic  and  inorganic  matter  by 
being  heated. 

*  For  detailed  explanations  of  the  methods  employed  in  testing  bituminous  road  materials, 
see  Bulletin  314  of  the  Office  of  Public  Roads  and  Rural  Engineering,  U.  S.  Department 
of  Agriculture,  Washington,  D.  C.,  1915. 

267 


268  BITUMINOUS   ROAD   MATERIALS  [CHAP.    VIII 

493.  Rock  Asphalt.    A  limestone  or  sandstone  naturally  impreg- 
nated with  asphalt.     Rock  asphalt  is  the  principal  form  of  asphalt 
used  in  Europe  for  paving  purposes,  and  there  is  usually  designated 
as  asphalt. 

494.  Asphaltic  Cement.    Refined  asphalt  which  has  been  mixed 
with  some  solvent  to  increase  its  plasticity,  adhesiveness,  and  tenacity. 

495.  CHARACTERISTICS  OF  ASPHALT.  As  usually  found  asphalt  is 

of  a  dark  brown  or  glistening  black  color.  It  varies  in  hardness  from 
a  viscous  liquid  to  about  3J  on  the  Dana  scale.  When  rubbed  or 
freshly  broken,  it  emits  a  peculiar  bituminous  odor,  and  has  a  slight 
sour  smell. .  Its  specific  gravity  in  the  natural  state  varies  from  0.96 
to  1.68  according  to  its  porosity  and  the  amount  and  the  character 
of  the  impurities  present.  It  is  insoluble  in  water;  but  is  more  or 
less  soluble  in  carbon  disulphide,  alcohol,  turpentine,  ether,  naphtha, 
and  petroleum. 

Asphalt  has  an  appearance  somewhat  like  coal  tar.  The  prin- 
cipal method  of  distinguishing  asphalt  from  coal  tar,  available 
to  the  layman,  is  the  odor.  The  tar  emits  a  sharp,  acrid  odor; 
while  both  the  crude  and  the  refined  asphalt  when  cold  give  a  weak 
clay-like  odor,  and  must  be  rubbed  to  obtain  the  distinctive  bitumi- 
nous odor.  If  tar  is  mixed  with  asphalt,  the  presence  of  25  per  cent 
will  be  revealed  by  the  odor.  When  being  laid  in  a  road  or  pave- 
ment, tar  gives  off  a  bluish  vapor,  while  asphalt  emits  a  white  vapor. 

496.  Asphaltic  limestone,  when  freshly  broken,  varies  in  color 
from  chocolate  brown  to  black,  the  color  being  darker  as  the  pro- 
portion of  asphalt  is  greater.    The  percentage  of  asphalt  permeating 
the  limestone  varies  in  different  deposits  and  in  different  parts  of  the 
same  mine,  usually  ranging  from  1  to  20  per  cent. 

Asphaltic  sandstone  contains  from  1  to  70  per  cent  of  asphalt. 
The  grain  is  sometimes  dense  and  sometimes  porous,  sometimes  very 
fine  and  sometimes  coarse. 

497.  SOURCES  OF  ASPHALT.    Liquid  asphalt,  or  maltha  as  it  is 
usually  called,  is  found  in  large  quantities  in  California;  but  solid  or 
natural  asphalt  is  not  found  to  any  great  extent  in  the  United 
States.     The  principal  kinds  of  the  natural  asphalts  used  in  this 
country  are:  Trinidad,  Bermudez,  and  California. 

498.  Trinidad  Asphalt.    The  Island  of  Trinidad,  near  the  north- 
east coast  of  Venezuela,  South  America,  supplied  something  like 
90  per  cent  of  all  the  asphalt  used  in  the  world  from  about  1875  to 
1900;   and  at  present  the  Island  of  Trinidad  is  the  main  source  of 
supply  of  the  native  asphalt  used  in  the  United  States.     In  south- 


ART.  1]  ASPHALT  269 

west  corner  of  the  Island  is  the  so-called  pitch-lake,  which  has  an 
area  of  about  115  acres.  The  surface  of  the  lake  has  an  elevation 
of  138  feet  above  the  sea-level,  and  near  the  center  the  asphalt  has  a 
depth  of  78  feet.  As  a  rule  the  surface  of  the  asphalt  is  sufficiently 
hard  that  teams  may  be  driven  over  it;  but  the  whole  mass  is  in 
constant  motion  around  several  vortices,  as  shown  by  trunks  of 
trees  which  rise  and  after  a  time  again  disappear.  Excavations 
made  during  the  day  close  up  during  the  night. 

The  asphalt  is  excavated  with  picks  and  shovels,  conveyed  to 
the  shore  in  carts,  and  lightered  to  vessels  off-shore.  On  the  sea 
voyage  it  becomes  compacted  into  a  solid  mass  and  must  be  again 
broken  up  with  picks.  The  crude  asphalt  is  mixed  with  much 
earthy  and  a  little  vegetable  matter  and  water,  and  is  dark  brown. 

The  crude  material  is  refined  by  placing  it  in  kettles  or  open 
tanks  and  heating  it  for  three  or  four  days,  during  which  time  the 
water  is  evaporated,  the  vegetable  matter  rises  to  the  surface  and 
is  skimmed  off,  and  the  earthy  material  settles  to  the  bottom.  Great 
care  is  required  in  the  refining  process  not  to  heat  the  apshalt  to  a 
point  where  chemical  changes  take  place.  The  refined  asphalt 
must  be  softened  by  the  addition  of  some  fluxing  material  before  it 
is  ready  for  use  in  the  pavement. 

499.  Bermudez  Asphalt.    A  lake  in  the  State  of  Bermudez, 
Venezuela,   South  America,   supplies  large   quantities  of  asphalt. 
The  crude  asphalt  consists  of  bitumen  mixed  with  sand,  clay,  and 
vegetable  matter;  and  is  refined  in  the  same  way  as  Trinidad  asphalt, 
but  more  rapidly,  since  it  contains  less  water  and  mineral  matter. 

500.  California  Asphalt.    California  is  the  principal  producer  of 
asphalt  in  the  United  States;   and  is  said  to  have  not  only  larger 
quantities  of  asphalt  than  any  other  equal  area  in  the  world,  but  a 
greater  variety  of  forms — solid  and  liquid  asphalt,  and  asphaltic 
limestones  and  sandstones — and  in  more  localities.     Maltha  ("  liquid 
asphalt ")  is  found  chiefly  at  Carpinteria,  Santa  Barbara  County, 
near  the  sea  shore;   and  solid  asphalt  is  found  at  La  Patera,  Santa 
Barbara  County,  also  near  the  sea  shore.    Asphaltic  limestone  and 
sandstone  are  found  at  a  number  of  places  in  California,  in  all  degrees 
of  richness  and  consistency.     The  principal  deposits  are  a,t  Santa 
Cruz,  San  Luis  Obispo,  and  Kings  City.     The  asphalt  is  extracted 
from  the  stone  by  heating  the  mass  in  a  tank  and  drawing  off  the 
liquid  asphalt. 

601.  The  base  of  the  California  petroleums  is  asphaltic,  as  dis- 
tinguished from  the  paraffin  base  of  the  eastern  oils;  and  the  process 


270  BITUMINOUS   ROAD   MATERIALS  [CHAP.   VIII 

of  refining  petroleum  leaves  the  asphalt  or  maltha  as  a  residue,  and 
at  several  places  asphalt  is  produced  in  this  way  from  crude  petro- 
leum. 

502.  Petroleum  Residue.    Some  crude  petroleums  on  distilla- 
tion have  an  asphalt  residue  (see  §  547).    Three  fourths  of  all  the 
asphalt  used  in  the  United  States  is  obtained  from  asphaltic  petro- 
leums.   Such  material  is  usually  called  oil  asphalt. 

503.  Other  American  Asphalts.    Considerable  asphalt  is  shipped 
to  the  United  States  from  mines  at  Inciarte  and  La  Paz,  State  of 
Zulia,  Venezuela,  South  America.     It  is  usually  called  Maracaibo 
asphalt  from  the  gulf  and  lake  of  that  name  near  the  mines. 

Asphalt  is  found  in  much  smaller  quantities,  but  sufficient  to 
be  of  considerable  commercial  importance,  in  Utah,  Colorado, 
Indian  Territory,  Texas,  and  Kentucky.  One  of  the  most  important 
of  these  is  Gilsonite,  a  solid  and  nearly  pure  native  bitumen  found  in 
Utah  and  Colorado. 

Several  deposits  of  natural  asphalt  exist  in  Cuba  and  along  the 
Gulf  Coast  of  Mexico. 

504.  SHIPPING  ASPHALT.     Refined  asphalt  is  shipped  in  barrels 

or  metal  drums  or  in  tank  cars. 
Fig.  91  shows  the  method  of 
unloading  asphalt  binder  from 
the  tank  car  to  the  machine 
which  distributes  it  upon  the 
asphalt-bound  macadam  road. 
In  the  left  foreground  is  the 

FIG.  QI.-UNLOADING  ASPHALT  BINDER.  mOt°r '  tl>Uck       distributor,       and 

behind  it  is  a  portable   heater 

for  heating  the  asphalt.  Notice  that  the  tank  is  covered  with 
blankets. 

505.  DESIRABLE  PROPERTIES  OF  ASPHALT.    The   character- 
istics required  in  an  asphalt  differ  according  to  the  purpose  for 
which  it  is  to  be  used;  but  in  general  any  asphalt  for  use  in  roads 
or  pavements  should  have  the  following  properties:    1,  chemical 
stability;    2,   freedom   from   decomposition   products;    3,   binding 
power;  4,  resiliency,  and  5,  waterproof  ness. 

Apparently  it  is  impossible  to  devise  any  tests  to  measure  directly 
some  of  these  properties;  and  the  difficulties  of  devising  a  series 
of  tests  is  increased  by  the  variation  in  the  character  of  the  different 
materials. 

506.  Chemical  Stability.     The  chemical  stability  of  a  bituminous 


ART.  1]  ASPHALT  271 

» 

«ement  is  indicated  somewhat  by  the  extent  to  which  the  material 
is  volatilized  under  standard  temperature  conditions.  The  harden- 
ing of  the  bituminous  cement  due  to  evaporation  and  oxidation  is 
determined  by  making  penetration  tests  before  and  after  volatiliza- 
tion. The  temperature  at  which  the  material  gives  off  vapor  enough 
to  ignite  gives  further  indication  of  chemical  stability. 

607.  Freedom  from  Decomposition  Products.  If  the  refining 
process  has  been  carried  on  at  a  too  high  temperature,  the  cement 
may  have  been  partially  decomposed.  This  condition  is  indicated 
by  the  amount  of  free  carbon  and  other  decomposition  products  that 
are  separated  by  certain  solvents.  If  the  material  is  a  fluxed  nat- 
ural asphalt,  these  tests  throw  some  light  upon  the  character  of  the 
flux. 

508.  Binding    Power.    It    is    important    that    the    bituminous 
material  shall  have  cementing  or  binding  power,  particularly  at 
summer  temperatures.     There  is  no  single  test  for  this  property. 
The  ductility  test  gives  some  indication  concerning  cementing  value. 

509.  Resiliency.     It  is  important  that  the  bituminous  cement 
shall  have  the  power  to  absorb  shock  and  thus  prevent  the  blow 
of  the  wheel  or  the  hoof  from  destroying  the  road  or  pavement  sur- 
face.    This  requires  that  the  cement  shall  have  resiliency  and  mal- 
leability, which  depend  somewhat  upon  consistency. 

510.  Waterproofness.    The  bituminous  material  should  be  water- 
proof so  as  to  prevent  water  from  penetrating  the  body  of  the  road 
and  doing  damage  by  freezing  or  softening  the  subgrade. 

511.  TESTS  OF  BITUMINOUS  MATERIALS.    Below  are  the  tests 
usually  applied   to   bituminous  materials,   and  a  brief  statement 
of  the  significance  of  each.     All  of  these  tests  (§  512-27)  are  applied 
to  asphalts,  but  only  the  first  eight  (§  512-19)  are  applied  to  oils  and 
tars.* 

512.  Foam  Test.    This  test  is  applied  to  asphalts  and  tars  to 
determine  the  presence  of  water.     Water  is  chiefly  objectionable 
since  it  makes  the  material  difficult  to  handle  when  heated  above  the 
boiling  point  of  water,  because  the  steam  makes  the  oil  or  tar  foam 
or  froth. 

513.  Specific  Gravity.    This  test  is  valuable  mainly  as  a  means  of 


*  For  a  detailed  account  of  the  method  of  making  the  testa  and  also  illustrations  of  the 
apparatus  used,  see  Bulletin  314  of  the  U.  S.  Department  of  Agriculture,  December  10,  1915, 
or  Hubbard's  Laboratory  Manual  of  Bituminous  Materials,  8vo,  p.  159,  John  Wiley  &  Sons, 
New  York,  1916;  and  for  a  description  of  the  tests  see  Proc.  Amer,  Soc.  of  Civil  Engineers, 
December,  1914,  p.  3036-50. 


272  BITUMINOUS   ROAD   MATERIALS  [CHAP.    VIII 

: . _ _ : — ^ 

identifying  the  material;  but  in  connection  with  other  tests  it  is 
sometimes  serviceable  in  determining  the  suitability  of  a  material 
for  road  purposes.  The  specific  gravity  of  crude  asphalt  varies  from 
1.04  to  1.40,  and  asphaltic  cement  from  0.96  to  1.06.  The  specific 
gravity  of  crude  petroleum  varies  from  0.73  to  0.98,  the  paraffin  oils 
being  the  lowest  and  the  asphaltic  the  highest.  The  specific  gravity 
of  crude  tar  varies  from  1.00  to  1.22,  the  water-gas  tars  ranging  from 
1.00  to  1.10,  and  the  coal  tars  from  1.10  to  1.22.  The  specific  gravity 
of  tar  depends  chiefly  upon  the  amount  of  free  carbon  it  contains, 
the  higher  the  specific  gravity  the  greater  the  percentage  of  free 
carbon.  Refined  tar  has  a  higher  specific  gravity  than  crude  tar, 
partly  because  the  light  hydrocarbons  and  the  water  have  been 
driven  off. 

514.  Flash  Point.    The  flash  point  is  determined  by  either  the 
open-cup  or  the  closed-cup  method,  the  latter  being  the  more  accu- 
rate.    This  test  is  mainly  valuable  as  a  quick  means  of  differentiating 
between  heavy  crude  oils  and  cut-back  products;*  but  it  also  indicates 
the  temperature  at  which  a  refined  oil  has  been  distilled.     Crude 
paraffin  oils  usually  flash  lower  than  crude  asphaltic  oils. 

515.  Consistency.    The   consistency  of  a  bituminous  material 
is  an  important  factor,  since  it  determines  the  grade  of  material 
suitable  for  a  particular  use,  and  since  this  test  is  a  means  of  securing 
uniformity  in  the  product. 

There  are  three  methods  or  instruments  in  common  use  for  deter- 
mining consistency,  viz.:  the  Engler  viscosimeter,  the  New  York 
Testing  Laboratory  float  apparatus,  and  the  penetrometer. 

516.  Viscosity.    The  viscosimeter  determines  the  viscosity ^i.  e., 
time  required  for  a  specified  amount  of  the  material  to  flow  through  a 
standard  aperture.    This  test  is  generally  used  for  liquid  bituminous 
materials.  * 

517.  Float  Apparatus.     This  apparatus  determines  the  time  for 
a  specified  quantity  of  semi-solid  or  solid  material  to  flow  through 
an  aperture;  and  is  generally  used  for  semi-solid  and  solid  tars  and 
pitches. 

518.  Penetration.     The    penetrometer    determines    the    distance 
a  needle  will  penetrate  the  material  in  a  specified  time.     Of  course, 
the  size  of  the  needle,  the  weight  on  the  needle,  and  the  temperature 
of  the  material  are  carefully  standardized.    The  penetration  is  usually 
stated  in  tenths  of  millimeters;  but  sometimes  in  degrees,  since  the 
index  finger  sweeps  over  an  arc  of  a  circle  graduated  to  degrees. 
This  apparatus  is  generally  employed  for  asphalts;  but  it  is  not  used 


ART.   1]  ASPHALT  273 

for  tars,  because  the  surface  tension  and  the  presence  of  free  carbon 
considerably  affect  the  results  without  materially  affecting  the  con- 
sistency. 

519.  Melting  Point.    The  determination  of  the  melting  point  is 
mainly  of  value  as  a  means  of  identification.     It  is  virtually  a  test 
of  consistency  (§515).     As  a  rule  as  the  melting  point  of  a  bituminous 
material  rises,  it  becomes  harder  and  more  brittle.     One  of  the  char- 
acteristics of  asphalt  which  peculiarly  fits  it  for  use  in  roads  and 
pavements  is  that  it  has  a  high  melting  point  without  being  brittle. 
Paraffin  also  has  a  high  melting  point,  but  it  is  brittle. 

520.  Loss  by  Evaporation.     This  test  determines  the  amount  of 
volitilization   under   standard   conditions.     The   residue  is  usually 
tested  for  penetration,  melting  point,  and  ductility.     The  compari- 
son of  the  results  of  these  tests  before  and  after  the  evaporation  test 
determines  the  amount  of  hardening,  which  is  an  indication  of  the 
stability  of  the  cement. 

521.  Distillation.    The  distillation  is  carried  on  at  considerable 
lower  temperatures  than  the  evaporation  test,  and  is  usually  applied 
only  to   tars.     The  melting  point  of  the  residue  is   determined, 
and  also  its  consistency  at  several  temperatures.     This  is  an  impor- 
tant test  of  tars  to  determine  their  road-building  qualities  and  also 
their  method  of  preparation. 

522.  Bitumen  Soluble  in  Bisulphide.     It  is  usually  assumed  that 
all  matter  soluble  in  cold  carbon  disulphide  is  bitumen.     Fluid  oils 
are  almost  wholly  soluble  in  this  material.     The  amount  and  char- 
acter of  the  insoluble  matter  are  of  most  interest  in  this  test.    The 
insoluble  matter  is  usually  free  carbon,  which  is  of  no  value  in  road 
work.     The  failure  to  pass  this  test  is  an  indication  that  the  material 
has  been  overheated,  i.  e.,  "  cracked." 

523.  Bitumen  Soluble  in  Naphtha.    This  test  is  chiefly  valuable 
in  determining  the  amount  of  body-forming  hydrocarbons  in  oil  and 
oil  products.     "  No  oil  having  less  than  4  per  cent  insoluble  in  naph- 
tha will  be  of  service  in  road  work  except  as  a  dust  preventive." 
Bitumens  insoluble  in  naphtha  are  commonly  called  asphaltenes, 
while  those  soluble  are  called  malthenes. 

524.  Bitumen  Soluble  in  Tetrachloride.    The  test  is  made  for 
purposes  of  identification  and  also  to  determine  whether  the  material 
has  been  over-heated  in  the  process  of  manufacture.     The  bitumen 
insoluble  in  carbon  tetrachloride  but  soluble  in  carbon  disulphide  is 
commonly  called  carbenes. 

525.  Fixed  Carbon.     The  amount  of  fixed  carbon  shows  much 


274  BITUMINOUS   ROAD   MATERIALS  [CHAP.   VIII 

the  same  results  as  the  bitumen  insoluble  in  naphtha.  The  amount 
of  fixed  carbon  present  is  an  indication  of  the  mechanical  stability  of 
a  road  oil.  Paraffin  oils  have  only  little  fixed  carbon,  while  asphaltic 
oils  have  more,  and  asphalts  still  more.  This  test  can  not  be  applied 
to  tar,  owing  to  the  error  introduced  by  the  presence  in  it  of  consid- 
erable free  carbon. 

526.  Ductility.     This  test  consists  in  forming  a  briquette  of  the 
material  and  observing  the  amount  of  elongation  before  rupture. 
It  is  the  only  test  for  determining  the  cementing  value  of  an  asphalt, 
and  hence  is  very  important. 

527.  Paraffin  Scale.    This  test  consists  in  determining  the  amount 
of  paraffin  present.     It  is  made  as  a  means  of  identification,  and  is 
not  a  very  accurate  test;    and  there  is  considerable  difference  of 
opinion  as  to  its  value. 

528.  THE  FLUX.    A  flux  is  a  heavy  oil  or  the  residue  from  the 
distillation  of  petroleum  which  is  mixed  with  refined  asphalt  to  make 
it  of  suitable  consistency  for  use  in  a  sheet  asphalt  pavement  or  as 
a  binder  for  asphaltic  macadam  or  concrete.     Fluxes  are  usually 
obtained  from  paraffin,  semi-asphaltic,  or  asphaltic  oils;  and  vary 
greatly  in  character  with  the  petroleum  from  which  they  are  derived. 
The  lower  the  specific  gravity  of  the  flux,  the  less  the  amount  required 
to  produce  an  asphalt  cement  of  the  desired  consistency.     Different 
asphalts  require  quite  different  amounts  of  flux.     For  example,  Ber- 
mudez  asphalt  requires  only  7  per  cent  of  a  light  flux,  while  Trinidad 
asphalt  requires  20  per  cent. 

529.  Specifications  for  Flux.    The  following  are  the  specifications 
of   the   American    Society   of   Municipal    Improvements,    adopted 
October  14,  1915,  for  the  flux  to  be  used  in  preparing  asphalt  for 
sheet  asphalt  pavements. 

1.  The  flux  must  have  a  penetration  greater  than  350  with  a  No.  2  needle 
at  77°  F.  under  a  50-gram  weight  applied  for  one  second. 

2.  It  shall  have  a  specific  gravity  at  77°  F.  between  0.92  and  1.02. 

3.  When  20  grams  of  the  flux  are  heated  for  5  hours  at  325°  F.  in  a  tin 
box  2?  inches  in  diameter  and  three  quarters  of  an  inch  deep  after  the  manner 
officially  prescribed,  the  loss  shall  not  exceed  5  per  cent  by  weight;  and  the 
residue  left  after  such  heating  shall  flow  at  77°  F. 

4.  The  flux  shall  not  flash  below  350°  F.  when  tested  in  a  closed  oil  tester. 

5.  It  shall  be  soluble  in  carbon  tetrachloride  to  the  extent  of  not  less 
than  99  per  cent. 

530.  ASPHALT    CEMENT.     Asphalt    cement    is    produced    by 
mixing  refined  asphalt  and  a  flux.     The  asphalt  should  be  heated  to 


ART.  1]  ASPHALT  .    275 

325  to  350°  F.  and  the  flux  to  150  to  200°  F.  before  they  are  mixed. 
The  mixing  is  done  by  blowing  air  or  steam,  preferably  the  latter, 
through  perforated  pipes  in  the  bottom  of  the  melting  tank.  The 
mixing  should  be  very  thorough,  and  usually  requires  three  or  more 
hours.  Care  should  be  taken  -that  the  cement  is  not  burned,  par- 
ticularly if  the  tank  is  heated  over  a  fire.  The  cement  will  harden 
if  kept  heated  for  a  long  time  or  if  the  agitation  is  kept  up  unduly 
long;  but  the  cement  can  be  softened  again  by  adding  more  flux 
and  mixing  further. 

531.  SPECIFICATIONS  FOR  ASPHALT  CEMENT.    Asphalt  cement 
is  used  for  a  bituminous  surface  on  water-bound  gravel  or  macadam 
roads  (§  589)  and  for  binder  in  bituminous  macadam  and  bituminous- 
concrete  roads  (§611  and  622);    but  principally  for  sheet  asphalt 
pavements  (Art.  1  of  Chap.  XVI),  asphaltic  concrete  (Art.  2  of  Chap. 
XVI),  and  also  for  joint    filler  for   brick,  stone-block,  and  wood- 
block pavements  (Chap.  XVI,  XVII,  and  XIX,  respectively). 

Usually  separate  specifications  are  drawn  for  each  of  the  above 
uses. 

532.  Whatever  the  purpose  for  which  the  asphalt  is  to  be  used, 
there  are  two  classes  of  specifications  for  it,  one  known  as  general 
or  open  or  blanket  specifications,  and  the  other  as  restricted  or 
special  or  alternate  specifications.     The  former  are  drawn  so  as 
to  include  all  kinds  of  asphalt  whatever  their  source  or  origin; 
and   the   latter   consist  of   special   requirements  for  each  type  or 
asphalt. 

Tables  31,  32  and  33,  page  278,  280,  and  281,  show  a  summary 
of  restricted  specifications  of  asphalt  for  different  uses;  and  §  543 
contains  an  example  of  general  specifications  for  asphalt  for  sheet 
asphalt  pavements. 

There  is  a  sharp  difference  of  opinion  as  to  the  relative  merits  of 
the  two  classes  of  specifications.  Those  who  favor  restricted  speci- 
fications claim  that  there  is  so  much  difference  between  the  different 
kinds  of  asphalt  that  it  is  impossible  to  make  general  requirements 
which  will  apply  to  all  and  at  the  same  time  define  any  quality  so  as 
to  make  it  a  real  test  for  any  particular  kind  of  asphalt.  For  exam- 
ple, assume  that  it  is  desired  to  permit  the  use  of  any  of  the  four 
asphalts  of  Table  31,  page  278,  and  that  it  is  stated  that  the  specific 
gravity  shall  be  from  0.96  to  1.06.  The  lower  limit  is  too  small  to 
fix  this  quality  in  some  of  the  asphalts,  and  the  upper  limit  is  too 
great  to  fix  it  for  others.  Again,  if  the  specifications  state  that  the 
penetration  shall  be  between  90  and  160,  they  will  permit  the  use 


276  BITUMINOUS   ROAD   MATERIALS  [CHAP.    VIII 

of  any  of  the  four  asphalts  of  Table  31;  but  the  limits  are  too  wide 
to  secure  uniformity  in  any  of  the  cements,  and  an  entirely  unsuitable 
material  could  be  supplied  under  such  specifications. 

On  the  other  hand,  equally  competent  asphalt  specialists  strongly 
dissent  from  the  above  statements;-  and  claim  that  the  different 
asphalts  on  the  market  are  so  nearly  alike  in  their  essential  qualities 
as  not  to  justify  separate  specifications.  They  claim  that  there  is 
no  more  reason  for  separate  specifications  for  the  different  asphalts 
than  for  separate  specifications  for  different  brands  of  portland 
cement.  They  claim  that  the  bitumen  in  all  asphalts  is  practically 
the  same,  and  that  the  seeming  difference  in  asphalts  is  due  to  the 
mineral  matter  which  they  contain.  For  example,  Trinidad  refined 
asphalt  (§  498),  one  of  the  best-known  and  most  extensively  used 
of  the  natural  asphalts,  contains  about  36  per  cent  of  mineral  matter; 
and  consequently  its  specific  gravity  is  greater  and  its  penetration 
is  less  than  a  more  pure  asphalt.  The  finely  divided  mineral  matter 
in  this  asphalt  does  not  injure  it  for  some  uses,  for  example,  sheet 
asphalt  pavements,  since  in  practice  a  considerable  amount  of 
fine  mineral  matter  is  added  to  the  asphalt  to  give  it  physical  stability 
(§  826).  Those  who  advocate  general  specifications  claim  that  the 
illustration  concerning  specific  gravity  in  the  preceding  paragraph  is 
wide  of  the  mark,  since  a  test  for  specific  gravity  is  valuable  only 
as  a  means  of  identifying  the  material,  and  in  no  way  aids  in  deter- 
mining any  essential  quality.  They  also  claim  that  the  penetration 
of  the  pure  bitumen  in  all  asphalts  is  substantially  the  same;  and 
that  the  difference  in  the  limits  is  only  to  provide  for  the  difference 
between  heavy  and  light  traffic,  a  difference  in  the  fineness  of  the 
sand,  and  differences  in  climatic  conditions. 

The  divergence  of  opinion  as  to  the  merits  of  the  two  types  of 
specifications  is  shown  by  the  fact  that  the  standard  specifications 
of  the  American  Society  of  Municipal  Improvements  for  bituminous 
macadam  (§  537-38),  bituminous  concrete  (§  539-40),  and  seal  coat, 
(§  541),  adopt  the  restricted  or  special  form  of  specifications  for  the 
asphalt;  while  the  standard  specifications  of  the  same  Society  for 
asphalt  concrete  and  sheet  asphalt  pavements  (§  542-43)  are  based 
upon  general  or  blanket  specifications  for  the  asphalt.  However,  a 
recent  vote  shows  that  the  weight  of  the  society  is  in  favor  of  the 
general  specifications. 

533.  The  writing  of  specifications  for  asphaltic  cement  requires 
thorough  laboratory  knowledge  of  the  chemical  and  physical  char- 
acteristics of  asphalt,  and  also  practical  experience  in  the  use  of  the 


ART.  1]  ASPHALT  277 

material.*  Great  care  must  be  used  in  changing  the  limits  in  speci- 
fications, since  a  change  in  the  value  for  one  element  may  require  a 
corresponding  change  in  some  other  factor.  Below  are  specifica- 
tions for  asphaltic  cement  that  have  been  successfully  used  for  roads 
and  pavements. 

534.  Asphalt  for  Bituminous  Surface  on  Water-bound  Macadam. 
The  following  are  the  specifications  of  the  Barber  Asphalt  Company 
for  liquid  asphalt  for  both  cold  and  hot-surface  application  to 
water-bound  macadam :  f 

635.  Liquid  Asphalt  A.  (For  Cold  Application.)  1.  Specific  Gravity:  The 
specific  gravity  at  60°  F.  shall  not  be  less  than  0.91. 

2.  Flash  Point:     The  flash  point  by  the  Tagliabue  open  cup  shall  not  be  less 
than  100°  F. 

3.  Viscosity:   The  specific  viscosity  by  the  Engler  apparatus  at  77°  F.,  for 
the  first  50  c.c.  shall  be  between  90  and  100. 

4.  Bitumen  Soluble  in  Bisulphide :  The  bitumen  soluble  in  carbon  disulphide 
shall  not  be  less  than  99  per  cent. 

5.  Paraffin  Scale:  The  paraffin  scale  determined  by  the  Holde  method  shall 
not  be  more  than  0.25  per  cent. 

6.  Distillation :  When  evaporated  to  80  per  cent  by  weight,  by  heating  in  an 
open  dish  at  150°  F.,  the  residue  shall  have  at  77°  F.  with  a  No.  2  needle  under  a 
weight  of  50  grams  in  1  second,  a  penetration  of  not  less  than  20  mm.;  and  its 
adhesiveness  by  the  Osborne  test  shall  not  be  less  than  200  seconds. 

536.  Liquid  Asphalt  B.  (For  Hot  Application.)  1.  Specific  Gravity:  The 
specific  gravity  at  60°  F.  shall  be  not  less  than  1.00. 

2.  Flash  Point.     The  flash  point  by  the  Cleveland  cup  shall  be  not  less  than 
325°  F. 

3.  Viscosity:   The  specific  viscosity  by  the  Engler  apparatus  at  212°  F.,  for 
the  first  50  c.c.  shall  be  from  23  to  33. 

4.  Bitumen  Soluble  in  Disulphide:  The  bitumen  soluble  in  carbon  disulphide 
shall  be  not  less  than  99.0  per  cent. 

5.  Paraffin  Scale:    The  paraffin  scale  by  the  Holde   method    shall   not   be 
more  than  0.25  per  cent. 

6.  Distillation:   The  loss  at  325°  F.  after  5  hours  of  50  grams  in  a  2£  by  1|- 
inch  dish  shall  not  be  more  than  1.0  per  cent. 

7.  Residue,  loss  by  evaporation:    The  residue   of  a  5.0  mm.  penetration  at 
77°  F.  under  a  load  of  100  grams  with  a  No.  2  needle  at  5  seconds,  when  evap- 
orated at  500°  F.  in  an  open  dish  shall  lose  not  less  than  75.0  per  cent. 

8.  Adhesiveness:  The  adhesiveness  at  77°  F.  by  the  Osborne  test  shall  not  be 
less  than  400  seconds. 


*  For  the  methods  and  results  of  tests  of  asphalts  see  Hubbard's  Dust  Preventives  and 
Road  Binders,  8vo,  p.  416,  John  Wiley  &  Sons,  New  York,  1910;  Richardson's  Modern  Asphalt 
Pavement,  8vo,  p.  629,  John  Wiley  &  Sons,  New  York,  1908. 

t  Letter  to  the  author  under  date  of  July  13,  1917. 


278 


BITUMINOUS    ROAD    MATERIALS 


[CHAP,  viii 


537.  Asphalt  Binder  for  Bituminous  Macadam.  The  American 
Society  of  Municipal  Improvements  on  October  12,  1916,  adopted 
restricted  specifications  for  four  kinds  of  asphalt,  any  one  of  which  is 
acceptable  as  a  binder  for  bituminous  macadam.  The  full  specifi- 
cations for  an  asphalt  cement  made  from  Gilsonite  and  asphaltic 
oil  follow.  The  specifications  for  the  three  other  asphalts  are  in 
exactly  the  same  form;  and  Table  31  gives  the  essential  elements 
of  the  specifications  of  all  four. 


TABLE  31 

COMPARISON  OF  SPECIFICATIONS  FOR  ASPHALT  CEMENTS  FOR  BITUMINOUS 

MACADAM 

Standards  of  American  Society  of  Municipal  Improvements,  Adpoted  October  1,  1915 


Ref. 
No. 

Items. 

KIND  OF  ASPHALT  CEMENT. 

Gilsonite 
and 
Asphalt  Oil. 

Texas  and 
California 
Oil  Asphalt. 

Mexican  Oil 
Asphalt. 

Bermudez 
Asphalt. 

1 

2 
3 

4 

5 

6 
7 
8 
9 
10 

Shall  not  foam  at 

177°  C. 
205°  C. 
0.96-1.00 

100-120 
>50 
>  60°  C. 

177°  C. 
205°  C. 
1.00-1.03 

90-110 
>15 
>30°C. 

177°  C. 
205°  C. 
1.025-1.045 

110-130 
>30 
>  40°  C. 

177°  C. 
163°  C. 
1.035-1.060 

130-160 
>30 

120-180  sec. 
<3.0% 
94-98.0 
98.5% 

75-85% 
11-14% 

Flash  point,  not  less  than  
Specific  gravity  at  25°  C  
Penetration  — 
100  grams,  5  sec.,  25°  C  
200  grams,  60  sec.,  40°  C.  .  .  . 
Melting  point  by  cube  method. 
Viscosity  by  N.  Y.  float  appa- 
ratus 

Distillation,  loss  after  5  hrs  
Bitumen  soluble  in  disulphide  .  . 
Bitumen  soluble  in  tetrachloride 
Bitumen  soluble  in  naphtha  .... 
Fixed  carbon. 

<2.0% 
99.5% 
99.5% 

75-85% 
8-12% 

<2.0% 
99.5% 
99.5% 
75-85% 
9-13% 

<2.0% 
99.5% 
99.5% 
70-80% 
12-17% 

638.  Gilsonite  and  Asphaltic  Oil.     1.  Foam:   The  asphalt  cement  shall  be 
homogeneous,  free  from  water,  and  shall  not  foam  when  heated  to  177°  C.  (350°  F.) 

2.  Flash  Point:  It  shall  show  a  flash  point  of  not  less  than  205°  C.  (400°  F.) 
when  tested  in  the  New  York  State  Board  of  Health  Closed  Oil  Tester. 

3.  Specific  Gravity:   Its  specific  gravity  at  a  temperature  of  25°  C.  (77°  F.) 
shall  be  not  less  than  0.960  nor  more  than  1.000. 

4.  Evaporation:   When  tested  with  a  standard  No.  2  needle  by  means  of  a 
standard  penetrometer,  it  shall  show  penetrations  within  the  following  limits 
for  the  conditions  stated,  the  penetrations  being  expressed  in  hundredths  of  a 
centimeter:   100-gram  load,  5  seconds  at  25°  C.  (77°  F.),  from  100  to  120;  200- 
gram  load,  1  minute  at  4°  C.  (39°  F.),  not  less  than  50. 

5.  Melting  Point:  Its  melting  point  as  determined  by  the  cube  method  shall 
be  not  less  than  60°  C.  (140°  F.). 

6.  Distillation:    When  50  grams  of  the  material  is  maintained  at  a  uniform 
temperature  of  163°  C.  (325°  F.)  for  5  hours  in  an  open  cylindrical  tin  dish  5£ 
centimeters  (about  2 1  inches)  in  diameter,  with  vertical  sides  measuring  approx- 
imately 3£  centimeters  (about  \\  inches)  in  depth,  the  loss  in  weight  shall  not 
exceed  2.0  per  cent  of  the  original  weight  of  the  sample, 


ART.  1]  ASPHALT  279 

Penetration  of  Residue:  The  penetration  of  the  residue  when  tested  as  de- 
scribed in  paragraph  4  with  a  standard  No.  2  needle  under  a  load  of  100  grams  for 
5  seconds  at  25°  C.  (77°  F.),  shall  be  not  less  than  one  half  the  penetration  of  the 
original  material  tested  under  the  same  condition. 

7.  Bitumen  Soluble  in  Bisulphide:    Its  bitumen  as  determined  by  its  solu- 
bility in  chemically  pure  carbon  disulphide  at  room  temperature,  shall  be  not 
less  than  99.5  per  cent. 

8.  Bitumen  Soluble  in  Tetrachloride :   It  shall  be  soluble  in  chemically  pure 
carbon  tetrachloride  at  room  temperature,  to  the  extent  of  not  less  than  99.5 
per  cent  of  its  bitumen  as  determined  by  paragraph  7. 

9.  Bitumen  Soluble  in  Naphtha:    It  shall  be  soluble  in  86  to  88°  Baume 
paraffin  naphtha,  of  which  at  least  85.0  per  cent  distills  between  35°  and  65°  C. 
(95°  and  149°  F.),  to  the  extent  of  not  less  than  75.0  per  cent  nor  more  than  85.0 
per  cent  of  its  bitumen  as  determined  by  paragraph  7. 

10.  Fixed  Carbon:  It  shall  yield  not  less  than  8.0  per  cent  nor  more  than  12.0 
per  cent  of  fixed  carbon. 

539.  Asphalt  Binder  for  Bituminous  Concrete.     The  American 
Society  of  Municipal  Improvements  on  October  12,  1916,  adopted 
restricted  specifications  for  four  kinds  of  asphalt  any  one  of  which  is 
acceptable  as  a  binder  for  bituminous  concrete.     The  full  specifi- 
cations for  asphalt  made  of  Gilsonite  and  asphaltic  oil  follow.     The 
specifications  for  the  four  other  materials  are  in  exactly  the  same 
form.     Table  32,  page  280,  gives  the  essential  elements  of  the  speci- 
fications of  all  five  asphalts. 

540.  Gilsonite  and  Asphalt  Oil.       1.  Foam:     The  asphalt  cement  shall  be 
homogeneous,  free  from  water,  and  shall  not  foam  when  heated  to  177°  C. 
(350°  F.). 

2.  Flash  Point:  It  shall  show  a  flash  point  of  not  less  than  205°  C.  (400°  F.) 

3.  Specific  Gravity:  Its  specific  gravity  at  a  temperature  of  25°  C.  (77°  F.) 
shall  be  not  less  than  0.970  nor  more  than  1.000. 

4.  Penetration:    When  tested  with  a  standard  No.  2  needle  by  means  of  a 
Dow  penetrometer  (or  other  penetrometer  giving  the  same  results  as  the  Dow 
machine),  it  shall  show  penetrations  within  the  following  limits  for  the  conditions 
stated,  the  penetrations  being  expressed  in  hundredths  of  a  centimeter:  100-gram 
load,  5  seconds,  at  25°  C.  (77°  F.),  from  75  to  90;  200-gram  load,  1  minute,  at 
4°  C.  (39°  F.),  not  less  than  35;   50-gram  load,  5  seconds,  at  46°  C.  (115°  F.) 
net  more  than  250. 

5.  Melting  Point:  Its  melting  point  as  determined  by  the  cube  method  shall 
be  not  less  than  55°  C.  (131°  F.). 

6.  Evaporation:   When  50  grams  of  the  material  is  maintained  at  a  uniform 
temperature  of  163°  C.  (325°  F.)  for  5  hours  in  an  open  cylindrical  tin  dish,  5£ 
centimeters  (about  2j  inches)  in  diameter,  with  vertical  sides  measuring  approxi- 
mately 3  \  centimeters  (about  \\  inches)  in  depth,  the  loss  in  weight  shall  not 
exceed  1.0  per  cent  of  the  original  weight  of  the  sample. 

Penetration  of  Residue:    The  penetration  of  the  residue,  when  tested  as 


280 


BITUMINOUS  fcoAD  MATERIALS 


[CHAP,  viii 


described  in  paragraph  4  with  a  standard  No.  2  needle  under  a  load  of  100  grams 
for  5  seconds  at  25°  C.  (77°  F.)  shall  be  not  less  than  one  half  the  penetration  of 
the  original  material  tested  under  the  same  conditions. 

7.  Bitumen  Soluble  in  Bisulphide:    Its  bitumen  as  determined  by  its  solu- 
bility in  chemically  pure  carbon  disulphide  at  room  temperature  shall  not  be 
less  than  99.5  per  cent. 

8.  Bitumen    Soluble  in   Tetrachloride :  It  shall  be  soluble  in  chemically  pure 
carbon  tetrachloride  at  room  temperature  to  the  extent  of  not  less  than  99.5 
per  cent  of  its  bitumen  as  determined  by  paragraph  7. 

9.  Bitumen  Soluble  in  Naphtha:  It  shall  be  soluble  in  86  to  88°  Baume  par- 
affin naphtha,  at  least  85  per  cent  of  which  distills  between  40  and  55°  C.  (104° 
and  131°  F.),  to  the  extent  of  not  less  than  70.0  per  cent  nor  more  than  80.0 
per  cent  of  its  bitumen  as  determined  by  paragraph  7. 

10.  Fixed  Carbon:    It  shall  yield  not  less  than  8.0  per  cent  nor  more  than 
12.0  per  cent  of  fixed  carbon. 


TABLE  32 

COMPARISON  OP  SPECIFICATIONS  FOR  ASPHALT  CEMENTS  FOR  BITUMINOUS 

CONCRETE 

Standards  of  American  Society  of  Municipal  Improvements,  Adopted  October  14,  1915 


1 

1 

1 

2 

a 

4 
6 

6 

7 
8 
9 
10 

Item. 

KINDS  OF  ASPHALT. 

Gilsonite 
and  Asphalt 
Oil 

Texas  Oil 
Asphalt. 

California 
Oil  Asphalt. 

Mexican 
Oil  Asphalt. 

Bermudez 
Asphalt. 

Shall  not  foam  at  
Flash  point,  not  less  than. 
Specific  gravity  at  25°  C.  . 
Penetration  —  100  grs.  .  .  . 
—  200  grams.  . 
Melting     point     by    cube 
method  
Viscosity   by   N.   Y.   float 
apparatus 

177°  C. 
205°  C. 
0.97-1.000 
75-90 
>35 

>55°C. 

177°  C. 
205°  C. 
1.000-1.030 
90-100 
>30 

>50°  C. 

177°  C. 
205°  C. 
1.030-1.040 
70-90 
>10 

>45°  C. 

177°  C. 
205°  C. 
1  .  025-1  .  050 
85-95 
>20 

>  50°  C. 

177°  C. 
165°  C. 
1.040-1.060 
140-160 
>40 

120-180  sec 
<3.0% 
93-98% 

98.5% 

75-85% 
11-15% 

Evaporation,    loss  after   5 
hrs 

<i.o% 

99.5% 
99.5% 

70-80% 
8-12% 

<i.o% 

99.5% 
99.5% 

72-78% 
11-15% 

<2.0% 
99.5% 
99  .  5% 

80-88% 
10-14% 

<2.0% 
99.5% 
99.5% 

67-77% 
12-18% 

Bitumen  soluble  in  disul- 
phide   
Bitumen  soluble  in  tetra- 
chloride   
Bitumen  soluble  in  naph- 
tha 

Fixed  carbon  

541.  Asphalt  for   Seal   Coat   for    Bituminous    Concrete.    The 

American  Society  of  Municipal  Improvements  adopted  restricted 
specifications  for  three  kinds  of  asphalt  for  the  seal  coat  of  bitumi- 
nous concrete  pavements  in  which  tar  is  the  cementing  material  of 
the  concrete.  These  specifications  are  of  the  same  general  form  as 
those  in  §  538  and  §  540.  The  essential  elements  of  the  specifi- 
cations are  shown  in  Table  33. 


ART.  1] 


ASPHALT 


281 


TABLE  33 

SPECIFICATIONS  FOR  ASPHALT  FOR  SEAL  COAT  FOR  TAB-CONCRETE  PAVEMENTS 

Standards  of  American  Society  of  Munisipal  Improvements,  Adopted  October  14,  1915 


Ref. 
No. 

Items. 

KINDS  os  ASPHALT. 

Gilsonite 
and 
Asphalt  Oil. 

Texas  Oil 
Asphalt. 

Mexican 
Oil  Asphalt. 

1 
2 
3 

4 

5 
6 

7 
8 
9 
10 

1  ' 

177°  C. 
205°  C. 
1  .  025-1  .  050 
85-95 
>20 
50°  C. 
<2.0% 
>50% 
99.5% 
99.5% 
70-80% 
8-12 

177°  C. 
205°  C. 
1.030-1.045 
60-70 
>18 
60°  C 
<1.0% 
>50% 
99.5% 
99.5% 
70-80% 
12-16 

177°  C. 
205°  C. 
1.025-1.055 
60-70 
>16 
55°  C. 
<1.0% 
>50% 
99.5% 
99.5% 
67-77% 
13-18 

200  grams                 

Distillation,  loss  after  5  hours  •.-.•••,  
Penetration  of  residue,  per  cent  of  original  

Bitumen  soluble  in  tetrachloride  
Bitumen,  per  cent  of  total  soluble  in  naphtha.  .  . 

542.  Asphalt  for  Sheet  Asphalt  Pavements.    The  following  are 
the  specifications  of  the  American  Society  of  Municipal  Improve- 
ments, adopted  October  14,  1915,  for  asphalt  cement  for  sheet  asphalt 
and  asphalt  concrete  pavements: 

1.  Homogeneous:  The  asphalt  cement  shall  be  thoroughly  homogeneous. 

2.  Penetration:    It  shall  have  a  penetration  at  77°  F.  of  from  30  to  55  for 
heavy  traffic  streets,  and  55  to  85  for  light  traffic  streets,  depending  upon  the  sand 
and  asphalt  used  and  the  local  climatic  conditions. 

3.  Flash  Point:   It  shall  not  flash  below  350°  F.  when  tested  hi  a  closed  oil 
tester. 

4.  Evaporation:    When  20  grams  of  the  asphalt  cement  are  heated  for  5 
hours  at  325°  F.  in  a  tin  box  2\  inches  in  diameter  and  f  of  an  inch  deep,  after 
the  manner  officially  prescribed,  the  loss  shall  not  exceed  5  per  cent  by  weight; 
and  the  penetration,  at  77°  F.,  of  the  residue  left  after  such  heating  must  not  be 
less  than  one  half  the  penetration,  at  77°  F.,  of  the  original  sample  before  heating. 

5.  Ductility:    Either  the  asphalt  cement  or  its  pure  bitumen  when  made 
into  a  briquette  in  the  Dow  mold  shall  have  at  50  penetration  at  77°  F.,  a  duc- 
tility of  not  less  than  30  centimeters,  when  the  two  ends  of  the  briquette  are 
pulled  apart  at  the  uniform  rate  of  5  centimeters  per  minute. 

When  the  asphalt  cement  as  used  has  a  penetration  other  than  50  at  77°  F., 
an  increased  ductility  of  2  centimeters  will  be  required  for  every  5  points  in  pen- 
etration above  50;  and  a  corresponding  allowance  will  be  made  for  a  penetration 
below  50. 

543.  There  are  two  marked  differences  between  the  preceding 
specifications  and  those  for  asphalt  binder  for  bituminous  macadam 
(§  538)  and  those  for  asphalt  binder  for  bituminous  concrete  (§  540). 

In  the  first  place,  the  preceding  specifications  are  briefer,  having 
only  five  items  while  the  others  have  ten;  but  the  longer  specifica- 


282  BITUMINOUS   ROAD   MATERIALS  [CHAP.   VIII 

tions  contain  several  items  of  only  minor  importance.  For  example, 
the  foam  test  determines  only  the  presence  of  water,  but  determines 
nothing  concerning  the  quality  of  the  asphalt.  Again,  the  specific 
gravity  test  is  of  value  in  identifying  the  asphalt,  but  is  of  no  value 
in  determining  its  quality.  Further,  the  melting  point  indicates 
consistency,  which  is  more  accurately  determined  by  the  penetration 
test.  On  the  other  hand,  notice  that  the  specifications  of  §  542  alone 
contain  a  test  for  ductility,  which  is  the  sole  test  to  determine  the 
cementing  value  of  the  asphalt. 

In  the  second  place,  the  specifications  of  §  542  are  the  general  or 
blanket  form,  while  those  of  §  538  and  §  540  are  restricted  or  special 
form.  For  a  discussion  of  the  merits  of  the  two  forms,  see  §  532. 

544.  Asphalt  Filler  for  Block  Pavements.  It  is  important  that 
the  asphalt  used  as  a  filler  in  block  pavements  shall  be  affected  as 
little  as  possible  by  temperature  changes;  and  therefore  the  manu- 
facturers have  prepared  a  material  especially  for  this  purpose, 
partly  by  refining  the  asphalt  and  partly  by  hardening  it  by  oxidation, 
i.  e.,  by  passing  air  through  it.  The  following  are  the  usual  speci- 
fications for  an  asphalt  filler  for  brick,  stone-block,  and  wood-block 
pavements.  * 

The  asphalt  paving  cement  shall  be  obtained  by  the  distillation  of  an  as- 
phaltic  petroleum  at  a  temperature  not  exceeding  700°  F.,  and  shall  comply 
with  the  following  requirements: 

1.  It  shall  be  homogeneous. 

2.  The  melting  point  shall  not  be  less  than  130  nor  more  than  145°  F. 

3.  The  solubility  in  carbon  tetrachloride  shall  not  be  less  than  98|   per 
cent. 

*  4.  The  penetration  at  77°  F.  shall  not  be  less  than  60  nor  more  than  100; 
and  the  penetration  at  100°  F.  shall  not  exceed  three  times  the  penetration  at 
77°  F.  The  contractor  before  beginning  work  shall  obtain  from  the  engineer  a 
statement  in  writing  as  to  the  penetration  desired,  and  a  variation  not  greater 
than  ten  points  either  way  from  this  penetration  will  be  permitted. 

5.  The  ductility  at  77°  F.  shall  not  be  less  than  40  centimeters,  the  rate  of 
elongation  being  five  centimeters  per  minute. 

6.  It  shall  not  lose  more  than  3  per  cent  by  volatilization  when  maintained 
at  a  temperature  of  325°  F.  for  5  hours;  nor  shall  the  penetration  of  the  residue 
after  such  heating  be  less  than  one  half  the  original  penetration. 

7.  The  asphalt  filler  shall  be  used  on  the  work  at  a  temperature  of  not 
less  than  275°  F.;  and  shall  at  no  time  be  heated  above  350°  F. 

8.  It  shall  be  delivered  where  directed  by  the  engineer  in  time  to  allow  for 
examination  and  analysis. 

*  Specifications  for  Stone  Block  Paving,  adopted  by  American  Society  of  Municipal  Im- 
provements, 1916. 


ART.  2] 


PETROLEUM 


283 


545.  COST.  The  cost  of  all  materials  is  abnormal  at  present 
owing  to  the  Great  European  War;  but  the  asphalt  market  is  further 
disturbed  by  the  unsettled  political  conditions  in  Mexico,  which 
make  it  difficult  to  obtain  Mexican  petroleum  for  fluxing.  The 
prices  of  solid  refined  asphalt  for  January,  1917,  which  prices 
obtained  for  the  year  1917,  f.o.b.  Maurer,  N.  J.,  in  tank  cars,  were 
about  as  follows : 


KINDS  OP  ASPHALT. 

Price  per  Ton 
(2000  Lb.) 

Per  Cent 
Bitumen. 

Value  for 
100  Per  Cent 
Bitumen. 

Bermudez      

$27.00 

95.0 

$25  65 

Mexican  
Residual 

15.00 
16  00* 

95.5 

14.32 

Texas  

18.00 

98.8 

17  78 

Trinidad 

19  00 

99  5 

18  90 

*  Varies  $3.00  to  $4.00  either  way  in  different  parts  of  the  country, 
advanced  100  per  cent  since  1914. 


The  average  price  has 


If  asphalt  is  bought  in  barrels  or  drums,  the  cost  is  usually  1.5 
to  2  cents  per  gallon  more  than  above,  with  sometimes  a  little  rebate 
on  the  returned  barrels. 

Liquid  asphalt  is  now  (1917)  7  cents  per  gallon  f.o.b.  Maurer,  N.  J. 

546.  For  current  prices  of  asphalts,  consult  the  price  lists  in  the 
technical  journals. 

ART.  2.     PETROLEUM 

547.  CLASSIFICATION.     There   are   two   types   of  crude   petro- 
leum— one   giving   a   paraffin   residue,    and   the   other   an   asphalt 
residue.     The  petroleums  from  Pennsylvania,   Ohio,   Indiana  and 
Illinois  have  a  paraffin  residue  or  base;  while  those  from  California 
and  Mexico  and  some  from  Texas  have  an  asphaltic  base.     The 
oils  from  Kentucky,  Louisiana,  and  some  from  Texas  have  a  mixed 
paraffin  and  asphalt  base,  and  are  usually  called  semi-asphaltic  oils. 

The  oils  having  a  paraffin  base  are  more  or  less  greasy,  and  have 
no  binding  qualities,  but  rather  a  lubricating  effect;  and  are  useful 
only  as  a  temporary  dust  layer.  The  oils  having  an  asphaltic  or 
bituminous  base  are  more  valuable  for  roads,  since  the  bituminous 
base  binds  the  particles  of  the  road  together,  even  after  the  more 
volatile  portions  of  the  oil  have  evaporated. 

Crude  petroleum  as  it  comes  from  the  well  is  an  oily  liquid,  vary- 
ing in  color  from  greenish  brown  to  nearly  black,  and  varying  in 
specific  gravity  from  0.73  to  0.97.  In  the  refining  process  the  more 


284  BITUMINOUS    ROAD    MATERIALS  [CHAP.    VIII 

volatile  and  more  valuable  constituents,  as  benzine,  gasoline,  kero- 
sene, and  lubricating  oils,  are  driven  off  by  heat.  It  is  the  residue 
that  is  used  in  oiling  roads  and  as  a  flux  for  softening  the  solid  native 
asphalts  (§  528-29).  The  character  of  the  residue  varies  with  the 
crude  petroleum  and  with  the  process  of  refining,  and  its  value  for 
road  purposes  depends  upon  its  specific  gravity  and  the  amount  of 
bitumen  it  contains. 

548.  METHODS    OF    REFINING.     There    are    two    methods    of 
refining  petroleum  to  produce  materials  for  use  on  roads.     One 
method  is  that  of  ordinary  distillation  by  the  use  of  external  heat, 
usually  steam.     The  distillation  is  carried  on  until  only  the  solid  or 
semi-solid  portion  remains.     This  method  is  not  usually  applied 
to  paraffin  petroleums  on  account  of  the  high  temperatures  necessary 
and  the  resulting  decomposition. 

The  second  method  consists  in  blowing  atmospheric  air  through 
petroleum  heated  to  a  temperature  below  that  required  for  distilla- 
tion. The  air  produces  oxidation  and  condensation  of  the  lighter 
hydrocarbons,  and  the  oil  gradually  thickens.  This  method  is 
usually  applied  to  oils  having  a  paraffin  base;  and  the  residue  is 
known  as  blown  oil,  or  sometimes  as  blown-oil  asphalt,  although  it  is 
usually  paraffin  or  at  most  a  semi-asphalt  mixture. 

549.  SHIPPING  THE  OIL.     Road  oil  is  usually  shipped  in  tank 
cars  holding  either  4000  to  6000  gallons  or  8000  to  10,000  gallons,  the 
latter  being  much  more  common.     Oil  may  be  had  in  barrels;  but  it 
is  then  more  expensive,  and  is  also  much  more  difficult  to  handle. 
The  tank  cars  are  equipped  with  steam  heating  coils,  so  the  material 
may  be  heated  in  the  tank  by  attaching  a  steam  pipe  or  hose. 

There  are  a  number  of  simple  pumps  on  the  market  that  will 
pump  either  hot  or  cold  oil.  The  ordinary  thresher's  water-tank 
pump  may  be  used  for  pumping  cold  oil;  but  not  hot  oil,  as  it  will 
soon  burn  out  the  valves.  Fig.  92,  page  285,  shows  the  method  of 
pumping  road  oil  from  the  dome  of  a  tank  car  by  means  of  a  hand 
diaphragm  pump;  and  a  thresher's  water-tank  pump  may  be  used 
in  the  same  way.  However,  this  method  is  nearly  obsolete,  since 
state  and  county  highway  departments  usually  own  outfits  made 
particularly  for  this  purpose. 

Power-driven  diaphragm  and  rotary  pumps  are  the  forms  gen- 
erally used;  and  are  attached  to  the  outlet  in  the  bottom  of  the  tank 
car.  These  pumps  are  driven  by  an  independent  gasoline  engine  or 
by  steam  from  a  steam  road-roller,  which  is  sometimes  needed  at  the 
tank  car  to  supply  steam  to  heat  the  oil.  Fig.  93  shows  the  method 


ART.  2] 


PETROLEUM 


285 


of  transferring  road  oil  from  the  tank  car  to  the  distributing 
wagon  by  means  of  a  gasoline-driven  rotary  pump. 

550.  ASPHALTIC  CONTENT  OF  ROAD  OILS.  Road  oils  are  fre- 
quently referred  to  as  containing  a  certain  per  cent  of  asphalt; 
and  the  oil  refineries  always  classify  road  oils  according  to  their 
asphaltic  content — see  §561. 

This  "  asphalt  "  is  not  a  definite  compound  that  can  be  deter- 
mined by  chemical  analysis.  To  determine  the  "  asphalt  "  in  a  road 
oil,  the  oil  is  heated  in  a  closed  oven  to  a  temperature  of  400°  F., 
which  drives  off  the  light  or  volatile  constituents  and  leaves  a  semi- 


FIG.  92. — UNLOADING  OIL  WITH  DIAPHRAGM 
PUMP. 


FIG.  93. — UNLOADING  OIL  WITH  ROTARY 
PUMP. 


solid  or  solid  residue.  This  residue  is  called  'asphalt,  although 
it  may  contain  many  different  bitumens.  If  the  residue  is  paraffin, 
it  is  useful  as  a  dust  layer,  but  worthless  as  a  road  binder.  Since  road 
oil  is  so  cheap,  it  is  not  likely  that  the  residue  contains  any  adulter- 
ation. The  amount  of  residue  in  a  road  oil  gives  no  indication  of  its 
value  for  use  on  a  road,  since  the  residue  may  be  wholly  paraffin,  or 
semi-asphalt,  or  asphalt.  However,  the  per  cent  of  "  asphalt  •"  does 
give  some  indication  as  to  the  viscosity  of  the  oil,  i.  e.,  as  to  the 
amount  of  body  it  contains.  The  method  employed  and  equipment 
used  in  determining  the  amount  of  "  asphalt  "  vary  considerably, 
and  are  not  usually  stated;  hence  the  result  is  quite  indefinite.  No 
reliance  should  be  placed  upon  such  a  loose  term;  but  the  oil  should 
be  bought  to  conform  to  definite  specifications. 

The  degree  of  hardness  of  this  residue  is  measured  by  the  depth 
of  penetration  of  a  No.  2  needle  under  a  load  of  100  grams  in  5  seconds. 
It  is  sometimes  specified  that  the  road  oil  shall  contain  a  certain  per 
cent  of  "asphalt"  having  a  stated  penetration.  For  example: 
"  The  oil  shall  contain  90  per  cent  of  'asphalt '  having  a  penetration 
of  80." 


286  BITUMINOUS   ROAD    MATERIALS  [CHAP.    VIII 

551.  SPECIFICATIONS  FOR  ROAD  OIL.     Road  oils  are  usually 
bought  without  any  specifications  as  to  their  composition  and  also 
without  inspection  or  analysis.     This  is  very  unfortunate,  since  the 
residue  may  be  paraffin,  which  is  a  lubricant  rather  than  a  binder; 
and  also  since  the  residue,  even  though  a  bitumen,  may  have  been 
burned  in  the  refining  process  until  it  possesses  little  or  no  binding 
qualities. 

Specifications  differ  considerably  according  to  the  purpose  for 
which  the  oil  is  to  be  used  or  the  conditions  under  which  it  is  applied ; 
and  practice  has  not  yet  established  a  standard  for  any  particular 
purpose  or  condition.  Below  are  the  specifications  of  several  grades 
of  road  oil  that  have  been  successfully  used. 

552.  For  Park  Drives.    The  following  are  the  specifications  of 
the  light  oil  used  as  a  dust  layer  (§  330),  in  Washington,  D.  C.,  on 
gravel  park  drives.* 

1.  The  oil  shall  be  a  viscous  fluid  product,  free  from  water,  and  showing 
some  degree  of  adhesiveness  when  rubbed  between  the  fingers. 

2.  It  shall  have  a  specific  gravity  of  not  less  than  0.940  at  25°  C. 

3.  It  shall  be  soluble  in  carbon  disulphide  at  air  temperature  to  at  least  99 
per  cent;  and  shall  not  contain  over  0.2  per  cent  of  insoluble  inorganic  matter. 

4.  It  shall  contain  not  less  than  3  per  cent  nor  more  than  10  per  cent  of 
bitumen  insoluble  in  86°  paraffin  naphtha  at  air  temperature. 

5.  When  240°  c.c.  of  the  oil  is  heated  in  an  Engler  viscosimeter  to  50°  C. 
and  maintained  at  this  temperature  for  at  least  3  minutes,  the  first  50  c.c.  shall 
flow  through  the  aperture  in  not  less  than  10  minutes  nor  more  than  20  minutes. 

6.  When  20  grams  of  the  material  is  heated  for  5  hours  hi  a  cylindrical  tin 
dish,  approximately  2|  inches  in  diameter  by  1  inch  high,  at  a  constant  tem- 
perature of  163°  C.,  the  loss  in  weight  by  volatilization  shall  not  exceed  20  per 
cent.     The  residue  should  be  decidedly  sticky. 

7.  Its  fixed  carbon  shall  not  be  less  than  3.5  per  cent. 

553.  For  Earth  Roads.    The  two  following  specifications  have 
been  adopted  by  the  Illinois  Highway  Department,!  and  have  prac- 
tically been  adopted  by  nearly  all  State  Highway  Departments  for 
the  surface  treatment  of  earth  roads.     The  oils  are  to  be  tested 
according  to  the  method  described  in  Bulletin  No.  314,  U.  S.  Depart- 
ment of  Agriculture,  December  10,  1915. 

554.  For  Loam  and  Clay.    The  following  light  oil  should  be  ap- 
plied cold,  for  a  first  application  on  loam  or  clay.     For  subsequent 
applications  use  the  oil  as  described  in  the  next  section. 

*  Paper  by  Col.  Spencer  Cosby,  U.  S.  Army,  in  charge  of  Buildings  and  Grounds,  Washing- 
ton, D.  C.,  presented  before  Section  D  of  the  American  Association  for  Advancement  of  Science, 
December  29,  1911. 

f  Private  letter  from  TV,  W,  Marr,  Chief  Highway  Engineer,  under  date  of  July  18,  1917, 


ART.  2]  PETROLEUM  287 

1.  The  oil  shall  be  homogeneous  and  free  from  water. 

2.  Specific  Gravity  25°  C.  (77°  F.) not  less  than  0.890 

3.  Bitumen  soluble  in  disulphide not  less  than  09.5% 

4.  Residue  of  100  penetration  * 40  to  60% 

5.  Specific  viscosity  at  40°  C.  (104°  F.) 10.0  to  25.0 

When  it  is  desired  to  apply  the  oil  hot,  oils  somewhat  more  viscous  and  having 
a  specific  viscosity  at  40°  C.  (104°  F.)  up  to  50  will  be  acceptable,  provided 
these  oils  conform  to  the  requirements  of  the  specifications  in  all  other  respects. 
An  oil  having  a  specific  viscosity  at  40°  C.  (104°  F.)  of  more  than  25.0  will  not  be 
accepted,  unless  it  is  to  be  applied  hot. 

555.  For  Sandy  Soil.     The  following  oil  should  be  applied  cold, 
and  is  suitable  for  a  dust  layer  on  a  sandy  earth  road,  for  a  second 
application  on  a  loam  or  clay  road,  and  also  for  a  surface  application 
on  a  water-bound  gravel  or  macadam  road. 

1.  The  oil  shall  be  homogeneous  and  free  from  water. 

2.  Specific  gravity  at  25°  C.  (77°  F.) not  less  than  0.910 

3.  Specific  viscosity  at  40°  C.  (104°  F.) 10.0  to  25.0 

4.  Loss  at  163°  C.  (325°  F.)  for  5  hours not  over  25% 

5.  Bitumen  soluble  in  disulphide not  less  than  99.5% 

6.  Bitumen  insoluble  in  86°  B.  naphtha not  less  than  5% 

7.  Fixed  carbon not  less  than  4.0% 

8.  Specific  viscosity  at  40°  C.  (104°  F.) 10.0  to  25.0 

When  it  is  desired  to  apply  the  oil  hot,  oils  somewhat  more  viscous  and 
having  a  specific  viscosity  of  40°  C.  (104°  F.)  up  to  50  will  be  acceptable,  pro- 
vided these  oils  conform  to  the  requirements  of  the  specifications  in  all  other 
respects.  An  oil  having  a  specific  viscosity  at  40°  C.  (104°  F.)  of  more  than  25.0 
will  not  be  accepted  unless  it  is  to  be  applied  hot. 

556.  For  Water-bound  Gravel  or  Macadam.    The  three  following 
specifications  have  been  adopted  by  the  Illinois  Highway  Department  f 
for  a  dust  layer  and  a  protective  coating  on  water-bound  gravel  or 
macadam  roads,  and  practical^  the  same  have  been  adopted  by 
most  of  the  State  Highway  Departments.     The  oils  are  to  be  tested 
according  to  the  methods  described  in  Bulletin  No.  314,  U.  S.  Depart- 
ment of  Agriculture,  December  10,  1915. 

The  light  oil  (§  557)  is  preferable  for  the  first  application  or  where 
the  road  is  somewhat  dusty,  since  it  penetrates  better  than  the 
heavier  oils;  but  for  subsequent  applications  a  heavier  oil  (§  558  or 
§  559)  is  preferable.  If  the  gravel  or  macadam  is  very  clean,  the 
heavier  oil  may  be  used  for  a  first  application. 

*  Am.  Soc.  Test.  Mat.  Standard  Test,  D-5-16. 

t  Private  letter  from  W.  W.  Marr,  Chief  Highway  Engineer,  under  date  of  July  18,  1917. 


288  BITUMINOUS  ROAD   MATERIALS  [CHAP.   VIII 

557.  Light  Oil.     The  following  oil  is  to  be  applied  cold. 

1.  The  oil  shall  be  homogeneous  and  free  from  water. 

2.  Specific  gravity  25°  C.  (77°  F.) 0.920  to  0.970 

3.  Loss  at  163°  C.  (325°  F.)  for  5  hours 20.0%  to  30.0% 

4.  Bitumen  soluble  in  disulphide not  less  than  99.5% 

5.  Bitumen  insoluble  in  86°  B.  naphtha 5.0  to  20.0% 

6.  Fixed  carbon 4.0%  to  10.0% 

7.  Specific  viscosity  at  25°  C.  (77°  F.) 30.0  to  70.0 

558.  Medium  Oil.     The  following  oil  need  not  be  applied  hot 
except  when  the  temperature  of  the  air  is  below  80°  F. 

1.  The  oil  shall  be  homogeneous,  free  from  water,  and  shall  not  foam  when 

heated  to  100°  C.  (212°  F.). 

2.  Specific  gravity  25°  C.  (77°  F 0.960  to  1.010 

3.  Flash  point not  less  than  100°  C.  (212°  F.) 

4.  Float  test  at  32°  C.  (90°  F.) 30  to  90  seconds 

5.  Loss  at  163°  C.  (325°  F.)  for  5  hours not  over  15.0% 

6.  Float  test  of  residue  at  50°  C.  (122°  F.) 90  to  180  seconds 

7.  Bitumen  soluble  in  disulphide not  less  than  99.5% 

8.  Bitumen  insoluble  in  86°  B.  naphtha 7.0  to  20.0% 

9.  Fixed  carbon 5.0%  to  10.0% 

10.  Specific  viscosity  at  100°  C.  (212°  F.) 5.0  to  15.0 

559.  Heavy  Oil.     The  following  oil  should  be  applied  hot. 

1.  The  oil  shall  be  homogeneous,  free  from  water,  and  shall  not  foam  when 

heated  to  150°  C.  (302°  F.) 

2.  Specific  gravity  25°  C.  (77°  F.) not  less  than  0.980 

3.  Flash  point not  less  than  150°  C.  (302°  F.) 

4.  Float  test  at  50°  C.  (122°  F.) ..100  to  200  seconds 

5.  Loss  at  163°  C.  (325°  F.)  for  5  hours, not  over  5.0% 

6.  Float  test  of  residue  at  50°  C.  (122°  F.) 120  to  240  seconds 

7.  Bitumen  soluble  in  disulphide not  less  than  99.5% 

8.  Bitumen  insoluble  in  86°  B.  naphtha 10.0  to  25.0% 

9.  Fixed  carbon ' 7.0%  to  15.0% 

10.  Specific  viscosity  at  100°  C.  (212°  F.) 30.0  to  70.0 

560.  COST.    The  price  of  road  oils  varies  greatly,  partly  because 
of  the  natural  variation  with  locality,  but  chiefly  because  a  large 
proportion  of  road  oils  is  sold  without  specifications  or  inspection. 
The  demand  for  oil  for  road  purposes  has  increased  so  rapidly  in 
recent  years  that  the  price  has  advanced  more  rapidly  than  most 
construction  materials. 

561.  The  following  are  the  market  quotations  in  Engineering 
News-Record,  July  5,  1917.     The  prices  are  for  road  oil  in  tank  cars 
(8000  gallons  minimum  capacity)  f.o.b.  places  named. 


ART.  3]  TAR  289 

New  York  City,  40-50  per  cent  asphalt 6|  cts.  per  gal. 

60-70  per  cent  asphalt 7    cts.  per  gal. 

dust  layer 71  cts.  per  gal. 

binder 8|  cts.  per  gal. 

St.  Louis,  asphalt 5  cts.  per  gal. 

Dallas,  40-50  per  cent  asphalt 6  cts.  per  gal. 

60-70  per  cent  asphalt 7£  cts.  per  gal. 

San  Francisco,  75-79  per  cent  asphalt  (barrel  =  42  gal.) $1.83  per  bbl. 

ART.  3.     TAR 

563.  The  tar  used  in  road  work  is  obtained  as  a  by-product  in 
the  destructive  distillation  of  bituminous  coal  in  the  manufacture 
of  illuminating  gas  or  in  the  production  of  coke,  as  well  as  in  the 
decomposition  of  petroleum. 

564.  DEFINITIONS.     Coal  Tar.    Tar  produced  from  the  destruc- 
tive distillation  of  coal. 

Coke-oven  Tar.     A  by-product  in  the  manufacture  of  coke. 

Gas-house  Tar.  A  by-product  in  the  manufacture  of  illuminating 
gas  from  coal. 

Oil-gas  Tar.  A  by-product  in  the  manufacture  of  illuminating 
gas  from  petroleum. 

Pitch.  The  solid  residue  produced  by  the  evaporation  or  dis- 
tillation of  tar. 

Refined  Tar.  A  tar  freed  from  water  by  evaporation  or  dis- 
tillation, which  process  is  continued  until  the  tar  is  of  the  desired 
consistency.  When  all  the  water  is  driven  off,  it  is  called  Dehydrated 
Tar.  Refined  tar  is  also  produced  by  fluxing  the  tar  residuum  with  a 
tar  distillate,  in  which  case  the  product  is  called  Cut-back  Tar. 

Water-gas  Tar.  A  by-product  in  the  manufacture  of  carbureted 
water-gas  from  petroleum. 

565.  CHARACTERISTICS  OF  TAR.    Most  of  the  tar  used  in  road 
work  is  coal  tar,  either  coke-oven  or  gas-house  tar.     In  some  partic- 
ulars the  characteristics  of  tars  overlap;  but  the  following  table  shows 
their  chief  differences: 


CHARACTERISTICS. 
Water,  per  cent 

KIND  OF  TAR: 
COKE-OVEN.       GAS-HOUSE. 
22                2.9 

Light  oil  up  to  200°  C 

34                40 

Creosote  oil,  per  cent  

14.5                8.6 

Naphthalene,  crude,  per  cent 

67                74 

Anthracene  crude  per  cent 

27  3              17  4 

Pitch,  per  cent.    ...        .... 

.  .,  .       44  3              58.4 

Free  carbon,  ner  cent  .  . 

5-8            15-25 

290  BITUMINOUS   ROAD   MATERIALS  [CHAP.    VIII 

The  quality  of  gas-house  tar  depends  upon  the  temperature  at 
which  the  distillation  takes  place.  The  distillation  usually  takes 
place  at  a  high  temperature;  and  consequently  the  tar  contains  less 
of  the  heavy  oils  and  more  of  the  solid  bitumen  and  more  free  carbon, 
and  is  not  desirable  for  road  work,  because  of  an  excess  of  free  carbon 
and  of  the  lack  of  the  heavy  oils. 

Coke-oven  tar  is  usually  formed  at  a  lower  temperature,  and 
hence  contains  more  of  the  heavy  oils  and  less  free  carbon;  and  is 
therefore  usually  more  suitable  for  road  work  than  gas-house  tar. 

Water-gas  tar  is  lighter  than  coal  tar,  contains  a  larger  percentage 
of  heavy  oils,  and  a  less  percentage  of  pitch.  It  is  usually  low  in  free 
carbon,  and  does  not  contain  ammonia.  Since  water-gas  tars  con- 
tain comparatively  small  proportions  of  pitch,  they  are  not  as  suitable 
for  a  road  binder  as  coal-gas  or  coke-oven  tars;  but  since  they  con- 
tain a  larger  percentage  of  the  heavier  oils,  they  are  desirable  materials 
for  use  as  dust  layers. 

566.  Crude  tar  is  refined  by  driving  off  the  lighter  oils.    The 
residue  may  be  liquid  or  solid  according  to  the  temperature  to  which 
the  distillation  was  carried  and  the  extent  to  which  the  heavy  oils 
have  been  removed.     Sometimes  the  distillation  is  carried  only  far 
enough  to  drive  off  the  water  and  the  lighter  oils.     Such  a  product  is 
known  as  dehydrated  tar;    and  it  is  more  suitable  for  road  work 
than  crude  tar,  since  it  contains  no  water  or  ammonia. 

567.  SHIPPING  TAR.    Tar  is  shipped  in  barrels  or  metal  drums 
or  in  tank  cars;  and  is  unloaded  and  distributed  the  same  as  asphalt 
and  oil — see  §  504  and  §  549. 

568.  SPECIFICATIONS  FOR  TAR.     Practice  has  not  established 
standard  specifications;  and  consequently  there  are  a  great  number 
in  use,  which  differ  according  to  the  source  or  character  of  the  tar 
and  also  according  to  the  opinion  of  the  one  writing  the  specifica- 
tions.    Only  an  expert  road  engineer  and  chemist  should  attempt 
to  prepare  specifications;   and  then  great  care  is  necessary,  since  a 
limitation  in  one  particular  may  affect  the  limits  of  some  other 
factor.     The  producers  of  bituminous  materials  make  a  variety  of 
grades  of  material,  which  are  sold  under  different  trade  names 
(see  §  578). 

Below  are  the  specifications  for  materials  that  have  been  suc- 
cessfully used  for  different  kinds  of  work  by  good  authorities. 

569.  For  Bituminous  Surfaces.    The  two  following  specifications 
have  been  adopted  for  refined  tar  for  bituminous  surfaces  on  water- 
bound  gravel  or  macadam  roads  (Chapter  IX),  and  on  bituminous 


ART.  3]  TAR  291 

bound  roads  (Chapter  X)  by  the  Illinois  Highway  Department,* 
and  are  practically  the  same  as  those  adopted  by  most  State  Highway 
Departments.  The  tests  are  to  be  made  as  described  in  Bulle- 
tin 314,  U.  S.  Department  of  Agriculture,  December  10,  1915. 

For  the  first  treatment  of  a  road  the  light  tar  of  §  570  is  to  be  pre- 
ferred; and  for  subsequent  application  the  heavier  tar  of  §  571  is 
better. 

570.  Hot   Application.     The    following  tar    should    be    applied 
hot. 

1 .  The  tar  shall  be  homogeneous  and  free  from  water. 

2.  Specific  gravity  25°  C.  (77°  F.) 1.120  to  1.200 

3.  Specific  viscosity  at  40°  C.  (104°  F.) 4.0  to  12.0 

4.  Total  distillate  by  weight  :f 

to  170°  C.  (338°  F.) not  over    5.0% 

to  300°  C.  (572°  F.) not  over  35.0% 

5.  Specific  gravity  of  total  distillate  25°  C.  (77°  F.) not  less  than  1.010 

6.  Melting  point  of  residue not  over  65°  C.  (149°  F.) 

7.  Bitumen  soluble  in  disulphide 88.0  to  96.0% 

8.  Inorganic  matter  (ash) not  over  0.5% 

571.  Cold  Application.     The    following   tar   should   be   applied 
cold. 

1.  The  tar  shall  be  homogeneous  and  free  from  water. 

2.  Specific  gravity  25°  C.  (77°  F.) 1.180  to  1.250 

3.  Float  test  32°  C.  (90°  F.) 90  to  150  seconds 

4.  Total  distillate  by  weight:f   to  180°  C.  (338°  F.) not  over  1.0% 

to  300°  C.  (572°  F.) not  over  25.0% 

5.  Specific  gravity  of  total  distillate,  25°  C.  (77°  F.) not  less  than  1.030 

6.  Melting  point  of  residue not  over  75°  C.  (167°  F.) 

7.  Bitumen  soluble  in  disulphide 78.0  to  88.0% 

8.  Inorganic  matter  (ash) not  over  0.5% 

572.  For    Bituminous    Macadam.     The    American    Society    of 
Municipal  Improvements  on  October  12,   1916,  adopted  standard 
specifications  for  two  grades  of  tar  for  bituminous  macadam  roads 
(Art.  1,  Chapter  X),  which  materials  are  optional  with  each  other 
and  also  with  any  of  the  four  kinds  of  asphalt  described  in  §  537-38 
and  Table  31,  page  278.     The  specifications  in  full  for  water-gas  tar 
are  given  in  §  573;    and  Table  34  shows  the  essential  features  of 
both  tars. 


*  Private  letter  from  W.  W.  Marr,  Chief  Highway  Engineer,  under  date  of  July  18,  1917, 
t  Amer.  Soc,  Test,  Mat.  Standard  Test,  D-20-16. 


292 


BITUMINOUS   ROAD    MATERIALS 


[CHAP,  viii 


573.  Water-gas  Tar.*     1.  Foam:    Refined  water-gas  tar  shall  be  homogene- 
ous, free  from  water,  and  shall  not  foam  when  heated  to  121°  C.  (250°  F.). 

2.  Specific  Gravity:  The  specific  gravity  at  a  temperature  of  25°  C.  (77°  F.) 
shall  be  not  less  than  1.150  nor  more  than  1.200. 

3.  Viscosity:    When  tested  by  means  of  the  New  York  Testing  Laboratory 
Float  Apparatus,  the  float  shall  not  sink  in  water  maintained  at  50°  C   (122°  F.) 
in  less  than  120  nor  more  than  150  seconds. 

4.  Bitumen  Soluble  in  Bisulphide:   The  bitumen  as  determined  by  its  solu- 
bility in  chemically  pure  carbon  disulphide  at  room  temperature,  shall  be  not  less 
than  95.0  per  cent;  and  the  material  insoluble  in  carbon  disulphide  shall  not  show 
more  than  0.2  per  cent  ash  upon  ignition. 

5.  Distillation:    When  distilled  according  to  the  tentative  method  recom- 
mended by  Committee  D-4  of  the  American  Society  for  Testing  Materials  in 
1911,   it  shall  yield  not  more  than  0.5  per  cent  distillate  at  a  temperature 
lower  than  170°  C.  (338°  F.);  not  more  than  12.0  per  cent  shall  distill  below 
270°  C.  (518°  F.);  and  not  more  than  25.0  per  cent  shall  distill  below  300°  C. 
(572°  F.). 

6.  Distillate,  specific  gravity  of:    The  total  distillate  from  the  test  made  in 
accordance  with  paragraph  5  shall  have  a  specific  gravity  at  a  temperature  of 
25°  C.  (77°  F.)  of  not  less  than  0.980  nor  more  than  1.020. 

7.  Distillate,  melting  point  of:    The  melting  point,  as  determined  in  water 
by  the  cube  method,  of  the  pitch  residue  remaining  after  distillation  to  300°  C. 
(572°  F.)  in  accordance  with  the  test  described  in  paragraph  5,  shall  be  not  more 
than  75°  C.  (167°  F.) 

TABLE  34 
COMPARISON  OF  SPECIFICATIONS  FOR  TARS  FOR  BITUMINOUS  MACADAM 

Standards  of  American  Society  of  Municipal  Improvements,  Adopted  October  12,  1916 


Ref. 
No. 

Items. 

Water-gas  Tar. 

Coal  Tar. 

1 

Shall  not  foam  at  

121°  C. 

121°  C 

?, 

Specific  gravity  at  25°  C  

1  150-1  200 

1  180-1  300 

3 

Viscosity  by  N.  Y.  float  apparatus  

120-150  sec. 

150-180  sec 

4 
5 

6 

7 

Bitumen  soluble  in  disulphide,  not  less  than  .  . 
Distillation,  yield  to  170°  C.,  not  more  than. 
Distillation,  yield  to  270°  C.,  not  more  than. 
Distillation,  yield  to  300°  C.,  not  more  than.  . 
Distillate,  total,  specific  gravity  of  
Residue,  melting  point  of,  not  more  than  .... 

95.0% 
0.5% 
12.0% 
25.0% 
0.98-1.020 
75°  C. 

80.0-95.0 
0.5% 
10.0% 
20.0% 
1.020 
75°  C. 

574.  For  Bituminous  Concrete.  The  American  Society  of 
Municipal  Improvements  on  October  12,  1916,  adopted  standard 
specifications  for  two  grades  of  tar  suitable  for  the  binder  of  bitumi- 
nous concrete  (see  Art.  2  of  Chapter  X),  which  materials  are  optional 
to  each  other  and  also  with  any  one  of  the  five  grades  of  asphalt 


*  Specifications  for  Broken  Roads  with  Bituminous  Surface,  adopted  by  American  Society 
of  Municipal  Improvements,  October  12,  1916,  p.  24-25, 


ART.  3] 


TAR 


293 


described  in  §  540,  and  Table  32,  page  280.  The  specification  in 
full  for  one  of  the  tars  is  given  in  §  575;  and  the  essential  features  of 
both  are  shown  in  Table  35. 

575.  Water-gas  Tar.     1.  Foam:  The  refined  tar  shall  be  homogeneous,  free 
from  water,  and  shall  not  foam  when  heated  to  150°  C.  (302°  F.). 

2.  Specific  Gravity:   Its  specific  gravity  at  a  temperature  of  25°  C.  (77°  F.) 
shall  not  be  less  than  1.160  nor  more  than  1.200. 

3.  Viscosity:    When  tested  by  means  of  the  New  York  Testing  Laboratory 
Float  Apparatus,  the  float  shall  not  sink  in  water  maintained  at. 50°  C.  (122°  F.) 
in  less  than  140  seconds  nor  more  than  170  seconds. 

4.  Bitumen  Soluble  in  Disulphide :    The  bitumen  as  determined  by  its  solu- 
bility in  chemically  pure  carbon  disulphide  at  room  temperature  shall  be  not  less 
than  95.0  per  cent;   and  the  material  insoluble  in  carbon  disulphide  shall  show 
nor  more  than  0.2  per  cent  ash  upon  ignition. 

5.  Distillation:    When  'distilled  according  to  the  tentative  method  recom- 
mended by  Committee  D-4  of  the  American  Society  for  Testing  Materials  in 
1911,  it  shall  yield  no  distillate  at  a  temperature  lower  than  170°  C    (338°  F.); 
not  more  than  7.0  per  cent  by  weight  shall  distill  below  270°  C.  (518°  F.);  and 
not  more  than  20.0  per  cent  by  weight  shall  distill  below  300°  C.  (572°  F.). 

6.  Distillate,  specific  gravity  of:    The  total  distillate  from  the  test  made  in 
accordance  with  paragraph  5  shall  have  a  specific  gravity  at  a  temperature  of 
25°  C.  (77°  F.)  of  not  less  than  1.000  nor  more  than  1.020. 

7.  Distillate,  melting  point  of:  The  melting  point,  as  determined  in  water  by 
the  cube  method,  of  the  pitch  residue  remaining  after  distillation  to  300°  C. 
(572°  F.),  in  accordance  with  the  test  described  m  paragraph  5,  shall  be  not 
more  than  75°  C.  (167°  F.). 

TABLE  35 

COMPARISON  OF  SPECIFICATIONS  FOR  TARS  FOR  BITUMINOUS  CONCRETE 

Standards  of  American  Society  of  Municipal  Improvements,  October  14,  1916 


Ref. 
No. 

Items. 

Water-gas  Tar. 

Coal  Tar. 

1 

2 
3 

4 
5 

6 

7 

Shall  not  foam  at  

150°  C. 
1.160-1.200 
140-170  sec. 
95.0% 
0.0 
7.0% 
20.0% 
1.00-1.020 

75°  C. 

150°  C. 
1.200-1.300 
140-170  sec. 
75.0-90.0% 
0.0 
10.0% 
20.0% 
1.030 

75°  C. 

Specific  gravity  at  25°  C  

Viscosity  by  N.  Y.  float  apparatus  

Bitumen  soluble  in  disulphide,  not  less  than.  . 
Distillation,  yield  to  170°  C.,  not  more  than.  . 
270°  C.,  not  more  than.. 
300  °C.,  not  more  than.. 
Distillate  total,  specific  gravity  of. 

Distillate,  residue,  melting  point  of,  not  more 
than             

576.  For  Joint  Filler  of  Block  Pavements.*    The  following  are  the 
specifications  for  a  tar  suitable  for  the  joint  filler  of  brick,  stone- 


*  P.  P.  Sharpies,  Manager  and  Chief  Chemist,  Tarvia  Department,  Barrett  Manufacturing 
Co.,  forwarded  for  this  use  under  date  of  July  13,  1917. 


294  BITUMINOUS   ROAD   MATERIALS  [CHAP.   VIII 

block,  or  wood-block  pavements.  The  specifications  may  need  a 
slight  variation  for  the  extremes  of  northern  or  southern  portions  of 
this  country. 

677.  1.  Pitch.  The  pitch  shall  be  straight-run  residue  from  the  distillation  of 
coal  tar. 

2.  Specific  Gravity:    The  specific  gravity  at  78°  F.  shall  not  be  less  than  1.24 
nor  more  than  1.32. 

3.  Melting  Point:    The  melting  point  shall  not  be  lower  than  115°  F.  nor 
higher  than  150°  F.    For  mastic  filler  the  melting  point  shall  be  115  to  135°  F.; 
for  brick  and  stone-block  125  to  140°  F.;  and  for  wood-block,  140  to  150°  F. 
The  contractor  before  beginning  work  on  any  contract  shall  obtain  from  the 
Chief  Engineer  in  writing  a  statement  as  to  the 'melting  point  desired  for  that 
particular  contract,  and  a  variation  of  5°  F.  either  way  from  this  value  will  be 
permitted;  but  the  melting  point  must  be  within  the  limits  indicated  above. 

The  melting  point  should  be  higher,  the  steeper  the  grade.  For  grades 
above  10  per  cent,  in  a  warm  climate,  the  melting  point  should  be  140°  to 
150°  F. 

4.  Free  Carbon:  The  free  carbon  shall  not  be  less  than  22  per  cent  nor  more 
than  37  per  cent. 

5.  Distillation:   The  specific  gravity  of  the  distillate  to  670°  F.  shall  be  not 
less  than  1.07  at  140°  F.  compared  with  water  at  the  same  temperature. 

578.  Trade  Names.    A  trade  name  is  in  a  sense  a  specification 
for  a  material;    and  hence  the  following  definitions  of  well-known 
trade  names  for  tar  products  are  appropriate  here : 

Tarvia  A.  A  refined  coal  tar  for  hot  surface  application  to  macadam  roads 
for  preserving  them  and  laying  dust.  Tarvia  A  in  distinction  from  Tarvia  B 
forms  a  perceptible  blanket  on  the  surface;  and  is  therefore  limited  for  successful 
use  to  roads  receiving  either  wholly  automobile  traffic  or  a  high  percentage  of 
such  traffic.  It  has  been  largely  used  in  park  work  in  the  neighborhood  of  large 
cities. 

Tarvia  B.  A  refined  coal  tar  for  cold  surface  application  as  a  dust  layer 
and  road  preservative.  Primarily  for  use  on  macadam  roads,  but  also  applicable 
to  gravel  and  other  hard-surfaced  roads. 

Tarvia  KP.  A  refined  coal-tar  binder  cut  back  to  permit  its  use  cold  in 
making  patches  and  in  other  maintenance  work  on  bituminous  surfaced  and 
bituminous  bound  roads. 

Tarvia  MF.  A  refined  coal  tar  prepared  for  use  as  a  mastic  with  sand  in 
filling  the  joints  of  brick,  stone-block,  and  lug  wood-block  pavements. 

Tarvia  X.  A  refined  coal  tar  prepared  for  use  as  a  binder  for  bituminous 
macadam  roads.  Modifications  are  made  to  permit  its  use  in  bituminous  con- 
crete. 

Tarvia  XC.  A  Tarvia  X  prepared  for  use  in  patching  and  maintaining  the 
joints  in  concrete  roads. 

579.  COST  OF  ROAD   TAR.     Cost  data  are  always  difficult  to 
handle  in  printed  matter,  since  the  record  is  liable  to  be  out  of  date 


ART.  3]  TAR  295 

before  it  is  presented  to  the  public;  and  this  seems  to  be  specially 
true  of  tar,  particularly  at  the  time  this  paragraph  is  written. 

The  cost  of  road  tars  meeting  the  preceding  specification,  in 
the  Middle  and  Eastern  States  where  the  conditions  are  more  uniform 
than  in  other  parts  of  the  country,  range  from  8  to  13  cents  per  gallon 
f.o.b.  siding  at  destination.  The  lighter  materials  suitable  for  cold 
application  cost  1  or  2  cents  per  gallon  less  than  those  applied  hot. 

580.  For  more  recent  data  consult  the  price  reports  in  the  current 
technical  journals. 


CHAPTER  IX 
BITUMINOUS  SURFACES  FOR  ROADS 

582.  Before  the  advent  of  motor-driven  vehicles  gravel  and  mac- 
adam roads  gave  good  service;    but  the  coming  of  the  automobile 
introduced  new  conditions  that  made  necessary  a  radical  change  in 
the  construction  of  a  gravel  or  macadam  road  having  any  considerable 
proportion   of   motor-driven    traffic.     The    low-hung    swift-moving 
automobile,  more  than  horse-drawn  vehicles,  throws  the  stone  dust 
into  the  air  and  thus  permits  it  to  be  blown  away,  and  besides  the 
rubber  tires,  unlike  steel  tires  and  horse  shoes,  do  not  make  any 
stone  dust  to  replace  that  blown  away.     Therefore  gravel  and  mac- 
adam roads  rapidly  deteriorate  under  any  considerable  motor-driven 
traffic.     This  state  of  affairs  led  to  the  introduction,  in  substantially 
the  past  ten  years,  of  several  new  forms  of  road  construction  in 
which  the  binding  power  of  clay  or  stone  dust  is  replaced  by  that  of  a 
bituminous  material  like  tar. 

There  are  two  general  types  of  such  construction,  viz.:  one  in 
which  a  superficial  coating  of  bituminous  material  is  laid  upon  a 
gravel,  macadam  or  concrete  road,  or  even  upon  a  brick  or  stone- 
block  pavement;  and  the  other  in  which  the  bituminous  material  is 
employed  as  a  binder  for  the  upper  stratum  of  the  road.  The  super- 
ficial layer  is  called  a  Protective  Coating  or  a  Bituminous  Carpet, 
according  to  its  thickness  and  construction.  This  type  of  con- 
struction will  be  considered  in  this  chapter. 

When  the  second  form  of  construction  is  employed  the  road  is 
known  as  either  a  Bituminous-Macadam  or  a  Bituminous-Concrete 
Road,  according  to  the  details  of  the  construction,  which  types  of 
construction  will  be  considered  in  the  next  chapter. 

583.  KINDS  OF  BITUMINOUS  SURFACES.    The  bituminous  sur- 
face may  consist  either  of  a  thin  bituminous  film  or  of  a  compara- 
tively thick   mat    composed  of   successive   layers  of   bituminous 

296 


ART.    1]  PROTECTIVE   COATING  297 

material  and  screenings,  sand  or  gravel.    The  former  is  usually  called 
a  Protective  Coating,  and  the  latter  a  Bituminous  Carpet. 

ART.  1.    PROTECTIVE  COATING 

584.  A  light  oil  is  sometimes  applied  to  an  earth  road,  a  gravel 
road,  or  a  water-bound  macadam  road  to  lay  the  dust.  It  is  not 
expected  that  the  oil  will  have  any  binding  power;  and  frequent 
applications  are  necessary  for  effectiveness.  But  when  a  water- 
bound  gravel  or  macadam  road  is  required  to  carry  only  a  small 
proportion  of  motor-driven  traffic,  it  is  sometimes  possible  to  pro- 
tect the  surface  with  a  thin  bituminous  coating  which  will  resist 
the  action  of  both  the  horse-drawn  and  motor-driven  traffic,  and  thus 
prolong  the  life  of  the  road  surface. 

685.  THE  BITUMINOUS  MATERIAL.  The  bituminous  material 
should  be  fluid  at  ordinary  temperatures  in  order  that  it  may  be 
applied  cold  and  spread  uniformly.  It  should  contain  a  small  amount 
of  volatile  oils  which  will  evaporate  and  leave  a  cementitious  film 
on  the  surface.  A  light  refined  tar  which  is  fluid  at  ordinary  tem- 
peratures (§  571-72)  is  generally  used.  An  asphaltic  oil  containing 
from  40  to  50  per  cent  of  asphalt  gives  fair  results.  Oils  are  cheap 
and  readily  applied ;  but  are  not  entirely  satisfactory  for  bituminous 
coatings  for  the  following  reasons:  1.  Most  petroleum  products,  even 
those  having  an  asphaltic  base,  while  in  a  fluid  state  act  to  a  certain 
extent  as  a  lubricant.  2.  Both  medium  and  heavy  asphaltic  oils 
require  considerable  time  to  set  up;  and  therefore,  if  the  road  is 
opened  to  travel  before  the  oil  has  set,  more  or  less  movement  of  the 
coating  will  take  place,  and  it  will  become  wavy  and  full  of  bumps. 

The  amount  of  tar  or  asphaltic  oil  should  rarely  exceed  0.2  of  a 
gallon  per  square  yard,  and  an  excessive  amount  is  specially  to  be 
avoided. 

586.  The  field  for  this  form  of  surface  is  comparatively  limited, 
and  the  effect  of  such  a  coating  is  only  temporary;  but  this  treatment 
is  often  a  valuable  means  of  carrying  an  old  gravel  or  macadam  road 
along  until  a  better  form  of  treatment  can  be  given.  It  is  more 
expensive  and  more  permanent  than  an  oil  dust-layer  (§  329-31); 
but  is  cheaper  and  less  permanent  than  a  bituminous  carpet  (§  588). 


298  BITUMINOUS   SURFACES    FOR   ROADS  [CHAP.    IX 


ART.  2.     BITUMINOUS  CARPET 

688.  When  the  proportion  of  motor-driven  traffic  on  a  water- 
bound  gravel  or  macadam  road  becomes  considerable  (see  Table  26, 
page  177),  it  is  more  economical  to  protect  the  road  surface  with  a 
bituminous  carpet  or  blanket  than  continually  to  add  screenings  or 
gravel  to  supply  binding  material.  In  some  cases  the  mat  or  carpet 
is  added  to  prevent  the  road  from  being  denuded  of  binder,  and  in 
other  cases  the  carpet  protects  the  stone  itself  from  excessive  wear, 
which  is  particularly  important  on  a  road  built  of  soft  limestone. 
The  bituminous  carpet  not  only  protects  the  road  but  eliminates 
practically  all  dust. 

In  consideration  of  the  large  mileage  of  water-bound  gravel  and 
macadam  roads  built  before  the  advent  of  the  automobile,  this 
method  of  treating  such  roads  is  very  important.  Under  some  con- 
ditions it  is  still  economical  to  build  new  water-bound  gravel  and 
macadam  roads,  and  cover  them  with  a  bituminous  carpet;  although 
owing  to  the  difficulties  of  maintaining  a  bituminous  carpet,  it  is 
usually  wiser  to  build  a  bituminous-macadam  or  a  bituminous- 
concrete  road. 

589.  THE  BITUMINOUS  MATERIAL.  Either  refined  tar  (§  568) 
or  asphaltic  oil  (§  551)  may  be  used.  The  particular  grade  of  tar 
or  oil  to  be  used  depends  upon  the  condition  of  the  road  and  the 
amount  and  character  of  the  travel.  If  the  road  has  begun  to  ravel 
and  most  of  the  stones  have  been  swept  bare  of  binding  material,  a 
refined  tar  like  that  in  §  571,  or  a  heavy  oil  like  that  in  §  559  should 
be  used.  If  some  bonding  material  remains  on  the  road  surface  and 
the  large  stones  are  not  entirely  exposed,  a  medium  oil  like  that  in 
§  558  would  be  better.  If  the  surface  is  tightly  bound  and  hard  to 
sweep  free  from  dust  and  fine  material,  a  tar  product  like  that  of 
§  570  or  a  light  oil  like  that  of  §  557  should  be  selected. 

690.  CLEANING  ROAD  SURFACE.  The  road  should  be  swept 
with  a  revolving  power-broom  and  then  with  a  hand-broom  until 
the  surface  is  entirely  free  from  dust  and  fine  particles.  The  bitumi- 
nous material  adheres  better  if  the  road  is  sprinkled  before  treatment, 
but  it  should  be  allowed  to  dry  before  the  bituminous  material  is 
applied.  Unquestionably  water  on  the  road  when  the  bituminous 
material  is  applied  is  harmful ;  but  the  sprinkling  washes  off  the  dust 
and  therefore  is  beneficial,  provided  the  road  is  dry  when  the  bitumi- 
nous material  is  applied. 


ART.    2] 


BITUMINOUS    CARPET 


299 


Fig.  94  shows  the  method  of  cleaning  an  old  macadam  road  pre- 
paratory to  applying  the  bituminous  surface. 


FIG.  94. — SWEEPING  AN  OLD  MACADAM  ROAD  BEFOKE  APPLYING  THE  BITUMINOUS  SURFACE. 

591.  APPLYING  BITUMINOUS  MATERIAL.  The  binder  may  be 
applied  either  by  hand  or  by  machine.  In  the  hand  method,  ordi- 
nary garden  watering  pots  or  special  pouring  cans  are  used,  being  filled 
from  a  large  supply  tank  that  is  driven  along  beside  the  work.  It  is 
very  difficult  to  apply  the  bituminous  material  evenly  with  a  hand 
pouring  car ;  ar?d  it  is  necessary  immediately  to  follow  the  applica- 
tion with  a  brush  broom  and  sweep  the  surplus  oil  ahead.  This 
method  of  applying  the  material  is  very  slow  and  expensive,  and  is 
now  seldom  used  except  for  small  jobs  and  for  patch  work. 

There  are  many  different  types  of  machines  for  distributing  the 
bituminous  material,  but  in  outward  appearance  they  do  not  differ 
greatly  from  the  oil  distributor  shown  in  Fig.  39  and  40,  page  137. 

There  are  a  number  of  hand-drawn  cart  gravity-distributors. 
Some  horse-drawn  distributors  have  gravity  feed ;  but  the  mechanical 
feed  or  pressure  distributor  is  the  more  common,  since  it  secures 
a  more  uniform  distribution  and  permits  more  accurate  regulation  of 
the  amount  applied.  Some  of  the  distributors  have  their  own 
heating  device;  but  some  are  made  for  spreading  cold  oil,  or  depend 
upon  an  auxiliary  heater.  Some  distributors  deliver  the  bituminous 
material  in  small  streams,  and  others  in  a  fan-like  sheet,  while 
still  others  deliver  it  in  a  fine  spray.  It  is  claimed  that  the  spray 
applies  the  material  more  uniformly,  and  that  it  strikes  the  surface 
of  the  road  with  enough  force  to  penetrate  all  interstices  and  to  blow 


300  BITUMINOUS   SURFACES  FOR  ROADS  [CHAP.   IX 

away  all  dust,  and  thus  secures  a  good  union  with  the  stone  of  the 
road. 

Fig.  95  shows  a  distributing  tank  which  is  drawn  with  horses  or 
behind  a  road  roller.  The  special  features  of  this  distributor  are: 
(1)  the  bituminous  material  is  heated  by  steam  from  a  road 
roller,  and  hence  can  not  be  burned;  (2)  the  material  is  dis- 
charged by  air  pressure  in  the  tank,  and  hence  there  are  no  pumps 
to  become  clogged;  and  (3)  the  first  cost  of  the  machine  is  low. 


FIG.  95. — HORSE-DRAWN  PRESSURE  DISTRIBUTOR. 

The  same  features  are  embodied  in  a  self-contained  automobile- 
truck  distributor,  which  permits  work  at  a  greater  distance  from  the 
central  heating  plant. 

Fig.  96  shows  a  motor-driven  distributor.  The  distributing 
head  is  in  three  sections,  either  outside  one  of  which  may  be 
turned  up  and  put  out  of  use.  With  both  arms  in  use  the  total 
spread  is  16  feet.  Fig.  97  shows  a  spray  in  use  making  a  patch. 
Views  1  and  5,  Fig.  100,  page  311,  show  a  distributor  spraying 
tar. 

592.  The  bituminous  material  may  be  delivered  in  railroad 
tank-cars  or  in  barrels  or  metal  drums.  The  first  method  is  objection- 
able owing  to  the  difficulty  of  having  enough  road  surface  ready 
to  receive  8,000  to  10,000  gallons  of  tar.  When  the  binder  is  delivered 
in  barrels  or  drums,  it  is  heated  in  a  large  kettle  while  the  tank  wagon 
is  distributing  a  load. 


ART.  2] 


BITUMINOUS   CARPET 


301 


The  heating  should  be  done  so  as  to  heat  evenly  the  entire  mass, 
and  the  heating  should  be  under  positive  control  at  all  times.  The 
tar  should  be  heated  to  93°  C.  (200°  F.)  and  not  above  121°  C. 


FIG.  96. — AUTOMOBILE  PRESSURE  DISTRIBUTOR. 

(250°  F.).     Any  material  heated  beyond  121°  C.  should  not  be  used. 
The  distributing  wagon  should  be  supplied  with  one  or  more  ther- 


FIG.  97. — SPRAYING  DISTRIBUTOR  MAKING  A  PATCH. 

mometers  to  insure  that  the  temperature  of  the  tar  when  applied  is 
between  the  above  limits.  It  is  unwise  to  apply  tar  when  the  air 
or  road  is  below  50°  F.  (10°  C.). 

593.  The  bituminous  material  is  applied  at  the  rate  of  J  to  J 
gallon  per  square  yard,  in  either  one  or  two  treatments.  The  carpet 
should  be  uniform  in  thickness,  as  otherwise  the  thin  places  will  cut 


302  BITUMINOUS   SURFACES   FOR   ROADS  [CHAP.    IX 

through,  and  the  thick  portions  will  bunch  if  soft,  and  crack  if  a 
hard  material  is  used.  The  thinner  the  carpet  that  can  carry  the 
traffic  the  better. 

594.  After  the  bituminous  material  has  had  a  few  hours  to  pene- 
trate the  surface  of  the  road,  and  after  it  has  set  up  a  little,  stone 
screenings  or  pea  gravel  is  added  at  the  rate  of  about  1  cubic  yard 
to  every  100  to  150  square  yards  of  road  surface.     The  size  of  the 
screenings  or  pea  gravel  should  be  such  as  will  pass  a  f-inch  screen 
and  be  caught  on  a  ^-inch.     The  screenings  should  be  from  hard 
stone,  and  should  be  free  from,  dust  and  fine  material.     The  harder 
the  stone  the  smaller  may  be  the  screenings.     The  screenings  are 
spread  by  hand  with  shovels  or  with  a  revolving-disk  mechanical 
spreader.     After  being  spread,  particularly  if  the  work  is  done  by 
hand,  the  screenings  should  be  carefully  broomed  to  secure  a  uniform 
thickness  of  not  over  f  of  an  inch. 

The  purpose  of  the  screenings  is  to  keep  the  bituminous  material 
from  being  picked  up  on  the  wheels  of  vehicles,  to  make  the  surface 
less  slippery,  and  to  increase  the  wearing  qualities  of  the  road. 

595.  After  the  screenings  have  been  spread  a  few  hours,  it  is 
advantageous  to  roll  with  a  roller,   preferably  the  tandem  type, 
weighing  between  8  and  15  tons;  but  the  rolling  is  not  vital. 

596.  VALUE  OF  BITUMINOUS  CARPETS.    Bituminous  carpets  on 
old  water-bound  macadam  roads  have  been  of  great  value  in  enabling 
such  roads  to  carry  a  considerable  amount  of  motor-driven  traffic; 
and  under  some  conditions  it  has  been  economical  to  build  a  new 
water-bound  macadam  road  and  cover  it  with  a  bituminous  mat  or 
carpet.     Such  a  surface  will  usually  last  from  6  months  to  2  years 
depending  upon  the  amount  and  kind  of  travel.     Bituminous  sur- 
faces have  not  been  as  successful  on  gravel  as  on  macadam,  perhaps 
because  the  former  are  more  difficult  to  clean;   but  with  care  a  fair 
degree  of  success  can  be  insured  on  gravel. 

597.  Many  attempts  have  been  made  to  add  a  bituminous  sur- 
face to  a  portland-cement  concrete  road,  but  with  widely  varying 
degrees  of  success.     The  concrete  road  ordinarily  has  a  large  amount 
of  travel,  and  therefore  usually  has  too  many  steel-tired  horse-drawn 
vehicles  for  a  bituminous  carpet.     It  seems  to  be  agreed  that  the 
following  conditions  are  important:    1.  The  concrete  itself  must  be 
good.     2.  The  concrete  surface  should  be  roughened  by  wear  before 
the  bituminous  coating  is  applied.     3.  The  surface  of  the  concrete 
must  be  warm,  dry,   and  clean  when  the  bituminous  material  is 
applied.     4.  A  preliminary  priming  or  paint   coat  of  thin  tar  is 


ART.    2]  BITUMINOUS   CARPET  303 

advantageous.  5.  Two  thin  coats  of  the  carpet  material  are  better 
than  a  single  thick  one.  6.  A  J-inch  coating  can  not  stand  up  under 
much  horse-drawn  traffic.  If  there  is  much  horse-drawn  traffic,  it 
may  be  necessary  to  make  the  coating  1  to  1^  inches  thick  by  applying 
several  successive  layers  of  tar  and  screenings.  Possibly  a  bitumi- 
nous material  will  yet  be  made  that  will  be  more  suitable  for 
such  use. 

The  advantages  of  a  bituminous  surface  on  a  concrete  road  are: 
1.  It  protects  the  concrete  from  wear.  2.  It  reduces  the  noise  from 
the  impact  of  horses'  shoes  and  steel-tired  wheels.  3.  It  removes  the 
glare  of  the  light-colored  concrete.  4.  It  hides  the  black  blotches 
made  in  rilling  the  cracks  and  joints. 

598.  MAINTENANCE  OF    BITUMEN    CARPETS.    The  work  of 
maintenance  consists  in  patching  the  carpet  where  it  wears  through 
or  peels  up,  and  in  removing  bunches  where  the  carpet  has  crawled. 
The  patching  is  easily  done  by  following  the  method  employed  in 
the  original  construction;    but  care  should  be  taken  that  the  spot 
to  be  covered  is  clean,  dry  and  warm. 

It  is  not  easy  to  remove  the  bunches.  If  the  surface  is  soft,  a 
scraping  grader  (§  155)  will  sometimes  smooth  the  surface  without 
peeling  up  the  carpet;  but  the  work  must  be  done  during  warm 
weather  and  immediately  after  a  rain.  The  bunches  may  be  re- 
moved by  hand  with  a  shovel  that  is  kept  hot  while  in  use;  but  the 
shovel  will  not  last  long.  A  sharp  chisel-like  cutting  tool  if  made  of 
heavy  metal  will  stand  heating  better  than  a  shovel,  and  will  remove 
the  bunches.  The  bunch  can  be  softened  by  building  a  small  fire  of 
twigs  over  it,  or  by  pouring  kerosene  over  it;  but  this  practice  is 
likely  to  ruin  the  material  for  some  distance  around  the  bunch. 
There  are  surface  heaters,  i.  e.,  a  hood  having  a  gasoline  flame  under 
it,  which  are  used  for  removing  sheet  asphalt  (Fig.  161,  page  451), 
which  can  be  employed  for  removing  these  bunches;  but  the  process 
is  slow  and  expensive,  and  the  flame  is  likely  to  damage  the  material 
which  is  not  removed. 

599.  COST  OF  BITUMINOUS    CARPET.     Before  the  recent  dis- 
turbance of  prices  by  the  Great  European  War,  the  cost  of  oils  or 
tars  for  bituminous  carpets  varied  according  to  the  grade  of  the 
material  from  4  to  16  cents  per  gallon,  but  usually  from  6  to  8  cents. 
In  some  states  the  total  cost  of  a  bituminous  carpet  has  been  as  low 
as  3  cents  per  square  yard,  while  in  others  it  has  been  as  high  as  15 
or  20  cents. 

Below  are  the  details  of  the  cost  of  applying  a  light  bituminous 


304  BITUMINOUS   SURFACES   FOR   ROADS  [CHAP.    IX 

carpet  to  a  gravel  road  and  to  a  water-bound  macadam  road  by  the 
Illinois  Highway  Department  in  1915. 

600.  Gravel  Road.     The  cost  at  Cairo,  Illinois,  of  applying  0.5 
gallon  of  cold  oil  (Aztec  liquid  asphalt)  containing  60  to  65  per  cent 
of  asphalt,  and  0.006  ton  of  torpedo  gravel,  stone  chips,  and  sand 
per  square  yard,  to  a  gravel  road  Ij  miles  long,  the  average  haul 
being  0.5  mile  and  the  rate  of  pay  for  laborers  being  15  cents  per 
hour  and  for  teams  40  cents,  was  as  follows:  * 

COST 

ITEMS.  Cts.  per 

Sq.  Yd. 

Oil,  8184  gallons  at  4.7  cents  f  .o.b.  siding 2 . 34 

Torpedo  gravel  at  59  cents  per  cubic  yard,  f.o.b.  siding 0.24 

Heating  and  applying  oil,  demurrage,  etc 0.32 

Hauling  gravel  0.5  mile  and  spreading 0.31 

Sweeping  and  cleaning  old  road 0 . 034 

Freight  on  equipment 0 . 54 

Superintendence,  engineering,  and  inspection 0.21 

Total,  exclusive  of  depreciation,  over-head  expense,  and  profits ......  3 . 99 

601.  Macadam  Road.     The  cost  of  applying  a  bituminous  carpet 
consisting  of  0.33  gallon  of  Trinidad  B  asphalt  and  0.016  ton  of 
torpedo  gravel  per  square  yard,  the  average  haul  being  1£  miles  and 
the  pay  of  laborers  being  25  cents  per  hour  and  of  teams  50  cents,  was 
as  follows:! 

COST 

ITEMS.  Cts.  per 

Sq.  Yd. 

Field  superintendence 0 . 35 

Bituminous  material  @  7.7  cts.  f.o.b.  siding 2 . 39 

Torpedo  sand  @,  1.825  per  ton  f.o.b.  siding  and  stone  chips  @  $.140 2.67 

Hauling  gravel  and  chips,  1£  miles 0 . 84 

Spreading  gravel  and  chips 0 . 56 

Sweeping  and  cleaning  old  road 0 . 05 

Heating  and  applying  material,  demurrage,  etc 1 . 04 

Freight  and  equipment 0.15 

Repairs  to  equipment 0 . 07 

Incidental  expense 0 . 22 

Patching  holes  and  repairing  culverts 0 . 27 


Total,  exclusive  of  engineering,  inspection  and  rent  of  equipment 8.61 

602.  State  Reports.     The  annual  reports  of  many  of  the  State 
Highway  Departments  give  detailed  data  of  the  cost  of  bituminous 

*  Illinois  Highways,  December,  1915,  p.  168;   or  Engineering  Record,  Vol.  73  (1916),  p.  806, 
j-  Illinois  Highways,  December,  1915,  p.  171, 


ART.    2]  BITUMINOUS   CARPET  305 

road  surfaces.  For  example,  the  1915  report  of  the  New  York 
Commissioner  of  Highways,  pages  177-94,  shows  the  kind  and  quan- 
tity of  bituminous  material  used,  its  cost,  the  amount  applied  per 
square  yard,  the  area  covered,  the  cost  of  labor,  and  the  total  cost 
for  each  road  in  each  county  treated  in  that  year — a  total  of  1800 
miles. 


CHAPTER  X 

BITUMINOUS  MACADAM  AND  BITUMINOUS  CONCRETE 

ROADS 

604.  A  bituminous-macadam  road  consists  of  two  or  more  courses 
of  broken  stone,  the  wearing  course  of  which  is  bound  with  bituminous 
cement  applied  on  the  surface.     Formerly  this  form  of  construction 
was  usually  called  bituminous  macadam  by  the  penetration  method, 
but  sometimes  simply  penetration  macadam. 

A  bituminous-concrete  road  consists  of  one  or  more  courses  of 
broken  stone,  the  wearing  course  of  which  is  bound  with  bitumi- 
nous cement  mixed  with  the  stone  before  it  is  placed.  Formerly  this 
type  of  construction  was  usually  called  bituminous  macadam  by  the 
mixing  method,  but  sometimes  simply  mixed  macadam. 

Since  in  both  of  these  types  of  construction  the  binder  may  be 
either  tar  or  asphalt,  it  would  be  appropriate  and  more  definite  to 
use  the  specific  terms  asphalt  macadam  or  concrete,  and  tar  mac- 
adam or  concrete;  and  further  it  would  not  be  inappropriate  to  use 
the  terms  native-asphalt  and  residuum-asphalt  macadam  or  concrete ; 
and  likewise  coke-oven  tar  macadam  or  concrete,  and  water-gas  tar 
macadam  or  «oncrete.  For  a  distinction  between  bituminous  con- 
crete and  asphalt  concrete,  see  §  891. 

The  present  use  of  the  terms  bituminous  macadam  and  bitumi- 
nous concrete  is  based  upon  the  analogy  between  the  method  of 
construction  of  these  roads  and  that  of  macadam  and  concrete, 
respectively. 

605.  The  two  methods  of  road  construction  considered  in  this 
chapter  have  come  rapidly  into  use  since  about  1910. 

ART.  1.     BITUMINOUS  MACADAM  ROADS 

606.  The  drainage  and  the  preparation  of  the  subgrade  is  sub- 
stantially the  same  as  for  the  forms  of  roads  already  discussed. 

607.  FOUNDATION.     The  foundation  is  often  an  old  gravel  or 
macadam   road,   usually  the  latter;    and   sometimes,   though   less 

306 


ART.    l]  BITUMINOUS   MACADAM   ROADS  307 

frequently,  a  new  gravel  or  macadam  road  is  constructed  for  the 
purpose;  and  occasionally  a  portland-cement  concrete  foundation 
is  used. 

If  the  foundation  is  an  old  water-bound  macadam  road,  the  sur- 
face should  be  swept  with  a  machine  broom.  All  fine  material 
that  is  caked  upon  the  surface  and  is  not  removed  with  the  machine 
broom  should  be  loosened  by  hand,  and  then  the  surface  should  be 
swept  perfectly  clean  with  a  hand  broom.  The  coarse  stone  should 
be  exposed,  so  the  bituminous  binder  may  adhere  well. 

If  the  foundation  is  a  new  macadam  road,  it  should  be  con- 
structed as  described  for  water-bound  macadam  roads  (Chapter  VI). 
Under  the  same  conditions,  the  total  thickness  of  the  road,  including 
the  bituminous  wearing  course,  may  be  a  little  less  than  that  of  a 
water-bound  macadam  road,  as  the  bituminous  top  will  not  wear  as 
rapidly  as  the  water-bound,  since  the  former  is  usually  built  where 
motor-driven  traffic  predominates  and  rubber  tires  have  but  little 
effect  upon  the  bituminous  top.  The  first  and  second  courses  of  stone 
are  laid,  rolled,  filled,  sprinkled,  and  again  rolled  as  described  for 
water-bound  macadam,  except  that  the  second  course  is  not  flushed, 
i.  e.,  is  not  filled  so  much  as  to  form  a  film  over  the  surface.  It  is 
essential  that  there  shall  be  no  voids  in  this  course  to  absorb  the 
bituminous  binder.  The  stone  should  be  clean  and  dry  when  the 
binder  is  applied. 

608.  WIDTH.     For  a  discussion  of  matters  relating  to  the  width 
and  position  of  the  improved  wheelway,  see  §  95-97. 

609.  MAXIMUM  GRADE.     For  a  discussion  of  maximum  grades, 
see  §  79-85;  and  for  recommended  values  for  the  maximum  grade,  see 
Table  15,  page  57. 

610.  THE  CROWN.     The  crown  for  bituminous  macadam  should 
be  less  than  for  water-bound,  f  of  an  inch  to  the  foot  being  enough. 
See  Table  16,  page  66. 

611.  WEARING  COAT.     The  wearing  coat  consists  of  a  layer  of 
1^-inch  to  2^-inch  stone  and  two  applications  of  asphaltic  cement 
or  refined  tar,  each  of  which  is  followed  by  a  layer  of  f -inch  to  ^-inch 
screenings.     If  the  stone  is  soft,  the  size  of  the  screenings  may  be  a 
little  greater  than  stated  above;  and  if  hard,  a  little  less. 

The  layer  of  stone  should  be  evenly  spread  to  such  a  depth  that 
after  rolling  it  will  have  a  thickness  of  2£  inches,  and  should  then  be 
rolled  dry  until  the  fragments  have  become  firmly  keyed  together  so 
that  the  stones  will  not  move  ahead  of  the  roller  or  so  that  they 
can  not  be  moved  by  the  thrust  of  a  man's  heel.  If  the  foundation 


308  BITUMINOUS  MACADAM  AND  CONCRETE  ROADS         [CHAP.   X 

is  new  macadam  (§  607),  this  course  should  be  spread  and  rolled 
while  the  top  course  of  the  foundation  is  still  moist  and  soft;  but 
should  not  be  rolled  so  much  as  to  force  the  slush  of  the  foundation 
more  than  half  way  up  into  the  voids  of  the  course  being  laid.  Fig. 
98  shows  the  rolling  of  the  layer  of  stone  of  the  wearing  coat  in 
progress.  It  is  important  that  this  course  be  not  rolled  so  much  as 
to  prevent  the  penetration  of  the  bituminous  binder;  and  .on  the 
other  hand,  the  course  should  not  be  so  open  as  to  require  too  much 


FIG.  98.     ROLLING  OF  LATEK  OF  STONE  FOR  WEARING  COAT. 

binder  to  fill  the  voids.  If  the  stone  is  hard,  it  may  be  necessary 
after  the  rolling  is  partly  completed,  to  fill  the  voids  by  applying  a 
coat  of  gravel,  see  Fig.  99. 

612.  BITUMINOUS  BINDER.  The  bituminous  binder  may  be 
either  asphaltic  cement  or  refined  tar.  The  asphaltic  cement  should 
meet  some  one  of  the  specifications  in  §  537-38;  and  the  tar  should 
meet  one  of  two  specifications  in  §  572-73.  Owing  to  its  greater 
cementing  value  asphalt  is  better  for  a  road  to  be  subjected  to  heavy 
horse-drawn  loads  than  tar. 

The  asphaltic  cement  when  applied  should  have  a  temperature  of 
135  to  177°  C.  (275  to  350°  F.);  and  the  tar  of  93  to  121°  C.  (200  to 
250°  F.). 

The  amount  of  bituminous  cement  for  the  first  application  should 
be  just  sufficient  to  penetrate  the  third  course  and  fill  all  of  the  voids; 
and  usually  this  will  require  about  1  gallon  per  square  yard  per  inch 


ABT.    1]  BITUMINOUS   MACADAM   HO  ADS  309 

of  thickness  of  the  upper  course;  and  for  the  second  application  from 
-J  to  |  gallons  per  square  yard.  An  excess  of  binder  is  not  only 
expensive,  but  causes  the  wearing  course  to  creep  and  form  waves. 

613.  Applying  the  Binder.  For  small  jobs  or  where  it  is  difficult 
to  operate  tank  wagons,  the  bituminous  material  is  shipped  in  barrels 
or  drums,  heated  in  open  kettles,  and  applied  by  hand  from  pouring 
cans.  For  the  best  results,  the  binder  should  be  hauled  in  tank 
wagons  provided  with  a  heater,  one  or  more  thermometers,  and  a 


FIG.  99.     FILLING  LAYER  OF  STONE  WITH  GRAVEL. 

pump  for  distributing  the  binder  under  pressure  in  the  form  of  a 
spray.  The  tank  wagon  must  have  wheels  with  tires  so  wide  as 
not  to  make  an  appreciable  rut  in  the  surface  of  the  road.  The 
spreading  must  be  done  so  as  to  secure  absolute  uniformity.  The 
area  covered  with  a  barrel-  or  a  wagon-load  should  be  measured,  and 
the  rate  of  application  computed.  After  the  binder  has  been  applied, 
the  surface  should  be  uniformly  black,  but  the  spaces  between  the 
stones  should  show.  The  temperature  of  the  stone  or  the  air  should 
not  be  less  than  50°  F.  (10°  C.)  during  the  application. 

After  the  first  coat  of  binder  is  applied,  a  layer  of  £-inch  to  f-inch 
stone  screenings,  not  over  f  of  an  inch  thick,  is  spread  over  the  sur- 
face; and  then  the  road  is  rolled.  If  there  is  an  excess  of  uncemented 
screenings  after  the  rolling,  they  should  be  removed  with  a  push 
broom,  for  an  excess  will  cause  the  seal  coat  (the  last  coat  of  binder) 
through  lack  of  adhesion  to  the  first  coat,  to  peel  off. 


310  BITUMINOUS   MACADAM   AND    CONCRETE   ROADS          [CHAP.   X 

After  applying  and  rolling  the  screenings,  a  seal  coat  of  -J  to  f 
gallons  of  binder  per  square  yard  is  spread  by  the  same  means  as  for 
the  first  coat;  and  the  utmost  care  should  be  taken  to  put  on  the 
material  uniformly. 

Finally,  the  road  receives  a  coat  of  screenings  of  hard  screenings 
or  pea  gravel,  not  over  f  of  an  inch  in  thickness;  and  is  again  rolled. 
The  rolling  is  continued  until  a  smooth  surface  is  produced.  The 
road  is  now  ready  for  travel. 

The  eight  views  in  Fig.  100  show  the  various  steps  in  construct- 
ing the  wearing  cost  of  a  bituminous  macadam  road. 

614.  ANOTHER  TYPE  OF  BITUMINOUS  MACADAM.    The  wear- 
ing course  of  the  preceding  type  of  bituminous  macadam  road  could 
be  appropriately  described  as  a  course  of  stone  grouted  with  bitumi- 
nous cement.     The  Massachusetts  Highway  Commission  and  others 
sometimes  use  a  mixture  of  tar  and  sand  for  the  grout  instead  of  a 
neat  bituminous  cement.     It  is  claimed  that  the  sand  materially 
stiffens  and  strengthens  the  road  crust  and  decreases  the  oxidation 
of  the  tar. 

Fig.  101,  page  312,  shows  two  views  of  the  construction  of  a  tar- 
sand  macadam  road  built  by  the  Massachusetts  Highway  Com- 
mission. 

615.  CHARACTERISTICS  OF  BITUMINOUS  MACADAM.    This  form 
of  construction  is  adapted  to  roads  having  a  moderate  amount  of 
travel  with  not  many  heavy  horse-drawn  loads  on  narrow  tires.     It 
has  a  pleasing  appearance,  and  is  well  adapted  to  both  horse-drawn 
and  motor-driven  traffic.     The  surface  seems  to  deteriorate  more 
rapidly  where  considerable  quantities  of  mud  are  tracked  on  it.     In 
warm  weather,  particularly  if  an  excess  of  binder  was  used,  there  i§  a 
tendency  for  the  surface  to  creep  and  develop  undulations. 

There  have  been  a  considerable  number  of  failures  of  bituminous 
macadam  roads,  apparently  because  of  the  neglect  to  observe  the 
proper  methods  of  construction, — perhaps  through  lack  of  knowl- 
edge, since  the  type  is  comparatively  new. 

616.  COST,    This  form  of  construction  usually  costs  about  15 
to  20  cents  per  square  yard  more  than  good  water-bound  macadam 
(§  388-93).     There  has  not  yet  been  sufficient  experience  to  deter- 
mine the  cost  of  maintenance  or  the  ultimate  life  of  the  bituminous 
layer.  '   •, '••  -j 

617.  MAINTENANCE.    Bituminous  macadam  is  peculiarly  resist- 
ant so  long  as  it  is  intact;   but  when  once  broken,  due  to  defective 
materials  or  workmanship,  or  to  wear  of  travel,  or  to  the  opening  of  a 


ART.    1]  BITUMINOUS  MACADAM   ROADS 


311 


1.  Spraying  Tar  on  Layer  of  Stone.  2.  Spreading  f-inch  Stone  on  First  Coat  of  Tar 


3.  Sweeping  Screenings  to  Secure  Uniform 
Distribution. 


4.  Rolling  after  Spreading  Screenings. 


5.  Spraying  Seal  .Coat  of, Tar.  6.  Putting  Screenings  on  Seal  Coat. 


7.  Final  Rolling  of  Road.  8.  Finished  Road. 

FIG.  100.— CONSTRUCTING  WEARING  COAT  OP  TAR-MACADAM  ROAD. 


312  BITUMINOUS   MACADAM  AND   CONCRETE   ROADS          [CHAP.   X 

trench,  it  disintegrates  rapidly.  Under  these  circumstances  the 
defective  portion  should  be  cut  out,  the  sides  and  bottom  of  the  hole 
coated  with  bituminous  cement,  and  the  hole  filled  with  stone  and 
cement  well  mixed  and  solidly  tamped  into  place.  It  is  well  to  leave 


1.  Roller,  Tank  Wagon  and  Mixer.  '    2.  Pouring  Tarvia-Sand  Grout. 

FIG.  101. — CONSTRUCTING  TAR-SAND  MACADAM  ROAD. 

the  patch  a  little  high,  so  it  will  not  be  low  when  finally  consolidated 
by  travel. 

If  the  surface  of  the  road  becomes  dry  and  lifeless,  a  new  seal 
coat  should  be  applied  at  once.  This  will  usually  occur  with  a  tar 
binder  in  two  or  three  years,  depending  upon  the  amount  and  char- 
acter of  the  travel;  and  with  an  asphalt  binder,  in  three  or  four 
years.  Applying  a  new  seal  coat  is  peculiarly  a  case  in  which  a 
stitch  in  time  saves  nine. 

ART.  2.    BITUMINOUS  CONCRETE  ROADS 

619.  In  the  form  of  construction  considered  in  this  article,  the 
wearing  course  consists  of  a  layer  of  crushed  stone  bound  together 
by  either  tar  or  asphalt.  The  stone  and  the  binder  are  usually 
heated  before  being  mixed,  and  are  laid  while  hot. 

The  distinction  between  bituminous  concrete  as  considered  in 
this  article  and  asphaltic  concrete  as  considered  in  connection  with 
asphalt  pavements  (Art.  2,  Chapter  XVI),  is  that  in  the  latter  the 
binder  is  always  asphalt,  and  the  aggregate  is  accurately  graded  so 
as  to  secure  a  minimum  of  binder  and  also  a  maximum  stability. 
One  form  of  construction  gradually  shades  into  the  other,  and  it  is 
impossible  to  draw  a  definite  line  between  them.  In  the  accurate 
gradation  of  the  aggregates  asphalt  concrete  is  closely  related  to  sheet 
pavements,  and  hence  the  two  are  considered  together  in  Chapter 
XVI. 


ART.    2]  BITUMINOUS   CONCRETE   ROADS  313 

620.  All  that  was  said  in  §  114-28  concerning  drainage,  sub- 
grade,  and  cro.wn  applies  also  to  bituminous  concrete  roads. 

621.  THE  AGGREGATE.     Gravel  may  be  used,  but  only  for  light 
traffic.     Broken  stone  is  generally  used  because  of  the  better  bond 
thus  secured.     The  broken  stone  should  be  hard  and  of  compact 
texture  and  uniform  grain,  be  free  from  adhering  fine  material,  and 
preferably  should  have  rough  surfaces  and  sharp  angles. 

Bituminous  concrete  is  sometimes  laid  with  crusher-run  stone; 
but  since  the  stone  is  not  so  uniform,  the  result  is  not  so  good  as 
where  graded  stone  is  used. 

For  the  best  results  the  aggregate  should  be  carefully  graded. 
"  Practice  has  demonstrated  that  a  mineral  aggregate  which  will 
comply  with  the  following  sieve  analysis,  using  screens  having  cir- 
cular openings,  will  produce  satisfactory  results:  All  the  material 
shall  pass  a  1^-inch  screen;  not  more  than  10  per  cent  nor  less  than 
1  per  cent  shall  be  retained  on  a  1-inch  screen;  and  not  more  than 
10  per  cent  nor  less  than  3  per  cent  shall  pass  a  ^-inch  screen."* 

For  a  more  full  discussion  of  the  grading  of  the  mineral  aggregate 
for  bituminous  concrete,  see  Art.  2,  Chapter  XVI — Asphalt  Pave- 
ments. 

622.  THE  BINDER.     The  bituminous  cement  used  in  the  mixing 
may  be  either  tar  or  asphalt  cement;  but  the  seal  coat  should  always 
be  asphalt  cement  (§  541).     The  tar  should  conform  to  the  specifi- 
cations in  §  574-75 ;  and  the  asphalt  cement  should  meet  the  require- 
ments stated  in  §  539-40. 

The  quantity  of  bituminous  cement  to  be  used  in  the  mix  will 
depend  on  the  gradation  of  the  broken  stone  and  the  character  of 
the  bituminous  cement,  the  climatic  conditions,  etc.  For  an  aggre- 
gate graded  as  in  the  preceding  section,  the  mixture  should  contain 
between  5  and  8  per  cent  by  weight  of  bitumen. 

623.  MIXING  THE  CONCRETE.     There  are  two  types  of  mixing, 
cold  and  hot. 

624.  Cold  Mixing.     This  method  is  employed  only  with  a  special 
grade  of  tar,  and  the  concrete  is  usually  mixed  by  hand.     This 
method  is  not  very  common,  and  great  care  is  necessary  in  using  it. 
The  stone  must  be  perfectly  dry,  the  weather  must  not  be  too  cool, 
and  there  should  be  a  considerable  period  of  warm  weather  imme- 
diately following  the  completion  of  the  road.     This  form  of  con- 
struction is  suitable  for  light  traffic;  but  not  for  heavy  traffic,  either 
horse-drawn  or  motor-driven. 

*  Report  of  Committee  of  Amer.  Soc.  of  Civil  Engineers,  Proc.  Vol.  42  (1916),  p.  1626. 


314  BITUMINOUS   MACADAM   AND    CONCRETE   ROADS         [CHAP.    X 

625.  Hot  Mixing.     Usually  the  stone   and   binder  are  heated 
separately  and  then  mixed  in  a  machine  mixer. 

Bituminous  concrete  can  be  mixed  in  an  ordinary  hydraulic- 
cement  concrete  mixer;  but  mixers  specially  designed  for  mixing  it 
are  much  more  satisfactory.  There  are  various  such  mixers  on  the 
market.  Some  mixers  are  heated  internally  by  an  open  flame,  and 
others  externally  by  a  flame  or  steam.  On  account  of  the  danger  of 
burning  the  cement,  the  flame  in  the  mixing  chamber  should  not 
be  used,  except  perhaps  on  small  jobs  and  in  repair  work;  and  even 
then  the  flame  should  never  be  allowed  in  the  mixer  after  the 
bituminous  material  has  been  added. 

The  heating  device  should  be  easily  regulated,  so  that  there 
will  be  no  danger  of  burning  the  binder.  The  most  common  cause 
of  failure  in  bituminous  concrete  roads  is  the  burning  of  the  binder. 
A  burned  batch  will  not  usually  show  when  laid,  but  after  a  few 
months  will  reveal  itself. 

626.  The  bituminous  cement,  if  asphalt,  is  usually  heated  to 
about  135°  to  177°  C.  (275°  to  350°  F.);    and  if  tar,  from   93°  to 
135°  C.  (200°  to  275°  F.).     The  stone  for  the  asphalt  mixture  is 
heated  to  about  150°  C.  (302°  F.),  and  that  for  the  tar  mixture  to 
about  100°  C.  (212°  F.).     Any  asphalt  or  tar  that  is  heated  more 
than  stated  above  should  not  be  used.     No  tar  should  be  heated  in  a 
kettle  containing  any  asphalt,  and  likewise  no  asphalt  should  be 
heated  in  a  kettle  containing  any  tar;  and  any  mixtures  resulting 
from  this  cause  should  be  rejected. 

When  discharged,  mixtures  of  asphalt  cement  and  broken  stone 
should  have  a  temperature  of  not  less  than  93°  nor  more  than  149°  C. 
(200-300°  F.).  When  discharged,  mixtures  of  refined  tar  and  broken 
stone  should  have  a  temperature  of  not  more  than  121°  C.  nor  less 
than  66°  C.  (250-150°  F.). 

The  mixer  should  be  designed  and  operated  so  as  to  produce  a 
thoroughly  coated  and  uniform  mixture  without  any  segregation 
of  the  stone  and  the  cement. 

Bituminous  concrete  should  not  be  mixed  or  laid  when  the  tem- 
perature of  the  air  in  the  shade  is  less  than  50°  F.  (10°  C.). 

627.  LAYING  THE  CONCRETE.     If  not  mixed  upon  the  street, 
the  concrete  should  be  hauled  in  canvas-covered  wagons  or  trucks; 
and  should  be  delivered  at  a  temperature  of  at  least  66°  C.  (150°  F.). 
The  hot  mixture  should  be  dumped  upon  platforms,  shoveled  into 
place  with  hot  shovels,  immediately  raked  to  a  uniform  thickness, 
and  then  thoroughly  compacted  by  rolling.     The  roller  should  be  of 


ART.    2]  BITUMINOUS   CONCRETE   ROADS  315 

the  self-propelled  tandem  type  weighing  from  10  to  12  tons,  and 
giving  a  compression  under  the  rear  roll  of  250  to  350  lb.  per 
linear  inch.  The  rolling  should  continue  until  all  roller  marks  dis- 
appear. 

After  rolling,  the  wearing  course  should  have  a  uniform  thickness. 
The  experience  with  this  type  of  construction  is  somewhat  limited, 
but  apparently  a  thickness  of  1J  or  2  inches  is  sufficient  to  stand  the 
heaviest  mixed  traffic;  and  apparently  a  greater . thickness  is  unwise, 
since  it  has  a  tendency  to  creep  and  form  bunches.  The  surface 
should  be  free  from  depressions  and  irregularities  exceeding  f  of  an 
inch  under  a  4-foot  straight  edge  laid  longitudinally. 

628.  SEAL  COAT.    As  soon  as  possible  after  the  completion  of 
the  rolling,  and  while  the  surface  is  dry  and  clean,  a  seal  coat  of  hot 
asphalt  cement  should  be  applied  with  a  hand-drawn  distributor, 
and  be  spread  with  a  squeegee.     The  asphalt  cement  should  meet 
the  requirements  in  §  541;   and  should  be  applied  at  a  temperature 
of  not  less  than  135°  nor  more  than  177°  C.  (275-350°  F.)  at  a  rate 
of  J  to  1  gallon  per  square  yard. 

As  soon  as  possible  after  the  application  of  the  seal  coat,  and 
not  more  than  20  minutes  thereafter,  a  thin  uniform  layer  of  stone 
chips  (f-  to  ^-inch  or  £-  to  ^-inch)  should  be  spread  and  thoroughly 
rolled  with  the  tandem  roller  described  in  §  378. 

Fig.  102,  page  316,  shows  six  views  of  the  construction  of  a  bitu- 
minous concrete  road  built  in  Pennsylvania.  The  maximum  size  of 
the  aggregate  in  this  case  was  that  passing  a  |-inch  mesh;  and  hence 
the  concrete  could  be  leveled  off  by  striking  with  a  template — see 
view  3.  Views  5  and  6  are  included  partly  to  fill  out  the  plate  and 
partly  to  show  the  two  forms  of  distributors. 

629.  MAINTENANCE.    See  §  617,  page  310. 

630.  COST.     Under  similar  conditions,   the   cost  of  this  type 
of  road  is  usually  about  20  to  25  cents  more  thaji  that  of  water- 
bound  macadam  (§  388-93). 

631.  COMPARISON  OF  BITUMINOUS  MACADAM  AND  BITUMINOUS 

CONCRETE.  Bituminous  macadam  is  the  more  common,  there  being 
seven  or  eight  times  as  much  in  use  as  bituminous  concrete. 

The  advantages  of  bituminous  macadam  are:  1.  It  is  compara- 
tively cheap.  2.  It  requires  no  expensive  machinery.  3.  It  is 
easily  and  quickly  laid.  4.  The  cost  for  labor  is  comparatively 
low.  The  disadvantages  are:  1.  Considerable  care  is  required  to 
prepare  properly  the  upper  course  of  the  foundation  to  receive  the 
bituminous  macadam.  2.  There  is  difficulty  in  securing  a  uniform 


316 


BITUMINOUS   MACADAM  AND   CONCRETE   ROADS       [CHAP.  X 


distribution  of  the  binder  through  the  wearing  coat.  3.  It  is  prac- 
tically necessary  to  use  an  excess  binder,  and  hence  the  surface 
may  creep  under  traffic.  4.  The  quality  of  the  binder  must  be  sac- 


1.  Loading  End  of  Mixers. 


2.  Discharging  End  of  Mixers. 


3.  Striking  Tar-Concrete. 


4.  Rolling  Tar-Concrete. 


5.  Applying  Seal  Coat.  6.  Applying  Seal  Coat. 

FIG.  102.— CONSTRUCTION  OF  TAB-CONCRETE  ROAD 

rificed  to  the  requirements  of   the   method   of  its  application.     5. 
The  method  is  not  applicable  in  cold  or  damp  weather. 

632.  The  advantages  of  bituminous  concrete  are:    1.  It  permits  a 
perfectly  uniform  distribution  of  the  binder.     2.  It  permits  the  use 


ART.    2]  BITUMINOUS   CONCRETE   ROADS  317 

of  a  suitable  quality  and  quantity  of  binder.  3.  It  can  be  laid  in 
comparatively  cold  weather.  The  disadvantages  are:  1.  The  cost  of 
the  labor  is  comparatively  high.  2.  It  is  slow  in  application.  3.  A 
considerable  quantity  of  expensive  machinery  is  required.  4.  The 
total  cost  is  somewhat  greater. 


PART    II 

STREET  PAVEMENTS 

633.  Good  pavements  are  necessary  to  the  highest  development 
of  the  commercial,  sanitary  and  esthetic  life  of  a  city.  The  large 
proportion  of  people  now  dwelling  in  cities  makes  the  subject  of  pave- 
ments an  important  one;  and  the  fact  that  the  urban  population  is 
increasing  much  more  rapidly  than  the  rural,  and  also  the  fact  that 
the  public  is  awakening  to  the  necessity  of  ameliorating  the  condi- 
tion of  life  in  the  city,  will  make  pavements  of  increasing  concern 
in  the  future. 


CHAPTER  XI 

PAVEMENT  ECONOMICS  AND  PAVEMENT 
ADMINISTRATION 

ART.  1.    PAVEMENT  ECONOMICS 

634.  BENEFITS  OF  PAVEMENTS.  The  effect  of  pavements  upon 
city  life  is  so  important  and  so  far  reaching  that  no  enumeration 
is  likely  to  include  all  of  the  benefits;  but  nevertheless  it  will  be 
of  advantage,  particularly  in  discussing  the  proper  distribution  of 
their  cost,  to  enumerate  some  of  the  more  important  of  the  benefits 
resulting  from  the  construction  of  pavements.  Briefly  the  principal 
advantages  are: 

1.  Good   pavements   lessen   the   tractive   power   required,    and 
decrease  the  cost  of  transportation.     See  §  4-9  for  a  discussion  of  the 
cost  of  transportation. 

2.  Good  pavements  increase  fire  protection  by  facilitating  the 
transportation  of  the  fire  apparatus. 

3.  Pavements  establish  a  permanent  grade,  which  is  an  important 
matter  when  other  improvements  are  to  be  made, 

318 


ART.    1]  PAVEMENT   ECONOMICS 


4.  Pavements  improve  the  appearance  of  the  street  by  giving  a 
uniform  surface  instead  of  the  irregular  one  of  an  unpaved  street. 

5.  Pavements  increase   cleanliness,   since  the  pavement  is  less 
dusty  in  a  dry  time  and  less  muddy  in  a  wet  time  than  an  unpaved 
street,  and  since  they  are  easily  cleaned. 

6.  Pavements   increase   healthfulness   by  removing  holes  rilled 
with  mud  and  filth. 

7.  Pavements  permit  pleasure  driving  at  all  seasons,  and  facili- 
tate social  intercourse. 

8.  Pavements  allow  the  use  of  bicycles,  which  furnish  to  many 
cheap  transportation  and  healthful  recreation. 

635.  In  discussions  of  this  subject  it  is  customary  to  include  the 
enhanced  value  of  the  adjacent  property  as  one  of  the  advantages  of 
a  pavement;  but  the  increase  in  the  value  of  the  property  is  simply  a 
measure  of  the  benefits  enumerated  above,  and  hence  should  not 
again  be  included. 

The  first  three  benefits  above  may  be  regarded  as  financial 
advantages  and  the  last  four  as  sanitary  and  esthetic.  It  is  im- 
possible to  compute  even  approximately  the  financial,  much  less 
the  sanitary  and  esthetic,  value  of  good  pavements;  but  it  is  safe 
to  say  that  they  are  an  absolute  necessity  to  both  the  business 
and  residency  of  the  larger  cities  and  also  for  business  districts  of 
the  smaller  cities,  and  that  on  residence  streets  of  small  cities  good 
pavements  add  greatly  to  the  health,  comfort  and  pleasure  of  life. 

636.  INVESTMENT  IN  PAVEMENTS.    The  table  on  page  320 
was    compiled    from    statistics    published    by   the   U.  S.    Census 
Bureau,  and  shows  the  total  areas  and  cost  of  the  different  kinds  of 
pavements  in   1909  in  the  158  cities  having  a  population  of  over 
30,000.*     The  areas  are  probably  reasonably  accurate,  but  the  unit 
prices  are  only  approximate  owing  to  failures  to  state  what  was 
included  in  the  cost,  i.  e.,  whether  or  not  it  included  grading,  foun- 
dation, curb,  gutter,  etc. 

From  this  table  it  appears  that  the  pavements  in  these  cities 
have  cost  $695,936,294;  and  as  the  total  population  is  25,603,949, 
the  pavements  have  cost  $27.10  per  capita.  The  area  of  pavements 
per  capita  varies  greatly  in  the  different  cities,  being  practically 
independent  of  the  size  and  location  of  the  city;  but  the  average 
seems  to  agree  fairly  well  with  the  area  of  pavements  in  a  number  of 
very  much  smaller  cities  investigated  by  the  author.  Therefore,  it 


*  General  statistics  of  cities  for  1909,  Bureau  of  Census,  Washington,  D.  C.,  1913. 


320  PAVEMENT   ECONOMICS   AND   ADMINISTRATION       [CHAP.   XI 

will  be  assumed  that  the  above  average  is  representative  of  the  entire 
country.  According  to  the  U.  S.  Census  Report  there  were  41,717,- 
853  people  dwelling  in  cities  of  8000  population  or  over  in  1916. 
Therefore  the  investment  in  pavement  in  these  cities  amounts  to 
$1,046,320,000.  Measured  by  the  money  invested,  street  pave- 
ments, except  steam  railroads,  are  probably  the  most  important  of 
any  single  class  of  engineering  construction. 

INVESTMENT  IN  PAVEMENTS 

Asphalt— sheet 83,227,011  sq.  yd.  at  $2 . 75  =$238,874,281 

block 5,418,666  "  2.75=  14,901,331 

Bithulitic .  .  . : 4,000,872  "  2.25=  9,001,962 

Brick 53,870,578  "  2 . 25  =  120,208,805 

Cobblestone 9,083,397  "  0.80=  7,166,718 

Concrete,  portland-cement 445,478  "  1 . 20  =  434,576 

Gravel— water-bound 43,634,491  "  0 . 20  =  8,726,898 

bituminous  bound 4,674,605  "  0.40=  1,869,842 

Macadam— water-bound 107,998,789  "  0 . 75  =  80,998,082 

tar-bound 3,008,919  "  1 . 00  =  3,008,919 

portland-cement  grouted.  303,069  "  1.00=  303,069 

Stone  block 51,414,901  "  3 . 50  =  179,950,183 

Wood  block— creosoted 2,936,047  "  3 . 00  =  8,808,141 

untreated 10,724,370  "  2.00=  21,448,740 

Other  kinds 4,367,708  "  .20=  873,541 


Total 385,409,889  =$675,936,294 

637.  Data  for  the  year  1899  similar  to  the  above  were  presented 
in  former  editions  of  this  treatise;  and  apparently  from  1899  to  1909 
the  area  of  pavements  increased  38  per  cent,  and  the  cost  71  per  cent. 
The  increase  in  area  is  due  to  the  increase  of  pavements  in  each  city 
and  to  the  increase  in  the  number  of  cities  from  129  to  158.  The 
increased  cost  is  due  chiefly  to  the  increase  in  the  area  of  pavements, 
but  partly  to  the  increase  in  the  quality  of  the  pavements  and  partly 
to  the  increased  cost  of  labor  and  materials.  The  quality  of  pave- 
ments has  increased  greatly  in  the  last  few  years.  For  example, 
formerly  stone-block  pavement  consisted  of  roughly  dressed  blocks 
laid  on  a  sand  or  gravel  subgrade  with  wide  joints  filled  with  sand  or 
pebbles;  while  now  most  stone-block  pavements  consist  of  accurately 
dressed  blocks  laid  on  a  concrete  foundation  with  close  joints  filled 
with  bituminous  or  hydraulic  cement.  A  corresponding  improve- 
ment has  taken  place  in  most  other  forms  of  pavements.  However, 
the  cost  of  pavements  has  not  usually  increased  proportionally, 
owing  to  improvements  in  methods  of  doing  the  work.  Some  of 


ART.    2]  PAVEMENT   ADMINISTRATION  321 

these  improvements  are:  The  cutting  of  granite  blocks  largely  by 
machinery  instead  of  wholly  by  hand;  the  use  of  the  4-wheeled 
scraper  and  the  steam  shovel  in  preparing  the  subgrade;  improve- 
ments in  the  methods  of  handling  and  delivering  the  sand  and  gravel 
for  the  foundation;  the  mixing  of  the  concrete  for  foundations  by 
machinery  instead  of  by  hand;  improvements  in  the  methods  of 
handling  the  pavement  materials,  etc. 

638.  According  to  Bulletin  No.  100  of  the  1890  census,  the 
average  annual  expenditure  for  pavement  construction  and  repairs- 
in  the  cities  of  the  United  States  having  a  population  of  10,000  or 
over,  was  $1.72  per  capita,  being  $1.54  in  the  cities  having  more  than 
100,000  population  and  $2.04  in  cities  from  10,000  to  100,000.  No 
later  data  seem  to  have  been  collected.  If  the  same  rate  of  expense 
obtained  in  1909,  the  total  annual  expenditure  for  pavements  in 
cities  of  8,000  or  more  population  was  $85,104,420.  In  some  smaller 
cities  the  average  normal  expenditure  for  pavements  is  four  to  five 
times  the  average  just  stated. 

The  first  cost  of  pavement  and  also  the  annual  cost  is  of  such 
magnitude  that  merely  as  a  financial  question,  street  pavements 
deserve  careful  attention  and  systematic  study, 


ART.  2.    PAVEMENT  ADMINISTRATION 

640.  IMPORTANCE    OF   PROBLEM.     The   importance   of   pave- 
ments as  an  element  in  municipal  finance  seems  not  to  be  fully  appre- 
ciated, and  this  subject  has  not  received  from  municipal  engineers 
and  city  officials  the  attention  and  study  its  importance  merits. 
Whether  measured  by  their  influence  upon  the  commercial,  sanitary 
or  esthetic  life  of  the  city,  or  by  the  amount  of  money  invested  in 
them,  street  pavements  belong  in  the  first  rank  of  importance  in 
municipal  affairs. 

641.  Present  Conditions.     The  following  quotation  *  shows  the 
surprising  attitude  of  the  public  and  municipal  officials  toward  this 
important  subject. 

"  One  would  suppose  that  a  subject  of  the  magnitude  and  impor- 
tance of  pavements  would  be  a  matter  of  the  most  critical  investiga- 
tion and  scientific  research;  but  it  is  safe  to  say  that  in  no  other 
branch  of  civil  engineering  is  there  expended  so  large  an  amount  of 

*  John  W.  Alvord,  C.E.,  in  "The  Street  Paving  Problem  of  Chicago."    A  Report  to  the 
Commercial  Club  of  Chicago,  1904. 


322  PAVEMENT   ECONOMICS   AND   ADMINISTRATION       [CHAP.   XI 

money  in  so  unsystematic  a  manner,  and  generally  with  such  unsat- 
isfactory results. 

"  Pavements  are  primarily  designed  to  accommodate  travel; 
but  scarcely  any  one  in  this  country  thinks  of  investigating  the 
travel  of  a  city  systematically  and  thoroughly  before  proceeding 
to  lay  pavements. 

"Pavements  are  financial  investments;  yet  few  city  officials 
before  proceeding  to  raise  the  necessary  capital,  undertake  to  com- 
pile data  from  which  to  compute  the  cost  of  maintenance  and  the 
length  of  life  or  depreciation. 

"  Street  pavements  are  by  far  the  most  expensive  single  improve- 
ment that  the  municipality  undertakes;  yet  in  hardly  any  of  the 
cities  of  this  country  are  there  suitable  laws,  proper  organization,  or 
sufficient  public  spirit  adequately  to  care  for  the  investment  after  it 
is  once  made. 

"  The  improvement  of  streets  is  a  legitimate  method  of  adorning 
our  cities;  yet  no  one  thinks  of  consulting  recognized  authorities  on 
good  taste  in  such  matters,  except  in  boulevards  and  parks. 

"  Pavements  have  been  a  necessity  of  civilization  since  Rome  was 
mistress  of  the  world;  but  cities  are  still  experimenting  with  the  sub- 
ject without  general  and  well-defined  policies.  Community  after 
community  repeats  the  fundamental  experiments,  and  copies  without 
reflection  or  study  what  it  sees  being  done  elsewhere. 

"  The  managers  of  the  railways  of  the  country  know  to  a  cent  the 
cost  and  the  comparative  utility  of  every  bolt  and  scrap  of  iron  that 
enters  into  their  road ;  and  they  can  tell  to  several  places  of  decimals 
of  a  cent  the  cost  of  moving  a  ton  of  freight  or  the  cost  of  transporting 
a  passenger  a  single  mile.  But  the  city  officials  in  this  country  that 
can  make  more  than  a  rough  guess  of  these  matters  in  connection 
with  the  enormously  greater  travel  of  cities,  can  be  counted  on  the 
fingers  of  one's  two  hands." 

642.  Causes  of  Present  Conditions.  The  present  anomalous  con- 
ditions are  due  to  the  following  causes  :* 

1.  "  The  administration  of  American  cities  change  every  few 
years;  and  seldom  do  officers  of  the  municipality  have  the  ambition 
or  opportunity  to  become  thorough  masters  of  the  broader  require- 
ments of  the  problems  with  which  they  are  confronted. 

"2.  In  a  republican  state  the  tax-payer  is  expected  to  have 
a  deciding  vote  in  the  expenditure  of  the  public  moneys,  especially 

*  John  W.  Alvord,  C.  E.,  The  Street  Paving  Problem   of   Chicago.     A  Report  to  the  Com- 
mercial Club  of  Chicago.  1904, 


ART.    2]  PAVEMENT   ADMINISTRATION  323 

those  raised  by  local  taxation.  As  a  result,  advancement  proceeds 
no  faster  than  the  education  of  the  whole  mass  of  tax-payers. 

"  3.  Different  kinds  of  street  pavement  rise  or  fall  in  public 
estimation  with  an  undue  amount  of  popular  fluctuation.  This  is 
because  there  is  no  pavement  that  is  perfect  for  all  classes  of  con- 
ditions; and  the  pavement  that  comes  the  nearest  to  meeting  one 
set  of  requirements  may  be  the  furthest  away  from  another  set  of 
requirements.  The  public  having  selected  a  pavement,  perhaps  ill 
adapted  to  a  particular  environment,  and  finding  it  lacking  in  im- 
portant particulars,  is  apt  to  thoughtlessly,  and  perhaps  pettishly, 
condemn  it  in  toto  when  such  a  sweeping  verdict  is  not  warranted. 
Even  city  officials  in  charge  of  such  matters  do  not  always  inves- 
tigate carefully  enough  the  causes  which  make  for  failure,  and 
allow  personal  impressions  to  take  the  place  of  carefully  investigated 
facts. 

"  4.  The  engineers  of  this  country,  up  to  within  a  few  years, 
have  not  generally  interested  themselves  in  the  subject  of  street 
paving,  because  they  have  not  been  given  very  good  opportunity 
to  properly  study  the  question.  Finding  the  tax-payer  a  self- 
appointed  and  sometimes  exacting  authority,  they  have  been  obliged 
more  or  less  to  abandon  the  field,  so  far  as  its  broader  questions  are 
concerned,  and  accept  his  dictum.  The  specialist  on  street  paving 
has  been  long  recognized  abroad  as  an  important  factor  in  municipal 
progress;  and  of  recent  years,  he  is  beginning  to  appear  in  the  United 
States.  Seldom,  however,  does  an  average  city  call  for  his  services; 
and  almost  never  do  municipalities  appoint  commissions  of  such 
men  with  ample  funds  to  make  an  exhaustive  study  of  the  street- 
paving  problem  in  its  broader  requirements. 

"5.  The  natural  distrust  for  municipal  authorities  is  the  normal 
condition  of  mind  of  the  American  tax-payer.  This  is  the  inevitable 
result  of  a  system  that  generally  produces  mediocre  results.  And 
nowhere  are  mediocre  results  more  apparent  and  unhappy  than  in 
.the  work  of  street  improvements.  As  a  result,  the  average  city 
administration,  however  honest  its  intentions,  feels  that  it  is  without 
moral  support.  It  does  not  initiate  broad  policies,  or  spend  public 
moneys  for  investigation  and  research;  but  it  gropes  its  way  in  the 
darkness  of  chance,  and  plays  its  cards  like  an  opportunist,  post- 
pones all  possible  trouble  to  its  successors,  and  blames  all  deficiencies 
onto  its  predecessors,  while  ever  pleading  for  revenue  for  new  experi- 
ments. A  distrustful  public  generally  refuses  to  cooperate  in  legis- 
lation tending  to  increase  taxation  for  the  future,  until  absolutely 


324  PAVEMENT   ECONOMICS   AND    ADMINISTRATION       [CHAP.    XI 

assured  of  wisdom  and  economy  in  the  present.  The  public  is  at 
times  a  harsh  judge,  and  does,  not  easily  overlook  glaring  imperfec- 
tions or  fully  analyze  deficient  results. 

"  6.  The  usual  method  in  this  country  of  assessing  the  cost  of 
improvements  to  the  abutting  property  has  tended  to  give  undue 
prominence  to  the  property  owner,  as  the  representative  of  the  public 
in  deciding  upon  and  paying  for  street  paving.  As  a  matter  of  fact, 
the  abutting  property  owner  is  an  agent.  Whenever  possible  he  passes 
on  to  the  rest  of  the  community,  in  the  form  of  increased  valuation 
and  rental  of  his  property,  the  cost  he  is  assessed.  Often  he  recoups 
himself  for  his  outlay  many  times  over;  and  yet  ordinarily  he  regards 
himself  as  a  public  benefactor,  and  as  such  claims  the  right  to  outline 
street  policies  which  usually  lead  to  his  own  pecuniary  advantage 
rather  than  to  that  of  the  public. 

"  7.  The  method  of  assessing  the  first  cost  of  pavements  on  the 
property  owner,  and  then  maintaining  the  pavement  out  of  the  public 
fund,  has  resulted  in  the  majority  of  cases  in  there  being  little,  if 
any,  maintenance.  No  municipality  has  ever  had  an  adequate 
1  general  fund.'  The  general  fund  is  the  common  prey  of  all  the 
more  novel  municipal  projects  and  ambitions;  and  the  common- 
place uses  to  which  it  might  be  put  will  always  be  unduly  curtailed. 

"  8.  There  is  no  general  public  appreciation  of  the  vital  necessity 
of  maintaining  pavements  after  they  are  once  laid;  and  as  a  conse- 
quence there  has  been  no  cooperation  on  the  part  of  the  public  in 
framing  legislation  and  raising  revenues  for  this  purpose.  In  this 
country,  the  practice  has  been  generally  to  build  pavements  at  high 
first  cost,  and  allow  them  to  wear  out  with  a  minimum  of  repair. 

"  9.  Finally,  in  this  country  the  street-paving  problem  is  every- 
where regarded  as  a  local  or  neighborhood  problem.  The  general 
public  has  not  yet  come  to  regard  it  as  a  national  problem,  or  even 
entirely  a  municipal  problem;  and  hence  the  lack  of  appreciation 
of  the  broader  municipal  requirements,  and  the  insufficiency  of  study 
of  its  fundamental  principles.  To  this  cause  may  be  assigned  the 
chaotic  condition  of  the  art  and  incoherence  of  the  data  now  existing 
and  the  absence  of  any  general  principles  which  should  govern  the 
subject  as  a  whole." 

643.  Remedy  of  Present  Conditions.  The  present  unfortunate 
conditions  could  be  largely  remedied  by  making  the  investigations 
and  by  carrying  out  the  policies  mentioned  below. 

1.  The  first  requirement  for  a  comprehensive  plan  for  street 
pavements  should  be  a  study  of  the  traffic  conditions  of  the  entire 


ART.    2]  PAVEMENT   ADMINISTRATION  325 

city,  which  should  include  a  census  of  the  origin,  amount,  character, 
direction,  and  density  (the  amount  per  foot  of  width  of  street  or 
pavement)  of  the  travel  on  representative  streets  in  all  parts  of  the 
city.*  Such  a  census  should  be  repeated  at  regular  intervals  so  that 
the  growth  and  tendency  of  the  traffic  may  be  known  with  as  much 
certainty  as  vital  statistics  or  the  census  of  population,  since  only  by 
so  doing  can  a  basis  be  found  for  sound  present  policies  or  for  fore- 
casting future  necessities.  It  is  not  possible  to  formulate  any 
scientific  and  adequate  plan  for  street  pavements  without  knowing 
the  present  and  prospective  use  to  be  made  of  the  pavements.  Un- 
fortunately, but  few  such  census  data  have  been  obtained  for  any 
American  city — see  §  34. 

2.  The  streets  should  be  classified  as  to  the  amount  and  char- 
acter of  the  travel,  the  width  of  pavement,  the  depth  of  foundation, 
the  kind  of  wearing  surface,  the  amount  necessary  for  maintenance, 
etc.     The  highway  departments  of  the  several  states  have  recently 
quite  generally  classified  the  rural  roads  according  to  the  amount  of 
travel,  and  have  specified  certain  types  of  road  surfaces  for  the  dif- 
ferent classes  of  roads — for  example,  see  Table  26,  page  177. 

3.  Careful  records  should  be  kept  of  the  cost  of  repairs  on  differ- 
ent kinds  of  pavements  under  different  traffic  conditions;    and  an 
inventory  should  be  made  at  stated  intervals  to  determine  whether 
or  not  a  particular  pavement  is  gradually  deteriorating,  which  would 
serve  as  a  rough  check  upon  the  sufficiency  of  the  annual  repairs. 

4.  Careful  records  should  be  kept  of  the  cost  of  cleaning  different 
kinds  of  pavements  under  different  traffic  conditions.     The  cost  of 
cleaning  is  a  part  of  the  cost  of  maintenance;  and  unless  the  annual 
cost  of  maintenance  is  known,  it  is  impossible  to  compare  accurately 
the  total  cost  of  different  kinds  of  pavements. 

5.  Observations  should  be  made  to  determine  the  tractive  force 
required  to  draw  loads  over  different  types  of  pavements.     There 
are  tables  of  tractive  resistance  for  different  road  and  pavements 
surfaces, — for  example,  see  Table  7,  page  20; — but  such  data  are 
quite  general  and  do  not  represent  with  sufficient  accuracy  the  pave- 
ments for  a  particular  city  which  are  built  under  certain  specifications 
and  of  materials  more  or  less  local.     The  railways  of  this  country 
spend  large  sums  to  determine  the  tractive  resistance  on  tracks  of 
different  types  and  under  different  conditions;    and  the  results  of 
such  observations  are  of  great  importance  in  maintaining  an  economic 

*  For  a  discussion  of  travel  census  for  rural  roads,  see  §  29-33. 


326  PAVEMENT   ECONOMICS   AND   ADMINISTRATION       [CHAP.    XI 

relation  between  the  cost  of  the  up-keep  of  the  track  and  the  cost  of 
drawing  trains  over  the  track.  Corresponding  data  for  street  pave- 
ments would  be  helpful  in  determining  when  the  condition  of  any 
pavement  renders  it  unfit  for  further  profitable  use. 

6.  The  public  should  be  educated  as  to  the  financial,  sanitary,  and 
esthetic  importance  of  street  pavements;  and  also  as  to  the  fact  that 
the  construction  and  maintenance  of  pavements  should  be  left  entirely 
to  those  who  have  made  a  careful  study  of  such  work. 

7.  The  laws  should  vest  the  power  to  determine  when  a  street 
should  be  paved  and  to  select  the  kind  of  pavement  in  either  an 
elective  board  of  long  tenure  so  that  its  members  may  have  an  oppor- 
tunity to  acquire  knowledge  of  paving  materials  and  policies,  or  in  a 
commission  of  real  experts  appointed  for  that  purpose. 

644.  APPORTIONMENT  OF  THE  COST.  There  is  much  discussion 
as  to  who  in  equity  should  bear  the  cost  of  the  pavement.  There 
are  three  distinct  views. 

1.  A  few  claim  that  as  they  own  neither  a  horse  nor  a  vehicle  and 
do  not  use  the  pavement,  they  should  not  be  required  to  pay  for  it. 
Although  a  resident  may  not  travel  upon  the  pavement,  it  is  used 
by  those  who  serve  him;    and  a  pavement  confers  other  benefits 
besides  those  relating  to  transportation.     It  is  entirely  impracticable 
to  distribute  the  expense  according  to  the  use  made  of  the  pavement. 

2.  Others  claim  that  the  pavement  is  for  the  benefit  of  the  general 
public  of  the  city  at  large,  and  hence  the  abutting  property  should 
pay  no  more  than  that  in  other  parts  of  the  city.     This  claim  ignores 
the  fact  that  the  abutting  property  secures  a  distinct  benefit  for  which 
it  should  be  required  to  pay.     Laying  at  least  part  of  the  cost  upon 
the  abutting  property  tends   to   discourage   a   demand   for   lavish 
expenditures  for  unnecessary  improvements,  that  possibly  might  be 
insisted  upon  if  the  city  contributed  the  entire  cost. 

3.  Many  hold  that  the  benefits  accrue  only  to  the  abutting  prop- 
erty, and  that  therefore  the  owner  of  the  abutting  property  should 
bear  the  entire  cost.     This  claim  disregards  the  fact  that  the  pave- 
ment is  for  the  use  of  the  general  public,  and  benefits  all  the  people 
and  all  those  having  business  interests  in  the  city.     An  improvement 
in  any  part  of  the  city  is  an  indirect  benefit  to  the  city  as  a  whole. 
In  excuse  of  this  method  of  payment,  it  is  sometimes  claimed  that, 
although  the  pavement  confers  a  general  benefit,   the  inequality 
will  be  compensated  when  all  the  streets  are  paved.     The  answer 
is  that  all  the  streets  may  never  be  paved,  and  besides  traffic  natu- 
rally concentrates  on  certain  lines  and  nearly  ignores  certain  others, 


ART.    2]  PAVEMENT   ADMINISTRATION  327 

and  therefore  some  pavements  will  require  much  more  care  and 
expense  than  others.  Further,  there  should  be  no  objection  to  letting 
every  property  holder  pay  a  part  of  his  ultimate  share  as  the  work 
progresses,  instead  of  paying  it  in  a  lump  sum  when  the  street  in 
front  of  his  own  property  is  paved.  The  second  or  third  view  or  a 
combination  of  them  usually  obtains  (§  645).  Table  36,  page  328, 
shows  the  method  of  apportioning  the  expense  in  fifty  American 
cities.* 

The  practice  is  slightly  different  for  the  grading,  the  original 
paving,  and  the  re-paving.  All  of  the  cost  of  grading  in  54  per  cent 
of  these  cities  is  paid  by  the  abutting  property,  in  32  per  cent  all 
by  the  city,  and  in  14  per  cent  part  by  each  in  varying  proportions. 
The  cost  of  the  original  'paving  in  62  per  cent  of  the  cities  is  charged 
entirely  to  the  private  owner,  in  22  per  cent  entirely  to  the  city,  and 
in  16  per  cent  it  is  divided  between  the  two.  The  cost  of  re-paving 
in  42  per  cent  of  the  cities  is  paid  wholly  by  the  property,  in  40 
per  cent  wholly  from  the  general  tax,  and  in  18  per  cent  it  is  divided 
between  the  two.  In  some  cities  a  street  in  an  addition  or  sub- 
division is  not  accepted  by  the  municipal  authorities  until  it  has  been 
graded,  and  hence  it  is  done  at  the  expense  of  the  abutting  property; 
but  on  the  other  hand,  some  cities  are  willing  to  bear  a  part  of  the 
cost  of  the  street  improvement,  and  therefore  pay  for  the  grading. 
Only  one  quarter  of  the  above  cities  pay  the  major  part  of  the  cost 
of  the  original  paving,  while  40  per  cent  pay  the  major  part  of  the 
cost  of  re-paving.  It  is  the  custom,  where  there  is  a  car  track  on  the 
street,  to  require  the  railroad  to  pave  an  8-foot  strip  for  each  track, 
the  remainder  being  divided  between  the  abutting  property  and  the 
city  at  large  in  the  same  proportion  as  on  the  streets  where  there  is 
no  track.  In  some  cities  intersecting  streets  are  regarded  as  municipal 
property,  and  the  cost  of  paving  the  intersection  is  assessed  against 
the  street,  i.  e.,  against  the  city;  but  in  others  the  cost  of  paving  the 
street  intersections  is  included  in  the  charge  against  the  abutting 
property.  In  most  cities  lots  owned  by  the  municipality  pay  the 
same  proportion  of  the  cost  of  the  street  improvements  as  private 
property,  although  usually  special  authority  is  required  thus  to  assess 
municipal  property. 

Table  36  also  shows  that  as  a  rule  the  eastern  and  southern 
cities  pay  a  larger  proportion  of  the  cost  of  pavements  than  do  the 
western.  This  difference  in  practice  is  probably  due  chiefly  to  the 

*  From  an  article  on  Theory  and  Practice  of  Special  Assessments  by  J.  L.  VanOrnum,  in 
Traus.  Amer.  Soc.  of  Civil  Engineers,  Vol.  38,  p.  336-422, 


328  PAVEMENT   ECONOMICS  AND   ADMINISTRATION       [CHAP.   XI 


TABLE  36 
APPORTIONMENT  OF  COST  OF.  PAVEMENTS  IN  FIFTY  CITIES 


Ref 
No. 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
40 
41 
42 
43 
44 
45 
46 
47 
48 
49 
50 

LOCALITY. 

Grading, 
Per  Cent 
Paid  by 

Original 
Paving, 
Per  Cent 
Paid  by 

Re-paving; 
Per  Cent 
Paid  by 

State. 

City. 

Prop- 
erty. 

City. 

Prop- 
erty. 

City. 

Prop- 
erty. 

City. 

50 
100 
100 

50 

ioo 

100 
100 
100 
50 
100 
50 

50 
100 
100 
67 
a 

"56" 
67 
50 
100 
100 
100 
100 
100 
75 

50 
33 

ioo' 

100 
50 
33 
50 

"d" 

c 

50 
100 

"b" 

'  'SO 
67 
50 
100 
100 

'ioo' 
'75' 

ioo 

100 
100 
100 
100 
100 

ioo 
ioo 

50 
100 

'ioo 

100 
98 
100 
100 
100 

50 

ioo 
ioo 
ioo 

50 

ioo 

100 

'ioo' 

100 
50 
33 
50 

d" 
100 

'ioo' 

25 

100 
100 
100 
100 
100 
c 
c 

100 

ioo 
d 

50 

'ioo" 

"2c" 
c 

'ioo' 

50 
100 
100 

100 
100 

ioo' 

Arkansas  
Califormia 

Little  Rock  

San  Francisco 

Hartford 

Dist.  of  Columbia  .  .  . 
Delaware  
Florida  
Georgia 

New  Haven  

Wilmington  

Jacksonville                    .... 

50. 

Atlanta 

Illinois  
Indiana 

Augusta  

50 
100 
100 

Peoria 

Indianapolis  
Burlington 

d 
100 
100 

'  '25' 

100 

'ioo 

100 

ioo 

50 
100 

Iowa  

Kansas  

Topeka  

Kentucky  

Louisville  

100 

75 

'ioo 

100 

"25 

100 

"ioo 

100 
100 
c 
c 

Louisiana  
Maine  
Maryland  

New  Orleans  ,  .  .  .  . 

Portland  

Baltimore. 

100 

ioo' 

100 
100 
100 
100 
100 

ioo 

100 
100 
100 
100 
100 
100 
100 
98 
100 
100 
100 
100 
100 

ioo' 
'166' 
ioo' 

100 

Massachusetts  

Michigan  
Minnesota  

Missouri  

Lowell  

Springfield.  ... 

Worcester 

100 
100 

Detroit  
Minneapolis 

St.  Paul  

100 
100 

"56' 
ioo 

100 
100 
100 
100 
100 
100 
100 
98 
100 
100 
100 

Kansas  City 

Nebraska  
New  Hampshire  
New  Jersey  

New  York  
Ohio  

St.  Louis  

100 

d   ' 

Omaha  

Manchester  
Newark  

Paterson 

d    ' 

"  2c' 
c 

'166' 
ioo 

100 
50c 
100 

c 

Albany  
Brooklyn.  . 

Buffalo  

New  York  

Rochester 

"2c" 
c 

'ioo 

100 
100 
100 

c 

Syracuse  

Cincinnati.  .  . 

Oregon 

Dayton  

Portland  
Harrisburg  

3ennsylvania  

Ihode  Island  
South  Carolina  
South  Dakota  
Tennessee  
Utah  
Virginia  
Washington  
Wisconsin  

Philadelphia  

Scranton  
Providence  

100 
100 

ioo' 
'56' 

Charleston  
Sioux  Falls  

Nashville  
Salt  Lake  

Richmond.  .  .  . 

Seattle  

ioo 

100 

Milwaukee  

a.  1  sq.  yd.  for  each  front  foot;  city  remainder 
6.  3f  sq.  ft.     "     "         "        "  ;     •• 

c.  City  pays  for  street  intersections. 

d,  City  does  not  pay  for  street  intersections, 


ART.    2]  PAVEMENT   ADMINISTRATION  329 

limited  revenues  of  new  cities  and  to  the  many  demands  upon  the 
general  tax  for  the  numerous  and  varied  necessities  of  rapidly  growing 
municipalities;  consequently  the  cost  of  pavements,  improvements 
having  a  definite  local  benefit,  has  been  charged  to  the  abutting  prop- 
erty. It  is  equitable  and  just  that  the  cost  should  be  borne  jointly 
by  the  private  property  and  the  city  at  large,  since  then  the  cost  falls 
upon  both  interests  which  directly  profit  by  the  improvement,  and 
neither  receives  a  substantial  benefit  without  sharing  in  its  cost. 

Ordinarily  the  proportion  of  the  expense  to  be  borne  by  the 
municipality  and  by  the  private  property  is  determined  wholly  by 
financial  considerations  or  usage,  and  is  made  uniform  over  the 
entire  city;  while  equity  and  justice  demand  that  a  distinction  should 
be  made  depending  upon  the  character  of  the  traffic.  The  interests 
of  the  general  public  in  a  street  vary  greatly  between  a  residence 
street,  a  business  street,  and  a  general  thoroughfare.  To  pave  the 
first  the  public  should  pay  only  a  small  share,  say,  20  or  30  per  cent; 
for  the  second,  say,  40  or  50  per  cent;  and  for  the  third  60  or  75  per 
cent.  Some  such  variation  in  the  proportion  to  be  borne  by  the  two 
interests  finds  further  justification  in  the  fact  that  if  the  street 
becomes  a  general  thoroughfare,  some  of  the  benefits  enumerated  in 
§  635  as  accruing  to  the  abutting  property  may  be  nullified  by  the 
noise  and  dirt. 

646.  SPECIAL  ASSESSMENTS.*  The  proportion  of  the  cost  of 
a  pavement  paid  by  the  private  property  is  usually  collected  as 
a  special  assessment,  which  has  been  defined  as  "  a  compulsory 
contribution  paid  once  and  for  all  to  defray  the  cost  of  a  special 
improvement  to  property,  undertaken  in  the  public  interest,  and 
levied  by  the  government  in  proportion  to  the  special  benefits 
accruing  to  the  property  owner. "  Special  assessments  differ  from 
taxes,  both  general  and  special,  in  that  the  former  are  based  upon  a 
direct  and  measurable  benefit  conferred  upon  the  contributor, 
which  is  the  measure  of  his  liability  to  be  taxed ;  while  taxes  are  levied 
for  the  maintenance  of  the  institutions  and  interests  of  the  govern- 
ment, without  reference  to  the  particular  benefits  conferred,  according 
to  the  ability  of  the  contributor  to  pay.  The  construction  of  pave- 
ments to  be  paid  for  by  special  assessment  must  be  done  under  the 
direction  of  the  public  officials. 


*  For  an  interesting  and  instructive  clscussion  of  the  history  and  theory  of  special  assess- 
ments, see  Special  Assessments  by  Victor  Rosewater — Vol.  2,  No.  3  of  Studies  in  History 
Economics  and  Public  Law.  152  p.,  6X9  inches,  Columbia  College,  New  York,  1893.  See 
also  the  article  referred  to  in  the  foot  note  on  page  327. 


330  PAVEMENT   ECONOMICS   AND   ADMINISTRATION        [CHAP.   XI 

In  a  general  way  it  may  be  said  that  there  are  two  distinct 
methods  of  apportioning  the  amount  to  be  paid  by  the  private 
property;  viz.:  (1)  according  to  the  frontage,  and  (2)  according  to 
the  area. 

647.  Frontage  Rule.  By  far  the  more  common  method  of  appor- 
tioning the  assessments  is  pro  rata  according  to  the  frontage  upon  the 
improvement.  This  method  is  often  designated  as  the  front-foot 
rule.  Of  the  forty-five  cities  in  Table  36,  page  328,  which  assess  the 
private  property  for  street  improvements,  thirty-eight  or  84  per  cent 
follow  the  frontage  rule,  three  use  a  combination  of  frontage  and 
area,  one  uses  area  alone,  one  value  alone,  and  in  two  of  the  cities 
the  method  employed  is  left  to  the  judgment  of  the  assessing  board. 

Ordinarily  the  frontage  is  an  equitable  basis  upon  which  to 
distribute  the  cost;  but  under  some  circumstances  a  rigid  appor- 
tionment according  to  frontage  gives  anomalous  results.  For 
example,  if  most  of  the  lots  have  their  shorter  side  on  the  improve- 
ment and  one  has  its  longer  side  thus  placed,  the  frontage  rule  will 
give  inequality — particularly  if  the  latter  lot  is  very  narrow.  This 
condition  frequently  occurs — for  example  where  the  most  of  the 
lots  front  upon  the  street  to  be  paved,  while  some  front  upon  an 
intersecting  street.  In  this  case,  it  is  customary  to  extend  the 
assessment  to  the  middle  of  the  block;  that  is,  assess  the  lots  between 
the  pavement  and  the  center  of  the  block,  in  which  case  it  becomes 
a  difficult  matter  to  determine  the  equitable  portion  for  each  of  these 
lots.  A  rigid  adherence  to  the  frontage  rule  sometimes  works 
injustice  near  the  intersection  of  two  streets  cutting  each  other  at  an 
acute  angle.  However,  no  method  can  be  devised  that  may  not 
require  modification  to  fit  unusual  conditions. 

648.  Area  Rule.    In  a  few  cities,  7  per  cent  of  those  in  Table  36, 
page  328,  the  cost  of  street  improvement  is  distributed  in  proportion 
to  the  area  of  the  abutting  lots;  but  usually  the  area  is  used  in  com- 
bination with  the  frontage.     Thus  in  Brooklyn,  N.  Y.,  60  per  cent 
of  the  cost  is  distributed  in  proportion  to  the  frontage  and  40  per  cent 
according  to  the  area.     An  amendment  to  the  charter  of  St.  Louis 
proposes  to  charge  25  per  cent  of  the  cost  of  the  pavement  according 
to  the  frontage  and  75  per  cent  according  to  the  area.     The  area  rule 
finds  its  greatest  justification  on  curved  streets. 

649.  Corner  lots  are  usually  the  cause  of  irritation  and  objection 
under  either  the  frontage  or  the  area  rule,  and  the  method  of  assess- 
ing them  differs  materially  in  different  cities.     In  some  cases  each 
margin  is  considered  a  front  on  its  proper  street,  without  any  modi- 


ART.    2]  PAVEMENT   ADMINISTRATION  331 

ification  in  the  rate. of  assessment;  in  a  few  cases  under  the  area 
rule,  an  additional  per  cent  is  imposed  upon  the  corner  lot  for  the 
pavement  of  either  street ;  but  usually  the  corner  is  assessed  according 
to  frontage  at  a  less  pro  rata  than  the  inside  lots,  since  it  may  be 
assessed  on  both  streets. 

650.  Terms  of  Payment.    There  are  various  methods  of  pay- 
ing  the   assessment.     1.  The   entire   amount   may   become   a  lien 
upon  the  property  as  soon  as  the  work  is  completed,  to  be  collected 
(a)  by  the  contractor,  or  (6)  by  the  city  acting  only  as  collecting 
agent  for  the  contractor,  or  (c)  by  the  city,  which  also  becomes 
responsible  to  the  contractor  for  the  payment  of  the  money.     2, 
The  amount  may  be  divided  into  equal  annual  installments,  usually 
five  or  ten,  with  interest  on  deferred  payments,  to  be  collected 
(a)  by  the  contractor,  or  (6)  by  the  city,  the  contractor  receiving 
special  paving-district  bonds,   or   (c)   by  the  city,  the  contractor 
receiving  general  city-bonds.     3.  The  city  may  raise  a  paving  fund 
by  general  tax  or  by  selling  bonds,  and  pay  for  improvements  as  made 
independent  of  the  collection  of  the  special  assessments.     The  second 
method  is  the  more  common.     The  first  is  objectionable  because  the 
amount  becomes  immediately  due;  and  the  third  is  objectionable  on 
account  of  the  difficulty  of  making  the  assessments  and  collections 
keep  pace  with  each  other,  and  also  because  of  a  tendency  to  produce 
extravagance. 

651.  Legality  of  Levy.     Special  assessments  can  be  levied  only 
under  explicit  authority  of  the   law.     The   different  states  have 
very  complete  and  explicit  statutes  governing  special  assessments; 
and  the  courts  always  hold  that  any  material  departure  from  the 
prescribed  procedure  invalidates  the  assessment. 

652.  GUARANTEEING  PAVEMENTS.    It  is  a  common   custom 
to  require  the  contractor  to  guarantee  the  pavement  for  a  term 
of  years,  which  guarantee  is  supported  either  by  an  indemnifying 
bond  or  by  a  portion  of  the  cost  of  the  pavement  retained  by  the 
municipality  until  the  expiration  of  the  specified  period.     In  some 
cases  the  guarantee  is  an  agreement  that  if  time  shall  reveal  that 
the  materials  or  the  method  of  construction  are  not  according  to 
the  contract,  the  contractor  shall  make  the  defect  good;    but  in 
other  cases,  the  so-called  guarantee  is  virtually  a  contract  to  main- 
tain the  pavement  for  the  specified  period  and  to  turn  it  over  in 
good  condition  at  the  end  of  that  time. 

Apparently  the  guarantee  originated  in  this  country  with  the 
introduction  of  sheet  asphalt  pavements.    The  material  was  new, 


332  PAVEMENT   ECONOMICS   AND    ADMINISTRATION       [CHAP.    XI 

the  method  of  laying  it  was  untried,  and  hence  no  city  would  run 
the  risk  of  paying  for  an  unknown  and  uncertain  pavement;  con- 
sequently the  contractor  agreed  to  guarantee  the  pavement  for  a 
period  of  years.  At  present  most  cities  continue  to  exact  a  guarantee 
for  asphalt  pavement,  ranging  from  five  to  fifteen  years,  on  the 
ground  that  the  method  of  testing  the  material  and  the  manner  of 
laying  it  are  too  little  understood  by  engineers  to  insure  good  and 
durable  work  without  a  guarantee.  At  the  beginning  of  the  use  of 
brick  as  a  paving  material  a  guarantee  was  sometimes  demanded; 
but  at  present  it  is  as  a  rule  not  required  with  this  material. 

653.  The  requirement  of  a  guarantee  of  the  pavement  is  justi- 
fiable when  the  material  to  be  used  is  new  and  there  is  little  or  no 
opportunity  for  the  engineering  department  to  acquire  the  knowl- 
edge necessary  for  an  effective  inspection  of  the  work;  but  as  a 
rule  a  guarantee,  particularly  for  a  long  time,  is  unwise  for  the 
following  reasons:  1.  The  contractor  has  no  control  over  the  street 
after  the  pavement  is  completed;  and  it  is  difficult  to  discriminate 
between  defects  due  to  improper  material  and  the  effects  of  ordinary 
wear,  which  may  differ  materially  on  different  streets.  It  is  also 
difficult  to  discriminate  between  defective  workmanship  and  damages 
due  to  causes  for  which  the  contractor  is  in  nowise  responsible,  as, 
for  example,  fires,  escape  of  illuminating  gas,  settlement  of  trenches 
made  after  the  completion  of  the  pavement,  etc.  2.  It  is  difficult 
to  enforce  the  guarantee  clause  if,  on  the  one  hand,  the  engineering 
department  inspects  the  material  and  accepts  the  workmanship; 
and,  on  the  other  hand,  if  a  representative  of  the  city  does  not  inspect 
*  the  work  there  is  liability  that  the  streets  may  be  needlessly  obstructed 
and  the  public  greatly  inconvenienced  by  a  bungling  experiment  by 
the  contractor.  The  difficulty  of  enforcing  a  guarantee  is  much  less 
in  a  large  city  where  there  is  more  work  to  be  had  and  where  the 
contractor  desires  to  protect  his  reputation  with  a  view  to  securing 
contracts  in  the  future,  than  in  a  small  city  having  but  little  work; 
and  the  difficulty  is  still  further  increased  if  the  law  requires  that  the 
contract  shall  be  let  to  the  lowest  responsible  bidder — as  is  usually 
the  case. 

The  contractor  objects  to  the  guarantee,  not  without  justice, 
on  the  following  grounds:  1.  The  specifications  are  prepared  by 
the  engineering  department  of  the  city,  and  as  the  quality  of  the 
material  and  the  method  of  construction  is  prescribed  by  the  city 
and  subject  to  the  approval  of  its  representatives,  the  contractor 
should  not  be  held  responsible  for  the  result.  However,  the  suffi- 


ART.    2]  PAVEMENT  ADMINISTRATION  333 

cient  answer  to  this  objection  is  that  the  contractor  accepts  the 
specifications  when  he  enters  into  contract,  and  is  therefore  right- 
fully bound  by  them.  2.  The  expense  is  needless  and  excessive, 
whether  an  indemnifying  bond  is  required  or  a  per  cent  of  the  con- 
tract price  is  retained,  which  expense  in  the  long  run  adds  to  the 
cost  of  the  pavement.  It  is  more  expensive  to  the  contractor  if 
the  city  retains  a  per  cent  of  the  contract  price,  since  a  portion  of 
his  capital  is  then  tied  up,  which  in  turn  drives  out  the  small  con- 
tractor, decreases  competition,  and  tends  to  increase  the  cost  of 
the  pavement.  On  the  other  hand,  the  interests  paying  for  the 
pavement  are  better  protected  if  the  city  retains  a  per  cent  than  if 
an  indemnifying  bond  is  accepted,  since  in  the  former  case  the  city 
has  the  money  in  hand  with  which  to  make  the  needed  repairs  in 
case  the  contractor  fails  to  do  so;  but  the  proper  care  of  such  de- 
ferred payments  adds  materially  to  the  labor  and  responsibility  of 
municipal  administration. 

The  contributing  property  holders  and  citizens  favor  the  guar- 
antee as  a  defense  against  incompetent  or  dishonest  city  officials 
and  employees.  The  guarantee  is  also  sometimes  defended  on  the 
ground  that  it  is  the  cheapest  method  of  securing  good  work,  since 
it  is  impossible  at  reasonable  cost  for  the  engineering  department 
to  inspect  all  stages  of  the  preparation  of  the  material  or  to  acquire 
the  knowledge  necessary  for  an  effective  supervision  of  the  con- 
struction; but  in  general  this  claim  is  not  true.  It  is  neither  credit- 
able to  the  engineering  profession  nor  economical  to  the  municipali- 
ties to  leave  all  exact  knowledge  of  paving  matters  in  the  hands  of 
the  paving  contractors. 

654.  The  proper  length  of  the  guarantee  period  is  a  matter 
about  which  there  is  considerable  difference  of  opinion.     For  asphalt 
pavement  a  guarantee  for  five  years  is  quite  common,  although  some- 
times a  fifteen-year  guarantee  is  required.     With  stone  block,  brick 
and  most  other  forms  of  pavements  nine  months,  or  at  most  a  year, 
is  sufficient  to  reveal  any  serious  defect  of  material  or  workmanship, 
and  therefore  a  long  guarantee  is  not  necessary. 

655.  Maintenance  by  Contract.    As  stated  above  it  is  common  to 
require  a  so-called  guarantee  which  is  virtually  a  contract  for  main- 
tenance for  the  specified  period.     Maintenance  by  contract  is  justi- 
fiable if  the  engineering  department  of  the  city  does  not  possess, 
or  can  not  reasonably  be  expected  to  obtain,  the  information  neces- 
sary in  repairing  the  pavement;   but  as  a  rule  maintenance  by  con- 
tract is  undesirable,  for  four  reasons:   1.  The  contractor  has  no  con- 


334  PAVEMENT   ECONOMICS   AND   ADMINISTRATION       [CHAP.   XI 

trol  over  the  streets,  and  the  repairs  required  are  dependent  upon  the 
restrictions  against  opening  the  pavements  and  also  upon  the  reg- 
ulations for  keeping  the  streets  clean.  2.  It  is  difficult  to  specify 
beforehand  the  amount  and  the  nature  of  the  repairs  that  may  be 
required  by  the  ordinary  use  of  the  pavements,  particularly  as  the 
opening  of  new  streets  or  the  paving  of  others  may  materially  alter 
the  amount  or  nature  of  the  traffic  on  any  particular  pavement. 
3.  It  is  impossible  to  determine  accurately  the  condition  of  the  pave- 
ment at  the  end  of  the  contract  period.  4.  With  a  new  and  untried 
material  it  is  impossible  to  determine  what  is  a  reasonable  expense 
for  maintenance. 

A  contract  for  maintenance  is  sometimes  defended  by  the  prop- 
erty holders  on  the  ground  that  thereby  some  one  is  secured  who  is 
admittedly  responsible  for  the  condition  of  the  pavement  and  who 
is  more  amenable  for  neglect  than  are  city  officials.  However,  if 
the  city  officials  can  not  be  trusted  to  repair  the  pavements  directly, 
it  is  doubtful  whether  they  may  reasonably  be  expected  to  super- 
vise the  repairs  to  be  made  by  the  contractor.  The  choice  between 
maintenance  by  contract  and  by  municipal  authorities  directly 
will  usually  depend  upon  the  local  conditions. 

The  pavements  of  Paris,  France,  were  formerly  maintained 
by  contract,  but  are  now  maintained  by  the  city  directly. 

656.  TEARING  UP  PAVEMENTS.  The  most  serious  cause  of 
the  destruction  of  pavements  is  the  frequency  with  which  they  are 
torn  up  for  the  introduction  or  repair  of  underground  pipes,  conduits, 
etc.  No  pavement  has  been  introduced,  and  probably  none  ever 
will  be,  which  is  not  seriously  injured  by  being  torn  up.  With  care 
and  intelligence  a  pavement  may  be  replaced  in  nearly  its  former 
condition;  but  it  almost  never  is  so  replaced",  and  under  the  condi- 
tions which  such  work  is  done,  it  is  almost  impossible  to  get  it  so 
replaced.  The  only  remedy  for  the  frequent  disturbance  of  pave- 
ments is  the  construction  of  a  subway  in  which  to  place  pipes, 
wires,  etc. ;  but  it  is  doubtful  if  any  such  remedy  would  be  lasting  for 
the  streets  are  continually  being  put  to  new  uses.  Formerly  it  was 
thought  sufficient  to  provide  for  water  and  gas  pipes  and  sewers; 
while  now  conduits  are  required  for  telegraph,  telephone,  and  electric 
light  wires;  and  street-car  tracks  are  constructed  on  the  surface, 
above  the  surface,  and  below  the  surface;  and  in  some  cities  space  is 
required  for  pneumatic  tubes,  and  pipes  for  distributing  heat,  com- 
pressed air,  cold  and  hot  water,  etc. 

The  only  thing  that  can  be  done  is  to  reduce  the  opening  of  the 


ART.    2J  PAVEMENT   ADMINISTRATION  335 

pavements  absolutely  to  a  minimum,  and  then  to  take  the  utmost 
care  to  see  that  as  little  damage  as  possible  is  done  in  making  the 
opening  and  that  the  pavement  is  restored  in  the  best  way  possi- 
ble. A  few  years  ago  in  New  York  City  a  quarter  of  a  mile  of  trench 
was  opened  for  each  mile  of  pavement,  and  in  addition  there  was 
an  opening  for  each  35  linear  feet  of  street.  The  year  stated  was  about 
an  average  for  those  immediately  before  and  after.  In  Chicago 
in  1902  200,000  square  yards  of  pavements  were  taken  up  by  public 
service  companies,  which  is  equal  to  about  10  miles  of  pavement  36 
feet  wide;  or  in  other  words,  the  equivalent  of  one  sixth  of  all  the 
new  pavements  laid  in  the  city  in  that  year  was  torn  up  by  the  public 
utilities  corporations.  This  did  not  include  the  pavements  taken 
up  by  the  city  itself  to  lay  water  pipes,  sewers,  etc. 

The  amount  of  money  spent  in  digging  up  the  streets  is  a  con- 
siderable item,  not  counting  the  interference  with  travel  and  busi- 
ness; but  the  expense,  being  distributed  among  various  interests, 
is  not  usually  sufficient  to  cause  any  one  company  to  re-construct 
its  system.  It  is  probable  that  the  interests  of  the  public  are  fre- 
quently sacrificed  to  the  interests  of  the  private  companies  using 
the  streets — usually  without  paying  for  the  privilege. 

Under  the  best  municipal  administrations  of  Europe  neither 
corporations  nor  individuals  are  permitted  to  disturb  the  pave- 
ments. All  removals  and  restorations  are  done  by  the  city's  own 
employees,  upon  the  deposit,  by  the  parties  who  require  the  streets 
to  be  opened,  of  a  sufficient  sum  to  cover  the  expense  of  each  piece 
of  paving  done,  at  a  fixed  price  per  yard  according  to  the  kind  of 
pavement.  Moreover,  interference  with  the  pavements  is  of  rare 
occurrence,  for  the  companies  having  pipes  underground  are  require 
thoroughly  to  examine  and  reinstate  their  mains  and  services  con- 
currently with  the  paving  of  a  street,  due  notice  of  the  execution  of 
which  is  given  by  the  city. 

657.  Nearly  all  cities  have  ordinances  governing  the  opening  of 
pavements,  which  differ  greatly  in  character  and  severity;  but  gen- 
erally the  result  is  unsatisfactory  owing  to  the  real  difficulties  of  the 
case,  or  to  inefficient  administration,  or  to  unexpected  emergencies. 
There  is  also  great  variation  as  to  the  method  of  doing  the  back- 
filling, replacing  the  foundation,  and  re-laying  the  pavement,  and 
also  as  to  who  shall  do  the  work, — whether  city  departments,  private 
parties,  public  utility  company,  or  contractor;  and  again  none  is 
satisfactory.  This  is  a  serious  unsolved  problem  in  American 
municipal  administration. 


CHAPTER  XII 
STREET  DESIGN 

660.  From  the  point  of  view  of  future  needs — commercial,  sani- 
tary, and  esthetic, — it  is  unfortunate  that  cities  grow  up  by  successive 
additions  under  the  stimulus  of  private  greed  and  real  estate  spec- 
ulation, without  any  comprehensive  or  well  considered  street  plan. 
In  some  instances — notably  Paris,  London,  and  Boston, — vast  sums 
have  been  spent  to  correct  what  might  have  been  prevented  in  the 
original  plan  of  the  streets.*     In  most  cities  transformation — slow 
and  expensive,  if  it  comes  at  all — is  the  only  remedy;  but  a  mended 
article  is  never  as  good  as  one  originally  well  made,  f 

Unfortunately  there  are  few  cities  in  this  country  having  adequate 
regulations  governing  suburban  development.  Municipal  authori- 
ties should  regulate  the  street  plan  of  subdivisions  and  additions  so 
as  to  secure  a  harmonious  whole,  and  particularly  with  a  view  of 
making  the  streets  continuous  and  to  afford  suitable  channels  of 
communication.  Where  such  regulations  do  not  exist,  streets  will 
be  laid  out  in  such  a  way  as  best  to  develop  the  particular  property, 
regardless  of  the  interests  of  the  public.  Washington  City,  which 
has  the  best  street  plan  of  any  American  city,  has  been  disfigured 
by  ill-planned  additions,  although  at  present  stringent  rules  govern 
the  width  and  the  arrangement  of  the  streets  of  additions  and  sub- 
divisions. 

661.  STREET  PLAN.     Since  an  engineer  is   occasionally   called 
upon  to  plan  a  city,  and  often  to  lay  out  additions  to  cities  and  vil- 
lages, the  various  street  plans  for  a  city  will  be  considered.     In 
planning  the  streets  of  a  city  three  objects  should  be  kept  in  mind; 


*  For  example,  Paris  spent  $14,000,000  in  improving  the  Rue  de  Rivoli,  and  London  $33,000,- 
000  on  the  Strand  Improvement. 

t  For  an  elaborate  and  abundantly  illustrated  treatise  from  the  view  point  of  an  engineer, 
of  many  of  the  things  considered  in  this  chapter,  see  The  Planning  of Jthe  Modern  City,  Nelson 
P.  Lewis;  John  Wiley  &  Sons,  New  York,  1916. 

336 


STEEET   PLAN 


viz.:  (1)  the  subdivision  of  the  area  in  such  a  manner  as  to  give 
the  maximum  efficiency  for  business  or  residence  purposes;  (2) 
sufficient  accommodation  for  the  pedestrian  and  vehicle  travel 
on  the  streets;  (3)  good  drainage;  and  (4)  easy  communication 
between  the  different  parts  of  the  city. 

662.  Size  of  Lots.     Owners  in  subdividing  property  are  anxious 
to  make  as  many  lots  as  possible;  and  in  some  other  respects  small 
lots  are  to  be  preferred.     It  is  desirable  to  make  the  lots  of  such  a 
size  that  few  of  them  will  be  subdivided,  as  clearness  of  identity  in 
transferring  or  assessing  the  lot  is  maintained  by  always  referring  to 
the  original  number.     A  frontage  of  25  feet  seems  the  best.     This 
width  is  suitable  for  business,  purposes,  and  for  residence  streets 
two  or  more  lots  will  give  proper  grounds.     Business  lots  are  some- 
times made  only  18  or  20  feet  wide,  but  25  feet  is  by  far  the  more, 
common. 

Lots  are  seldom  less  than  100,  nor  more  than  180,  feet  deep; 
and  usually  vary  from  100  to  150  feet.  A  lot  more  than  150  feet 
deep  is  objectionable,  because  of  the  temptation  to  build  unsightly 
residences  fronting  on  the  alley  and  because  of  the  usual  indifference 
to  keeping  a  deep  lot  in  good  sanitary  condition. 

663.  Size   of  Blocks.     With   a   rectangular   system   of  streets, 
the  blocks  are  preferably  long  and  narrow;    since  the  distance  re- 
quired between  streets  in  one  direction  is  only  that  necessary  to 
give  the  proper  depth  of  lots,  while  in  the  other  direction  the  streets 
need  be  only  close  enough  to  provide  convenient  channels  for  the 
traffic. 

For  convenience,  especially  in  business  districts,  it  is  best  to 
have  an  alley  run  lengthwise  through  the  block.  The  alley  varies 
from  10  to  30  feet,  but  is  usually  from  16  to  20  feet. 

The  above  depth  of  lot  and  width  of  alley  makes  the  width  of 
the  block  220  to  330  feet.  The  length  of  the  block  will  depend  upon 
the  requirements  for  traffic  perpendicular  to  the  principal  streets. 
Sizes  of  blocks  vary  much  in  any  particular  city,  and  still  more 
between  different  cities.  The  following  are  the  dimensions  of  typical 
blocks  in  several  cities:  Boston,  220X400  feet,  and  100X550  feet; 
New  York,  200X900  feet,  and  200X400  feet;  Philadelphia,  400X550 
feet,  and  500X800  feet;  Washington,  400X600  feet,  and  300X800 
feet;  Montreal,  250X750  feet;  Chicago,  300X350  feet,  and  300X500 
feet. 

Fig.  103,  page  338,  illustrates  the  advantages  to  be  derived  from 
a  careful  study  of  the  best  size  of  blocks  and  of  the  most  advan- 


STREET   DESIGN 


[CHAP,  xil 


tageous  arrangement  of  streets.  The  left-hand  side  of  the  diagram 
shows  the  typical  arrangement  of  streets  and  blocks  in  the  residence 
district  of  New  York  City,  the  shaded  portions  representing  the 
usual  buildings.  The  right-hand  side  shows  a  much  superior  arrange- 
ment.* The  three  center  blocks  of  the  present  plan  comprise  an 
area  of  720X800  feet,  and  contain  480,000  square  feet  of  building 
area  and  96,000  square  feet  of  streets,  and  in  the  corresponding  area 
of  the  proposed  plan,  there  are  481,000  square  feet  of  building  area 


FIG.  103. — IMPROVED  ARRANGEMENT  OF  STREETS  AND  BLOCKS. 


and  94,200  square  feet  of  streets;  therefore  the  two  plans  give  sub- 
stantially  the  same  area  for  buildings  and  for  streets.  In  the  first 
case  the  length  of  streets  is  1600  feet,  in  the  second  1520  feet; 
therefore  the  two  plans  have  practically  equal  light  and  air.  The 
proposed  arrangement  is  the  better  in  the  following  particulars: 
1,  number  of  corner  sites;  2,  accessibility  of  rear  entrances  for  delivery 
of  provisions,  coal,  etc.,  and  the  removal  of  garbage,  ashes,  etc.,  and 
in  case  of  fire;  3,  removal  from  the  street  of  dangerous  and  cramped 


*  Proposed  by  Mr.  J.  F.  Harder,  in  Municipal  Affairs,  Vol.  2,  p.  41-44      Reform  Club 
New  York  City,  1898. 


STREET  PLAN  33Q 


cellar  entrances;  4,  removal  from  the  main  or  primary  streets  of  the 
loading  and  unloading  of  trucks;  and  5,  increased  transportation 
facilities  in  a  direction  perpendicular  to  the  length  of  the  original 
blocks. 

664.  Location  of  Streets.     In  planning  a  system  of  streets  there 
are  two  objects  that  should  be  carefully  considered,  viz. :  the  drain- 
age and  easy  communication  between  the  different  sections  of  the 
city.     Not  infrequently  these  elements  have   been  overlooked  or 
neglected.     The  surface  drainage,  the  sewerage  and  the  travel  must 
follow  the  general  slope  of  the  land;   and  therefore  if  there  is  much 
irregularity  of  contour  in  the  site,  a  location  of  the  streets  with 
reference  to  the  contours  will  afford  at  once  the  best  drainage  and 
the  easiest  communication  between  different  parts  of  the  city.     If 
the  site  is  nearly  level,  the  relationship  between  the  slope  of  the  land 
and  the  direction  of  the  streets  is  comparatively  unimportant;   but 
the  arrangement  of  the  street  plan  to  afford  the  greatest  facilities  for 
communication  between  the  different  parts  of  the  city  is  still  an 
important  matter.     Therefore  the  conclusion  is  that  on  a  site  of 
irregular  contour  the  streets  should  be  located  with  reference  chiefly 
to  the  topography,  and  on  a  level  site  primarily  to  secure  the  most 
direct  and  easiest  intercommunication. 

665.  Location  with  Reference  to   Topography.     Unfortunately  in 
this  country  our  very  desirable  rectangular  system  of  public  land 
survey  has  frequently  led  to  the  adoption  of  a  very  undesirable  rect- 
angular system  of  streets  which,  though   convenient  for  dividing 
property  into  the  greatest  number  of  rectangular  lots  upon  which 
can  be  built  the  greatest  number  of  rectangular  buildings,  has  little 
else  to  recommend  it.     Surface  drainage  sewerage  and  travel  should 
follow  the  slope  of  the  country,  and  any  attempt  to  deviate  from  this 
becomes  a  serious  question  in  the  building  of  a  city  upon  any  but 
nearly  level  ground.     The  streets  are  of  necessity  the  drainage  lines 
of  the  city  and  should  be  placed  in  the  natural  valleys,  and  the  failure 
so  to  locate  the  streets  in  many  cities  where  the  land  is  very  irregular 
in  contour  has  led  to  great  expense  in  the  construction  of  the  streets 
and  of  a  system  of  storm-water  sewers. 

The  upper  half  of  Fig.  104,  page  340,  shows  an  actual  case  of  a 
system  of  rectangular  streets  located  without  any  reference  to  the 
topography;  and  the  lower  half  of  the  same  diagram  shows  a  pro- 
posed arrangement  *  that  would  save  much  expense  in  grading  the 

*  By  W.  D.  Elder  in  Proc.  Michigan  Engineering  Society,  1898,  p.  52. 


340 


STREET   DESIGN 


[CHAP.   XII 


streets  and  at  the  same  time  give  a  quick  entrance  into  the  center 
of  the  city,  and  also  give  long  easy  grades  from  the  heart  of  the 
city  to  the  higher  outlying  district. 


Center  of 
City 

FIG.  104. — LOCATION  OF  STREETS  WITH  REFERENCE  TO  CONTOUBS. 


STREET   PLAN  341 


666.  The  original  rectangular  street  system  of  San  Francisco 
was  laid  out  without  much  attention  to  the  resulting  street  grades, 
some  of  which  are  55  per  cent.     As  rapidly  as  possible  these  excessive 
grades  are  being  reduced.     Recently  $30,000  was  spent  to  reduce  the 
grade  from  29  to  16  per  cent  through  one  block.     The  cost  was 
something  like  15  per  cent  of  the  value  of  the  abutting  property. 
This  extreme  case  involved  several  unique  but  expensive  features.* 

667.  Location   with   Reference   to    Directness   of  Communication. 
There  are  three  distinct  general  plans  for  city  streets  with  refer- 
ence to  directness  and  ease  of  communication. 

668.  One  consists  of  a  system  of  parallel  streets  crossing  a  similar 
system  at  right   angles.     This  is  often   called   the   checker-board 
system,  but  more  properly  the  rectangular  system,  since  the  blocks 
are  not  necessarily  squares.     This  arrangement  gives  the  maximum 
area  for  blocks,  and  also  furnishes  blocks  of  the  best  form  for  sub- 
division into  lots.     The  rectangular  system  is  the  most  common, 
and  has  its  most  marked  exemplification  in  Philadelphia. 

669.  A  second  arrangement  of  streets  consists  of  the  rectangular 
system  with  occasional  diagonal  streets  along  the  lines  of  maximum 
travel.     This  system  was  employed  by  L 'Enfant  in  planning  the  city 
of  Washington.     Fig.  105,  page  342,  shows  a  portion  of  that  city.     To 
a  limited  degree,  the  same  plan  was  adopted  in  laying  out  the  city 
of  Indianapolis,  which  has  four  broad  diagonal  avenues  converging 
to  a  circular  park  in  the  center.     These  two  are  the  only  cities  of 
any  importance  in  which  this  system  was  adopted  in  advance  of 
building.     This  system  is  usually,  but  somewhat  improperly,  called 
the  diagonal  system. 

The  chief  advantage  of  the  diagonal  street  is  the  economy  due 
to  the  saving  of  distance  by  traversing  the  hypothenuse  instead  of 
the  two  sides  of  a  right  triangle.  In  Rome,  london,  Paris,  and 
in  numerous  other  smaller  places  in  Europe,  whole  districts  have 
been  razed  to  make  way  for  new  streets  to  serve  as  arteries  for  in- 
creased traffic. 

A  second,  and  by  no  means  an  unimportant,  advantage  of  the 
combination  of  the  diagonal  and  the  rectangular  system  is  the  open 
squares  and  spaces  so  grateful  to  the  eye  and  of  no  little  sanitary 
value  in  compactly  built  cities.  New  York  City  has  recently  been 
spending  a  million  dollars  a  year  to  create  such  spaces  by  purchas- 
ing land  and  demolishing  the  buildings. 

*  Engineering  News,  Vol.  75  (1916),  p.  12-13. 


342 


STREET   DESIGN 


[CHAP,  xii 


vJuJt  _  l'/3l_JL_mgl  _  II  _  lOSfLll 


'J 


omaaaacnciaajannoanssaa  acrna 


ODflO 


anaczinao 


WIDTH   OF   STREETS    •  343 

Although  the  diagonal  avenue  occupies  ground  that  might 
otherwise  be  used  for  building  purposes,  there  is  a  compensating 
advantage  in  the  greater  length  of  street  front  obtained.*  In  many 
cases  the  total  cost  of  cutting  diagonal  streets  through  built-up 
districts  has  been  paid  by  the  increased  value  of  the  property  on  and 
near  the  street  thus  opened  up. 

670.  The  third  arrangement  of  city  streets  is  the  ring  or  concen- 
tric plan,  which  is  very  popular  in  Europe.     The  most  noted  example 
is  Vienna  with  its  Ring-strasse  (ring  street)  within  and  its  Gtirtel- 
strasse  (girdle  street)  without.     The  former  is  187  feet  wide  and 
encircles  the  public  buildings  and  the  leading  houses  of  business  and 
amusement.     The  enclosed  network  of  streets  intersect  the  Ring- 
strasse  at  forty  points,  and  outward  from  it  extend  fifteen  main 
radial  avenues. 

671.  WIDTH  OF  STREETS.     The  width  of  city  streets  is  important 
on  account  of  its  influence  upon  the  ease  with  which  traffic  may  be 
conducted  and  also  because  of  its  effect  upon  the  health  and  com- 
fort of  the  people  by  determining  the  amount  of  light  and  air  which 
may  penetrate  into  thickly  built-up  districts.     The  streets  of  nearly 
all  large  cities  are  too  narrow,  being  crowded  and  dark.     A  more 
liberal  policy  in  planning  streets  would  probably  be  of  pecuniary 
advantage,  since  there  is  usually  an  enhanced  financial  value  due 
to  wide  streets.     A  lot  100  feet  deep  on  a  street  80  feet  wide  is  usually 
more  valuable  than  a  lot  110  feet  deep  on  a  street  60  feet  wide; 
that  is  to  say,  within  reasonable  limits  land  is  usually  more  val- 
uable in  the  street  than  on  the  rear  of  the  lot.     Wide  streets  are 
especially  needed  where  they  are  bordered  by  high  buildings  or  are 
to  carry  street-railway  lines,  f 

In  order  properly  to  accommodate  the  traffic  in  business  dis- 
tricts of  cities  of  considerable  size,  a  street  should  have  a  width 
of  100  to  140  feet,  the  whole  of  it  being  used  for  roadway  and  side- 
walks; while  residence  streets  in  a  city  of  considerable  size,  where 
the  houses  are  set  out  to  the  property  line  and  stand  close  together 
should  have  a  width  of  60  to  80  feet.  Although  it  is  advantageous 
to  have  a  wide  street,  it  is  not  necessary,  nor  even  desirable  that 
the  whole  width  be  paved;  the  central  portion  may  be  paved,  a 


*  For  a  discussion  of  this  phase  of  the  subject,  see  an  article  by  L.  M.  Haupt  in  Jour.  Franklin 
Inst.,  Vol.  103,  p.  252. 

t  For  an  elaborate  and  instructive  article  on  this  subject  see  a  paper  by  John  Nolen  before 
the  1911  National  Conference  on  City  Planning,  Engineering  and  Contracting,  Vol.  35  (1911), 
p.  621-622. 


344  STREET  DESIGN  [CHAP.   XII 

strip  on  either  side  being  reserved  for  grass  plats.  The  width  of 
the  pavement  should  be  adjusted  to  the  amount  of  travel,  which 
varies  greatly  accordingly  as  the  street  is  a  business  street,  a  thor- 
oughfare, or  an  unfrequented  residence  street. 

The  width  of  the  streets  in  different  cities  varies  greatly.  In 
the  older  places  in  New  England  and  the  Central  States,  many  of 
the  streets  are  only  30  to  40  feet  wide;  but  in  the  West  a  street  is 
seldom  less  than  60  to  66  feet  wide.  In  both  regions  the  princi- 
pal streets  are  often  80  to  100  feet  wide,  and  in  many  of  the  larger 
cities  the  boulevards  and  great  avenues  are  150  to  180  feet.  The 
main  avenues  in  Washington  are  160  feet  wide,  in  New  York  135, 
and  in  Boston,  180  feet. 

At  present  the.  regulations  governing  the  width  and  the  arrange- 
ments of  additions  and  subdivisions  of  Washington,  a  city  which 
has  the  best  street  plan  of  any  in  America  (see  §  669)  are:  "  No 
new  street  can  be  located  less  than  90  feet  in  width,  and  the  lead- 
ing avenues  must  be  at  least  120  feet  wide.  Intermediate  streets 
60  feet  wide,  called  places,  are  allowed  within  blocks;  but  full- 
width  streets  must  be  located  not  more  than  600  feet  apart." 

672.  AREA  OF  STREETS.  The  proportion  of  the  area  of  the 
city  devoted  to  streets  varies  greatly,  particularly  between  the 
older  and  the  newer  cities.  The  following  is  the  per  cent  of  street 
area  in  a  few  extreme  cases  of  American  cities :  * 

MINIMUM  STREET  AREA  MAXIMUM  STREET  AREA 

1.  Taunton,  Mass 3.20  per  cent  Duluth,  Minn 86.7  per  cent 

2.  Worcester,  Mass. ...  5.43     "  "  Dallas,  Tex 78.3     "  " 

3.  Binghamton,  N.  Y.  .  7.55     "  "  Denver,  Colo 73.9     "  " 

4.  Philadelphia,  Pa. ...  8.42     "  "  Indianapolis,  Ind 56.4     "  " 

5.  Boston,  Mass 8.76     "  "  Washington,  D.  C 43.5     "  " 

6.  Lowell,  Mass 8.92     "  "  Davenport,  la 42. 1     "  " 

7.  Fall  River,  Mass 9.17     "  "  Evansville,  Ind 40.8     "  " 

The  area  devoted  to  streets  and  alleys  in  a  few  of  the  principal 
cities  of  the  world  is  as  follows : 

AREA  OF  STREETS  AND  ALLEYS 

1.  Washington 54  per  cent 

2.  Vienna 35  «  « 

3.  New  York  City 35  «  « 

4.  Philadelphia 29  "  " 

5.  Boston 26  "  " 

6.  Berlin 26  "  " 

7.  Paris .25  "  " 

*  Census  Bulletin  No.  100— July  22,  1891,— p.  16. 


WIDTH   OF   PAVEMENTS  345 


673.  WIDTH  OF  PAVEMENTS.     It  is  wise  to  make  the  streets  of 
residence  districts  of  liberal  width  for  sanitary  and  esthetic  reasons; 
and  also  because  the  future  of  the  street  can  not  be  certainly  fore- 
seen,— the  residence  street  may  become  a  business  street,  or  an 
unfrequented  street  a  thoroughfare.     However,  it  is  not  necessary 
that  the  whole  width  should  be  devoted  to  wheelways  and  side- 
walks, particularly  in  small  cities.     A  grass  plat  between  the  side- 
walk and  the  pavement,  in  which  shade  trees  are  set  (§  696),  adds 
to  the  beauty  of  the  street  and  to  the  comfort  of  the  residents  by 
removing  the  houses  farther  from  the  noise  and  dust  of  the  pave- 
ment.    The  grass  plat  or  parking  also  affords  an  excellent  place  in 
which  to  place  water  and  gas  pipes,  telephone  and  electric-light 
conduits,  etc.     In  large  cities  where  the  street  front  is  built  up  solid 
with  houses  of  several  stories,  it  may  be  necessary  to  dispense  with 
the  grass  plat,  and  to  devote  the  entire  street  to  sidewalks  and 
roadway. 

It  is  universally  admitted  that  pavements  are  desirable;  but 
often,  owing  to  the  unwillingness  of  at  least  some  of  the  people  to 
pay  for  them,  it  is  difficult  to  secure  them.  Except  for  the  cost, 
the  wider  the  pavement  the  better;  but  length  is  more  valuable  than 
width.  An  excessive  width  is  a  needless  expense,  and  delays  or 
prevents  the  getting  of  any  pavement  at  all;  hence  one  help  toward 
securing  pavements  is  to  make  the  pavement  only  wide  enough  to 
accommodate  the  traffic.  Not  infrequently  the  pavements  of 
suburban  and  residence  streets  are  needlessly  wide.  A  narrow 
pavement  not  only  costs  less  to  construct,  but  also  costs  less  to 
clean  and  maintain;  while  the  cost  of  maintenance  depends  chiefly 
(or,  with  a  pavement  not  subject  to  natural  decay,  wholly)  upon 
the  amount  of  traffic,  and  hence  is  nearly  (or  entirely)  independent 
of  the  width. 

674.  Without  Car  Track.     A  width  of  18  feet  affords  sufficient 
room  for  a  vehicle  to  pass  when  another  is  standing  on  each  side 
of   the   pavement — a   rare   occurrence; — and   therefore   it   appears 
that  a  pavement  18  feet  wide  is  sufficient  for  the  less  frequented 
residence  streets.     The  only  objection  to  a  very  narrow  pavement 
is  the  difficulty  of  turning  a  vehicle  in  such  a  street.     The  serious- 
ness of  this  objection  depends  upon  the  construction  of  the  vehicle. 
Many  delivery  wagons,  express  wagons,  etc.,  may  be  turned  on  an 
18-foot  pavement.     If  occasionally  a  vehicle  is  compelled  to  go  to 
the  corner  turn,  to  or  even  to  drive  around  the  block,  the  incon- 
venience is  not  very  serious,  and  is  so  infrequent  as  not  to  justify 


346  STREET   DESIGN  [CHAP.   XII 

any  considerable  expense  to  prevent  it.  A  width  of  20  to  24  feet  is 
probably  sufficient  for  a  majority  of  residence  and  suburban  streets. 
When  a  residence  street  is  an  artery  of  travel,  it  may  be  necessary 
to  make  the  pavement  wider  than  stated  above.  In  a  number 
of  cities,  there  has  been  a  marked  tendency  in  recent  years  to  reduce 
the  width  of  pavements  on  residence  streets. 

Thirty  feet  affords  sufficient  room  for  two  vehicles  to  pass  each 
other  where  two  others  are  standing  at  the  curb;  and  therefore 
this  width  of  pavement  is  ample  for  business  streets  in  small  places. 
On  a  narrow  business  street  it  may  be  necessary  to  curtail  the  width 
of  the  pavement  to  prevent  the  sidewalk  space  from  being  unduly 
encroached  upon. 

In  many  of  the  cities  the  width  of  the  pavement  is  uniformly 
a  fractional  part  of  the  total  width  of  the  street,  regardless  of  the 
needs  of  traffic.  In  many  cities,  both  American  and  European, 
the  pavement  is  three  fifths  or  60  per  cent  of  the  width  of  the  street. 
In  New  York  City  and  Brooklyn  the  rule  seems  to  be  to  make  the 
pavement  half  the  width  of  the  street.  In  Washington  City  there 
is  no  hard-and-fast  rule,  but  the  following  is  the  usual  relation: 
on  streets  60  feet  wide  or  less,  the  pavement  is  25  feet  or  40  per  cent 
of  the  width  of  the  street;  on  streets  from  60  to  90  feet  wide,  the 
pavement  is  25  to  35  feet,  or  40  per  cent;  and  for  streets  130  to  160 
feet  wide,  the  pavement  is  40  to  50  feet,  or  30  per  cent. 

675.  With  Car  Track.  For  a  residence  street  containing  a  car 
track,  the  minimum  width  permissible  is  28  feet,  which  will  allow 
a  car  to  pass  with  a  vehicle  on  each  side  of  the  track.  In  Brooklyn 
a  great  many  streets  only  34  feet  wide  between  curbs  contain  a 
double  line  of  street-car  tracks,  which  leaves  a  space  of  only  9£ 
feet  between  the  track  and  the  curb.  This  is  astonishingly  small, 
but  seems  to  do  fairly  well. 

On  a  business  street  containing  a  car  track,  it  is  wise  to  make 
the  pavement  wide  enough  to  permit  a  vehicle  to  pass  between 
the  car  and  another  vehicle  standing  at  the  curb.  This  will  require 
about  48  feet.  If  the  street  is  too  narrow  to  permit  this  width 
of  pavement  and  also  the  proper  width  of  sidewalks,  only  one  track 
should  be  allowed  in  the  street;  if  a  double  track  is  necessary 
the  cars  should  be  required  to  make  the  return  trip  by  another 
street. 

At  Rochester,  N.  Y.,  the  car  tracks  on  residence  streets  are 
located  on  the  parking  at  the  side  of  the  street.  This  is  an  unusual 
arrangement,  but  it  possesses  some  advantages.  1.  It  separates 


STREET   GRADES  347 


the  vehicle  and  car  traffic,  and  prevents  mutual  interference.  2. 
It  permits  a  narrower  pavement.  3.  It  prevents  disturbance 
of  the  pavement  to  repair  the  car  track.  4.  It  lessens  the  danger 
of  a  passenger's  being  struck  by  another  car  or  a  vehicle  in  leaving 
a  car.  The  objection  to  this  arrangement  is  that  it  interferes  with 
the  grade  of  the  driveways  to  private  grounds. 

676.  STREET  GRADES.     The  fixing  of  street  grades  is  one  of 
the  most  important  functions  of  municipal  engineering,  since  the 
grade  system  of  the  streets  is  the  foundation  of  all  municipal  engi- 
neering  matters.     The   grades    should    be   established    before   the 
sewer  system  is  planned;    and  if  they  are  established  before  the 
property  is  improved  the  problem  is  comparatively  simple,  since 
they  may  be  laid  chiefly  with  reference  to  obtaining  within  proper 
limits  of  cost  desirable  gradients  for  the  street.     But  when  build- 
ings have  been  erected,  sidewalks  constructed  and  trees  planted,  it  is 
often  extremely  difficult  to  secure  grades  which  will  harmonize  the 
various  and  conflicting  interests. 

677.  Elements     Governing    Grades.     The    grades    necessarily 
depend  mainly  upon  the  topography  of  the  site;   but  in  general  the 
determination  of  the  proper  grade  for  a  street  requires  the  consid- 
eration of   the   following  elements:    (1)  the   drainage,   (2)  the  cost 
of    earthwork,    (3)    the    accommodation    of    travel,    (4)    the  effect 
upon  the  abutting  property,  and  (5)  the  general  appearance  of  the 
street. 

678.  Drainage.     The  streets  are  the  natural  drainage  channels 
of  the  city;    the  lots  must  drain  into  them,  and  the  house  must 
drain  into  the  sewers  placed  in  the  streets.     When  no  storm-water 
sewers  are  to  be  constructed,  the  grades  become  very  important, 
since  the  streets  must  provide  for  the  surface  drainage  of  the  city, 
and  particular  consideration  must  be  given  to  relative  grades  and 
gutter  capacities  in  order  to  prevent  the  excessive  concentration 
of  storm  water  at  the  lower  levels  and  to  provide  for  its  proper 
distribution  and  disposal. 

679.  Cost  of  Earthwork.     Not  infrequently  the  cost  of  making 
the  excavations  and  embankments  is  given  undue  weight.     The 
balancing  of  cuts  and  fills  is  often  properly  a  controlling  element 
in  country  road  construction,  but  it  should  have  relatively  little 
weight  in   determining  the  grades   of  city  streets.     The   expense 
for  earthwork  is  incurred  once  for  all,  and  a  few  hundred  dollars 
more  or  less  is  usually  unimportant  in  comparison  with  the  expense 
of  maintaining  the  street  surface  and  the  drainage  system,  and 


348  STREET   DESIGN  [CHAP   XH 

the  cost  of  conducting  traffic  over  the  grades,  and  also  in  com- 
parison with  a  better  general  appearance  of  the  street. 

680.  Accommodation  of  Travel.    The  question  often  is  whether 
or  not  to  secure  ease  of  traction  at  the  expense  of  increased  cost  of 
construction.     The  discussion  in  Chapter  II,  §  65-86,  sheds  a  little 
light  and  only  a  little,  as  to  the  proper  method  of  answering  this 
question.    Apparently  engineers  are  inclined  to  overestimate  the 
disadvantage  to  travel  of  a  slight  grade.     Practical  experience  has 
demonstrated  that  there  is  not  much  difference  in  effect  upon  the 
cost  of  transportation  between  level  roads  and  those  having  grades 
of  2  or  3  per  cent  unless  such  grades  are  very  long  or  have  an  unusu- 
ally smooth  surface. 

681.  Effect  upon  Abutting  Property.     The  private  interests  of 
the  property  holder  should  be  carefully  considered;    although  it 
is  frequently  impossible  to  establish  proper  grades  without  injury 
to  the  adjoining  property.     The  general  question  is  how  far  private 
interests  should  be  sacrificed  to  the  general  good.      It  is  better  that 
the  city  or  the  other  residents  on  the  street  should  pay  the  owner 
damages  than  that  lasting  detriment  should  be  done  to  the  appear- 
ance of  the  street  or  to  the  traffic. 

682.  General   Appearance.     Some  attention   should  be   paid   to 
the  appearance  of  a  longitudinal  view  of  the  pavement.      It  is  desir- 
able that  the  longitudinal  grade  be  not  changed  so  frequently  as 
to  give  the  street  a  wavy  appearance.     Further,   the   transverse 
grades  at  street  intersections  and  on  side  hills  should  be  so  arranged 
as  not  to  produce  a  confused  appearance  in  looking  along  the  street. 
The  grades  of  the  streets,  both  longitudinal  and  transverse,  have  a 
material  effect  upon  the  general  appearance  and  beauty  of  the  city. 

683.  Maximum  Grade.     In  a  general  way  the  principles  gov- 
erning the  determination  of  the  permissible  maximum  grade  of  a 
city  street  are  the  same  as  for  a  country  road,  i.  e.,  it  is  a  question 
between  the  cost  of  operation  on  the  one  hand  and  the  cost  of  con- 
struction and  maintenance  on  the  other,  except  that  for  a  country 
road  the  cost  of  construction  is  chiefly  the  cost  of  moving  the  earth, 
while  for  a  city  street  the  cost  of  construction  should  also  include 
the  effect  upon  abutting  property  of  high  embankments  or  deep 
excavations,   and  except  further  that  usually  in  the  city  heavy 
loads  can  take  a  circuitous  route  and  avoid  the  maximum  grade 
entirely.     In   determining  the  maximum   grade   for   a   street,    the 
fact  should  not  be  overlooked  that  the  smoother  the  pavement  the 
more  serious  is  a  steep  grade. 


STREET   GRADES  349 


684.  In  the  Borough  of  Manhattan,  New  York  City,  are  some 
business  streets  having  grades  as  steep  as  6  per  cent,  and  a  num- 
ber of  residence  streets  have  10  per  cent  grades,  and  some  have 
grades  of  12,  15  and  18  per  cent.  Brooklyn,  N.  Y.,  has  4  per  cent 
grades  on  business  streets  and  12  on  residence  ones.  A  number 
of  cities  have  maximum  grades  on  paved  streets  of  20  per  cent — for 
example,  Worcester,  Mass..  Syracuse,  N.  Y.,  Borough  of  Rich- 
mond, New  York  City,  and  Pittsburg,  Pa.  Burlington,  Iowa,  has 
an  80-foot  street  with  a  24  per  cent  grade  up  which  is  laid  a  zigzag 
brick  pavement  18  feet  wide  having  a  maximum  grade  of  14  J  per 
cent  with  a  minimum  radius  of  the  inside  curb  of  16  feet.  San 
Francisco  has  some  extremely  steep  street  grades,  for  one  example 
see  §  666. 

For  a  discussion  of  the  maximum  grade  for  each  kind  of  pave- 
ment, see  the  heading  Maximum  Grades  in  the  chapter  treating  that 
particular  pavement. 

It  is  usually  considered  that  a  grade  steeper  than  15  per  cent 
is  impracticable  and  dangerous  even  for  light  traffic;  and  there- 
fore if  this  grade  can  not  be  obtained,  the  street  should  be  divided 
into  two  parts  separated  by  a  terrace  or  stone  wall,  each  portion 
being  entered  only  at  its  intersection  with  the  cross  street — see  Fig. 
118,  page  358.  A  10  per  cent  grade  is  usually  considered  prohibitive 
for  heavy  loads;  and  5  or  6  per  cent  is  considered  the  limit  on  busi- 
ness streets. 

685.  The  selection  of  the  proper  pavement  for  the  maximum 
grade  is  a  matter  of  great  importance.     For  the  recommendations 
of  a  committee  of  the  American  Society  of  Civil  Engineers  concerning 
maximum  permissible  grades,  see  Table  15,  page  57.     It  is  usually 
held  that  sheet  asphalt  should  not  be  laid  on  grades  steeper  than  2  to  3 
per  cent,  although  it  has  often  been  laid  on  6  or  7  per  cent  grades, 
and  in  one  instance  on  a  17  per  cent  grade  (see  §  887).     Brick,  or 
hard  sandstone,  or  granite  may  be  used  upon  the  maximum  grade. 
The  sandstone  and  the  granite  blocks  should  be  narrow  and  should 
be  of  a  quality  that  does  not  wear  smooth.     It  has  been  recom- 
mended to  chamfer  the  corners  of  rectangular  stone  or  wood  blocks 
when  laid  upon  steep  grades,  to  give  the  horses  a  good  foot-hold; 
but  it  is  at  least  doubtful  whether  the  benefit  of  a  good  footing  is 
not  neutralized  by  the  increased  tractive  resistance.     The   joints 
should  be  filled  with  tar  or  hydraulic  cement. 

686.  Minimum  Grade.    The  street  surface  should  have  enough 
longitudinal  slope  to  drain  its  surface  well.    With  a  smooth  and 


350 


STREET   DESIGN 


[CHAP.    XII 


impenetrable  pavement  no  ruts  will  be  formed,  and  hence  the 
determination  of  the  minimum  permissible  grade  is  mainly  a 
question  of  the  grade  of  the  gutter.  If  the  drainage  is  carried 
away  by  underground  storm-water  sewers,  the  street  may  be 
perfectly  level  longitudinally,  since  the  necessary  grade  for  the 
gutters  may  be  obtained  by  making  them  deeper  as  they  approach 
the  inlet  to  the  sewer.  For  a  further  discussion  of  this  phase  of  the 
subject,  see  Grade  of  Gutter — §  711. 

If  it  is  inexpedient  to  vary  the  depth  of  the  gutter  (§  710)  or  to 
increase  the  grade  by  constructing  additional  inlets  and  catch  basins, 
it  is  necessary  to  secure  the  proper  slope  for  the  gutter  by  inserting 
a  summit  in  the  street  solely  for  drainage  purposes — usually  referred 
to  as  an  accommodation  summit.  However,  it  is  undesirable  that 
there  should  be  frequent  changes  in  the  grade,  as  they  give  the 
pavement  an  unpleasant  wavy  appearance  when  one  looks  along 
the  street. 

687.  Elevations  at  Street  Intersections.  One  of  the  most  impor- 
tant parts  of  the  establishment  of  a  system  of  street  grades  is  the 

arrangement  of  the  grades 
W/fo  at  street  intersections.    It 

is  a  common  practice  to 
establish  only  the  eleva- 
tion of  the  intersection  of 
the  center  lines  of  the 
streets;  but  this  often  re- 
sults in  much  confusion  in 
determining  the  elevation 
for  the  curb  at  the  corner, 
particularly  where  the  two 
streets  have  considerably 
different  grades.  For  ex- 
ample, in  Fig.  106,  assum- 
ing (for  the  present  at 
least)  that  the  curb  is  to 
be  at  the  same  elevation 

as  the  center  of  the  street  opposite,  the  elevation  of  the  corner  of 
the  curb,  D,  as  computed  from  the  grade  of  CB  is  90.20  feet; 


(< 2?' 


Curb 


..L 


Center  Line  ofSfreef 

FIG.  106. — ELEVATION  OF  CURB  AT  CORNER. 


while  the  elevation  of  the  same  point  as  computed  from  the  grade 
of  BA  is  91.20  feet— a  difference  of  1.0  foot.  To  obviate  this 
source  of  confusion,  the  elevation  of  each  corner  of  the  curb  and 
also  of  the  intersections  of  the  center  lines  should  be  established, 


STREET   GRADES 


351 


•10- 


A  similar  confusion  occurs  in  attempting  to  compute  the  elevation 
of  the  corner  of  the  property,  from  the  grade  of  the  corner  of  the 
curb.  For  example,  in  Fig.  107,  assuming  that  the  grade  of  the  top 
of  the  curb  is  the  same  as  that  of  the  center  of  the  street,  and  assum- 
ing that  the  sidewalk  has  a  downward 
slope  away  from  the  property  of  0.24 
inch  per  foot  (2  per  cent),  and  also 
assuming  that  the  grade  of  the  corner 
of  the  curb,  D,  has  been  established 
as  80.00,  then  the  elevation  of  the 
corner  of  the  property,  G,  as  com- 
puted from  the  grade  of  the  curb.  DE 

80.30  feet,  while   the  elevation   of 


IS 


Q0.3O 


6O.IO 


Curb 


FIG.  107. — ELEVATION  AT  CORNER  OF 
PROPERTY. 


the  same   point   computed   from   the 
grade  of  the  curb  DF  is  80.80  feet. 

Some  engineers  advocate  estab- 
lishing the  elevation  of  the  corner  of 
the  property  and  the  determination  of 

the  grades  of  the  curb  and  of  the  street  therefrom;  while  others 
advocate  establishing  the  elevation  of  the  corner  of  the  curb  and  from 
that  determining  the  elevation  of  the  corner  of  the  property  and  also 
of  the  center  of  the  street  intersection.  To  be  legal  the  elevation 
must  be  fixed  by  ordinance.  The  courts  hold  that  the  "  elevation  '* 
is  the  top  of  the  pavement  in  the  center  of  the  street;  therefore 
it  is  necessary  to  establish  by  ordinance  the  elevation  of  the  center 
of  the  street  intersection.  Further,  to  prevent  misapprehension  and 
error  in  computing  the  elevation  of  the  corners  of  the  curbs,  and  also 
to  save  the  labor  of  computing  them  anew  each  time  a  lot  is  to  be 
surveyed,  it  is  wise  to  establish  also  the  elevation  of  the  corner  of 
the  curb.  The  ordinance  should  distinctly  state  the  method 
to  be  employed  in  computing  the  auxiliary  elevations  of  the 
sidewalk  and  of  the  corner  of  the  property.  Often  the  grades  are 
established  for  only  one  street  without  due  consideration  of  the 
intersecting  street;  and  then  when  the  second  street  is  improved, 
the  result  is  confusion,  disputes,  and  sometimes  suits  for  damages. 

688.  When  the  rate  of  grade  of  both  streets  is  small,  it  is  desir- 
able that  the  entire  street  intersection  from  property  line  to  prop- 
erty line  should  be  level,  a  condition  which  permits  the  continuation 
of  the  section  of  each  roadway  until  they  intersect,  makes  the  top 
of  the  curb  at  the  four  corners  of  the  same  elevation,  and  also  allows 
the  sidewalks  at  the  corners  to  be  level.  That  is  to  say,  in  Fig.  108, 


352 


STREET  DESIGN 


[CHAP.   XII 


the  four  points  marked  b  and  all  the  points  marked  a  are  in  the 
same  horizontal  plane.  Each  street  has  its  full  crown  on  the  line 
bb,  and  consequently  there  is  a  slight  rise  from  b  to  c. 

Where  either  or  both  streets  have  much  inclination,  it  may  not 
be  wise  to  flatten  out  the  intersection,  and  thereby  increase  the 
grade  on  the  remainder  of  the  street.  Under  these  conditions, 
the  best  arrangement  of  the  intersection  is  a  matter  requiring 
careful  study  and  is  one  upon  which  there  is  much  diversity  of  opin- 
ion. If  steep  grades  are  continued  across  intersections,  they  intro- 
duce side  slopes  in  the  streets  thus  crossed,  which  are  troublesome 
and  possibly  dangerous — particularly  to  vehicles  turning  the  upper 
corners.  Such  intersections  are  also  objectionable  on  account  of 

the  difficulty  of  properly  caring 
for  the  storm  water.  In  resi- 
dence districts  it  is  usual  to  make 
the  intersection  "  level  from  curb 
to  curb";  that  is,  in  Fig.  lb8,  the 
four  points  marked  b  are  in  the 
same  horizontal  plane.  The 
level  places  serve  as  breathing 
places,  and  lessen  the  danger  of 
collision  at  the  intersection. 
However,  if  the  street  has  a  con- 
siderable grade,  a  level  intersec- 
tion appears  to  have  a  decided 
pitch  toward  the  hill,  which 
gives  the  street  an  unpleasing 

appearance;  and  therefore  under  these  conditions,  it  is  better  to 
apply,  even  in  residence  districts,  the  principle  of  the  succeeding 
paragraph  and  give  the  intersection  a  moderate  inclination  down 
hill.  If  the  intersection  has  only  enough  inclination  to  seem  level, 
the  general  appearance  of  a  series  of  such  intersections  is  pleasing 
having  the  effect  of  a  succession  of  terraces. 

The  following  rule  *  for  adjusting  the  grades  at  street  inter- 
sections is  frequently  employed  and  apparently  is  the  most  com- 
plete of  any  that  has  been  proposed.  "  In  the  business  section  all 
the  street  grades  of  3  per  cent  or  less  should  be  continued  unbroken 
over  the  intersection;  and  streets  having  a  steeper  grade  than  3 


Fio.  108. — ELEVATIONS  AT  LEVEL  STREET 
INTERSECTION. 


*  Proposed  by  Messrs.  Rudolf  Hering  and  Andrew  Rosewater  for  the  streets  of  Dulutht 
Minn.,  in  a  report  dated  March  7,  1890.  Engineering  News,  Vol.  25,  p.  148-49;  Engineering 
Record,  Vol.  22,  p.  53. 


STREET    GRADES 


353 


FIG.  109. — ELEVATIONS  AT  INCLINED  STREET 
INTERSECTION. 


per  cent  should  have  an  intersection  of  3  per  cent  between  curb  lines. 

The  grade  of  the  curb  between  the  other  curb  line  and  the  property 

line  should  in  no  case  be  greater  than  8  per  cent.   The  elevation  at  the 

corner  of  the  property  should  be 

determined  by  adding  to  each  of 

the  elevations  of  the  curb  opposite 

the  corner,  the  rise  of  the  sidewalk 

and  taking  the  mean."     Fig.   109 

shows  the  elevations  of   a  street 

intersection  adjusted  according  to 

the    above    rules,     assuming    the 

transverse  slope  of  the  sidewalk  to 

be  2  per  cent   (practically  £  inch 

per  foot — the  usual  value). 

The  difficulty  of  adjusting  ele- 
vations at  an  intersection  is  con- 
siderably increased  if  the  two 
streets  do  not  intersect  at  right 

angles.  It  is  impossible  to  formulate  any  general  rule,  since  each 
case  must  be  decided  according  to  the  local  conditions.  Close 
observation  and  good  judgment  are  required  to  secure  a  reasonably 
satisfactory  adjustment. 

689.  Notice  that  if  either  street  has  a  grade  and  is  carried  past 
the  intersection  nominally  unchanged,  the  area  between  the  four 
curb  corners  and  that  immediately  adjacent  will  be  a  warped  surface. 

For  example,  in  Fig.  110,  if  the  street  S 
has  a  descent  as  indicated  and  the  street 
W  is  level,  and  the  unchanged  crowns  of 
the  street  intersect  at  C,  the  area  marked 
w  must  be  raised  to  carry  the  upper  side 
of  the  street  W  over  the  intersection,  and 
the  portions  marked  v  must  be  raised  to 
carry  the  street  S  over  the  lower  side  of 
the  street  W.  If  the  grade  of  either  street 
is  small  this  adjustment  can  be  made  by 
"  warping  in  "  or  ''boning  in  "  the  surface 
for  a  short  distance. 

Curves  at  Grade  Intersection.     It  is  frequently 
should  be  carried  straight  through  from 


FIG.  110.— A  WARPED  STREET 
INTERSECTION. 


690.  Vertical 

claimed  that  the 


grade 


street  intersection  to  street  intersection,  i.  e.,  that  the  grade  should 
not  be  broken  in  the  block.     Apparently  the  reason  for  this  practice 


354 


STREET   DESIGN 


[CHAP,  xii 


is  the  claim  that  a  break  of  grade  between  streets  is  unsightly.  As 
usually  put  in,  the  angle  of  intersection  is  simply  rounded  off  a 
little  by  eye;  and  if  the  change  of  grade  is  considerable,  the  appear- 
ance is  not  good.  A  change  of  grade  in  the  block  is  nowise  different 
from  a  change  at  the  street  intersection,  except  that  the  former  is 
a  little  more  conspicuous.  For  both  appearance  and  the  comfort 
of  the  travel,  wherever  there  is  considerable  change  of  grade,  the 
two  grade  lines  should  be  connected  by  a  vertical  curve;  and  if 
this  is  properly  done,  a  break  of  grade  in  the  block  or  elsewhere  is 
unobjectionable.  A  vertical  curve  should  be  inserted  at  a  change 
of  grade  either  of  the  pavement  or  of  the  curb. 

By  breaking  grade  in  the  block,  it  is  possible  to  fit  the  grade 
line  more  closely  to  the  natural  surface,  and  thereby  to  decrease  the 
cost  of  construction,  to  lessen  the  damage  to  abutting  property,  and 
to  improve  the  general  appearance  of  the  street. 

691.  A  parabola  is  the  best  form  for  a  vertical  curve  and  is 
most  easily  put  in.  In  Fig.  Ill,  AB  and  AC  represent  two  grade 


Fio.   111.  —  VERTICAL  CURVE. 

lines  meeting  in  the  apex  A,  joined  by  the  vertical  parabola  B  C, 
which  is  tangent  to  the  straight  grade  line  at  B  and  C.  The  curve 
may  be  located  by  measuring  ordinates  vertically  below  the  points 
1,  2,  3,  etc.  The  tangent  distances  A  B  and  A  C  are  equal.  D  E 
is  equal  to  the  rise  in  half  the  length  of  the  curve,  i.  e.,  from  B  to 
A]  and  D  C  is  equal  to  the  fall  in  the  second  half,  i.  e.,  from  A  to  C. 
If  n  represents  the  number  of  equidistant  points  to  be  established 
on  the  curve  (including  the  second  tangent  point,  C),  then  the 

ordinate  at  the  first   point 


x  = 


•     The   ordinate   at 


any  other  point  is  equal  to  x  times  the  square  of  the  number  of 
equal  divisions  between  B  and  that  point;  that  is,  the  ordinate 
from  2  is  4z,  from  3  is  9x,  from  4  is  16x,  etc.  In  actual  work,  the 


STREET   GRADES  355 


grade  elevation  of  the  points  1,  2,  3,  etc.,  are  to  be  worked  out  in 
the  usual  manner;  from  these  elevations  subtract  the  ordinates  as 
computed  above,  and  the  remainder  is  the  grade  elevation  of  the 
respective  points  on  the  parabola  B  C.  The  agreement  of  the  eleva- 
tion of  the  last  point  on  the  curve,  6  in  Fig.  Ill,  with  the  point  C 
on  the  tangent,  checks  the  work  of  computing  the  elevations. 

If  the  second  tangent,  A  C,  is  level,  D  C  in  the  above  value 
for  x  is  0;  and  if  the  second  tangent  has  an  up  grade,  D  C  is  minus, 
and  the  numerator  =  D  E  —  D  C.  If  the  first  tangent  is  level 
D  E  =  0;  and  if  the  first  tangent  has  a  down  grade,  D  E  is  minus, 
and  the  numerator  =  D  C  —  D  E.  The  principles  deduced  for 
Fig.  Ill  are  equally  true,  if  that  diagram  be  turned  upside  down. 

To  secure  the  best  results,  there  should  be  15  feet  of  curve  for 
each  1  per  cent  of  change  of  grade,  although  10  feet  for  each  1  per 
cent  will  give  fair  results.  Long  vertical  curves  make  a  graceful 
street.  The  effect  of  any  proposed  curve  in  lowering  (or  raising) 
the  apex  can  be  judged  of  beforehand  by  remembering  that  the 
distance  from  the  apex  A,  Fig.  Ill,  to  the  curve  is  equal  to  half  of 
the  difference  in  elevation  between  A  and  the  mean  of  the  elevations 
of  B  and  C. 

692.  CROWN  OF  PAVEMENT.     The  only  reason  for  crowning  a 
pavement,  i.  e.,  for  making  the  center  higher  than  the  sides,  is  to 
afford  surface  drainage;  and  therefore  the  proper  crown  to  be  given 
to  pavements  will  be  considered  under  the  head  of  Street  Drainage 
—see  Chapter  XIII. 

To  make  intelligible  the  discussion  of  the  succeeding  section, 
it  is  necessary  to  state  here  that  in  general  the  surface  of  the  pave- 
ment consists  either  of  two  planes  meeting  at  or  near  the  center,  or 
of  a  flat  convex  curve,  usually  the  latter;  and  for  present  purposes 
it  is  sufficient  to  say  that  the  average  transverse  slope  is  usually 
between  1  and  3  per  cent  (see  §  720-24).  The  smoother  the  pave- 
ment and  the  better  the  construction,  the  less  should  be  the  crown. 

693.  CROSS  SECTION  OF  SIDE-HILL  STREETS.    The  arrange- 
ment of  the  cross  section  of  a  street  upon  a  side  hill  is  a  matter 
requiring  good  judgment,  that  needless  damage  may  not  be  done  to 
the  abutting  property  or  that  the  general  appearance  of  the  street 
may  not  be  uselessly  sacrificed.     In  solving  this  problem  no  fixed 
rules  can  be  laid  down;    but  each  case  must  be  treated  by  itself, 
taking  into  account  the  local  conditions.     Fig.  112  shows  the  normal 
arrangement  for  a  residence  street  on  level  ground;   both  footways 
are  at  the  same  elevation,  the  slope  of  the  parking  is  the  same  on 


356  STREET  DESIGN  [CHAP.   XII 

the  two  sides,  the  tops  of  the  curbs  are  at  the  same  level,  the  gutters 
are  of  the  same  depth,  and  the  surface  of  the  street  rises  equally  from 
each  side  to  the  center.  The  normal  section  for  a  business  street 


66ft 


soft  -------- 


FIG.  112. — CROSS  SECTION  OF  STREET  ON  LEVEL  GROUND 

would  be  the  same  except  that  the  sidewalk  would  occupy  all  of  the 
space  between  the  curb  and  the  building  line.  On  a  side-hill  street 
the  above  conditions  can  not  always  be  realized;  and  various  expe- 
dients must  be  resorted  to,  depending  upon  the  difference  in  eleva- 
tion of  the  two  sides  of  the  street.  The  following  are  some  of  the 
common  expedients. 

1.  If  the  difference  is  not  very  great,  the  curbs  may  be  set  at 
the  same  level,  and  one  sidewalk  may  be  placed  higher  than  the  other, 
the  grade  of  the  parking  being  different  on  the  two  sides.     On  a 
business  street,  where  there  is  no  parking,  the  slope  of  the  footway 
may  be  different  on  the  two  sides.     With  sidewalks  consisting  of 
stone  slabs,  cement,  or  asphalt,  a  slope  of  at  least  i  of  an  inch  per 
foot  (1  in  96)  is  required  for  drainage;  and  a  slope  of  more  than  f 
of  an  inch  per  foot  (1  in  32)  is  dangerous  when  covered  with  ice  or 
snow. 

2.  A  slight  difference  of  level  may  be  overcome  by  raising  the 
curb,  i.  e.,  by  increasing  the  depth  of  the  gutter,  on  the  high  side, 
and  lowering  the  curb  on  the  low  side,  the  crown  of  the  pavement 
remaining  symmetrical  about  the  longitudinal  center  line.     Fig.  113 

tfo//r    i  m  Roadway 

- .  -       -L^  -.--.- 


FIG.  113. — CROSS  SECTION  OF  SIDE-HILL  STREET. 

shows  an  actual  section  of  a  street  arranged  on  this  plan.*  Except 
under  extreme  conditions,  the  curb  should  not  show  more  than  10 
inches  because  of  the  difficulty  of  stepping  to  or  from  the  pavement, 
nor  less  than  three  inches  because  of  the  danger  of  its  being  over- 
flowed when  the  gutter  is  full  of  melting  snow. 

*  Trans.  Amer.  Soc.  of  Civil  Engineers,  Vol.  42,  p.  5. 


CROSS   SECTION   OF  SIDE-HILL   STREETS  357 

Sometimes  a  double  curb  is  employed  with  a  horizontal  tread 
about  1  foot  wide  between  the  two  risers.  The  combined  con- 
crete curb  and  gutter  (§  737)  lends  itself  most  readily  to  this  form 
of  construction.  Fig.  114  shows  such  an  arrangement.*  The  objec- 


Walk 


fr-     i*      ~%r-         20;  _  ^.  _  zo>        nW-   »z;  _  -4 
Fia.  114.  —  DOUBLE  CURB  FOB  SIDE-HILL  STHEET. 

tions  to  the  double  curb  are:  1,  its  cost;  2,  the  difficulty  of  keeping 
the  step  neat  and  sanitary;  and  3,  it  lessens  the  width  available  for 
roadway  and  sidewalk.  In  practice  these  objections  have  not  proved 
to  be  serious.  Instead  of  the  double  curb,  it  has  been  proposed  to 
place  the  second  step  at  the  area  line  or  property  line,  to  which 
arrangement,  particularly  on  a  business  street,  the  owner  is  liable  to 
object. 

3.  A  slight  difference  may  also  be  overcome  by  making  the  upper 
side  of  the  pavement  nearly  level  and  giving  the  lower  half  the  normal 
slope. 

4.  The  crown  may  be  moved  toward  the  high  side  of  the  street, 
the  profile  for  each  side  being  determined  in  the  usual  way;  that  is, 
the  surface  of  the  pavement  may  be  two  planes  meeting  at  the 

ffoacttYay  * 


66._. 
Fio.  115.  —  CROSS  SECTION  OF  STREET  ON  A  SIDE  HILL. 

crown  with  the  intersection  rounded  off  a  little,  or  it  may  be  two 
arcs  of  a  circle  or  a  parabola  tangent  to  a  horizontal  line  at  the 
high  point  (see  §  717  and  §  718).  Fig.  115  is  an  actual  example 
of  this  method  of  solution.*  If  the  longitudinal  grade  is  consid- 


FIG.  116. — CROSS  SECTION  OF  STREET  ON  A  SIDE  HILL. 

erable,  as  it  usually  is  under  such  circumstances,  there  is  no  objec- 
tion to  the  upper  side  of  the  street's  being  exactly  level  transversely. 
The  extreme  of  this  solution  is  to  make  the  surface  of  the  pave- 
ment a  right  line  from  the  upper  to  the  lower  side — see  Fig.  116. 

*  Trans.  Amer.  Soc.  of  Civil  Engineers,  Vol.  42,  p.  5. 


358 


STREET   DESIGN 


[CHAP.    XII 


This  arrangement  has  been  objected  to  on  account  of  its  throw- 
ing all  of  the  drainage  to  one  side  of  the  street;  but  this  is  not  a 
serious  objection,  particularly  if  there  is  a  considerable  longitudinal 
grade,  as  usually  there  is. 

5.  Where  there  is  a  considerable  difference  of  elevation  on  a 
residence  street,  it  is  sometimes  wise  to  place  the  footway  next 
to  the  curb,  and  to  allow  the  slope  of  the  parking  to  unite  with  that 
of  the  property — see  Fig.  117. 


FIG.  117. — CROSS  SECTION  OP  SIDE-HILL  STREET. 

6.  When  any  or  all  of  the  above  solutions  fail,  it  may  be  neces- 
sary to  terrace  the  street  and  to  construct  an  upper  and  a  lower 
roadway  as  shown  in  Fig.  118.  For  an  example  of  the  application 
of  this  method  of  treatment  and  some  other  interesting  features, 
see  §  666. 

694.  When  the  street  contains  one  or  more  street-car  tracks, 
the  problem  of  arranging  a  cross  section  on  the  side  of  a  hill  is  still 
more  complicated.  It  is  necessary  that  the  two  sides  of  a  track 
shall  be  at  least  nearly  on  the  same  level;  but  it  is  not  necessary 
that  the  two  tracks  shall  be  at  the  same  elevation.  A  difference  in 
elevation  of  f  of  an  inch  between  rails  of  the  same  track  and  of  3 
inches  between  adjoining  tracks  is  permissible. 


Fia.  118. — CROSS  SECTION  OF  SIDE-HILL  STREET. 

696.  STREET  TREES.  It  is  always  desirable  both  for  the  shade 
and  for  the  appearance  to  have  residence  streets  lined  with  trees  on 
each  side.  Although  trees  in  the  streets  have  an  important  sanitary 
and  aesthetic  value,  opinions  differ  regarding  the  proper  responsi- 
bility for  them.  One  view  vests  all  right  and  title  to  the  tree  in  the 
owner  of  the  property  before  which  it  stands;  and  the  other  asserts 
that  the  trees  belong  to  the  city  at  large,  and  that  the  individual 


STREET   TREES  359 


has  no  more  right  to  the  tree  in  front  of  his  property  than  has  any 
other  citizen.  In  the  first  case,  the  planting  of  the  tree,  its  kind, 
position,  and  care  depend  upon  the  public  spirit  of  the  property 
holder;  and  as  a  result  the  street  presents  a  motley,  straggling 
appearance  often  with  no  trees  where  they  are  most  needed  for  the 
best  general  effect.  Without  some  degree  of  public  control,  it  is 
impossible  even  to  approximate  the  best  results  of  tree  planting; 
but  fortunately  the  number  of  cities  in  which  the  street  trees  are 
under  the  control  of  the  municipal  authorities  is  rapidly  increasing. 

In  planning  a  system  of  streets,  the  location  of  the  trees  should 
be  definitely  provided  for.  They  should  be  located  in  the  grass 
plats  between  the  sidewalk  and  the  edge  of  the  pavement,  and 
at  a  sufficient  distance  from  both  the  sidewalk  and  the  pavement 
that  there  will  be  no  danger  of  the  roots  lifting  either.  The  trees 
should  be  spaced  in  the  row  so  as  to  permit  each  when  fully  grown 
to  spread  to  its  natural  dimensions,  which  usually  requires  a  space, 
center  to  center,  of  25  to  40  feet.  Not  infrequently  trees  are  planted 
much  too  close — particularly  in  the  fertile  and  originally  treeless 
prairies  of  the  Mississippi  Valley; — and  crowd  each  other  and  pre- 
vent a  symmetrical  growth.  In  planting  trees,  it  is  well  to  alternate 
those  of  rapid  growth  with  those  which  mature  more  slowly;  and 
then  as  the  latter  increase  in  size  and  demand  more  room,  the  former 
having  served  their  temporary  purpose,  can  be  removed.  Increased 
stateliness,  impressivene'ss,  and  charm  is  secured  if  the  trees,  at  least 
the  permanent  ones,  on  any  one  thoroughfare  are  of  one  variety. 
Different  streets  can  have  different  kinds  of  trees,  since  in  nearly  all 
cities  there  is  a  large  number  of  suitable  varieties  available. 

697.  In  most  states  there  are  one  or  more  cities  that  have  ob- 
tained— either  officially  or  by  volunteer  civic-improvement  societies 
— valuable  experience  as  to  the  varieties  best  suited  to  the  environ- 
ment, from  whom  data  can  doubtless  be  obtained  by  those  desiring 
information  concerning  the  kind  of  trees  to  plant  in  the  streets  of 
any  particular  city. 

698.  The  following  are  the  requirements  for  a  street  tree  adopted 
by  a  commission  of  experts  for  Washington  City.*     "  1.  A  some- 
what compact  stateliness  and  symmetry  of  growth,  as  distinguished 
from  a  low  spreading  or  pendant  form,  so  that  the  stem  may  reach 
a  sufficient  height  to  allow  free  circulation  of  air  below  the  branches. 
2.  An   ample  supply  of  expansive  foliage  of  bright  early  spring 

*  Proc?  Amer,  SPQ,  Municipal  Improvements,  Vol.  5,  p.  97, 


360  STKEET  DESIGN  [CHAP.   XII 

verdure,  and  rich  in  the  variety  of  colors  and  tints  assumed  during 
autumn.  3.  Healthiness,  so  far  as  being  exempt  from  constitu- 
tional diseases,  as  well  as  from  maladies  frequently  engendered  by 
peculiarities  of  soil  and  atmosphere  impurities.  4.  Cleanliness, 
characterized  by  a  persistency  of  foliage  during  the  summer,  freedom 
from  falling  flowers,  and  exemption  from  the  attacks  of  noxious 
insects.  5.  It  should  be  easily  transplanted,  of  moderately  vigorous 
growth,  and  not  inclined  to  throw  up  shoots  from  the  root  or  lower 
portion  of  the  stem.  A  tree  of  extremely  rapid  growth  is  generally 
short-lived.  6.  The  branches  should  be  elastic  rather  than  brittle, 
that  they  may  withstand  heavy  storms;  and  lastly,  there  should  be 
no  offensive  odor  from  foliage  or  flowers." 

Of  course,  no  tree  planted  amid  the  artificial  conditions  found 
in  a  large  city  will  fully  meet  such  rigid  requirements.  In  1872, 
at  the  commencement  of  systematic  tree  planting,  the  above  com- 
mission recommended  the  following  list  of  trees.  The  Silver  Maple 
(Acer  dasycarpum),  the  American  Linden  (Tilia  americana),  the 
European  Sycamore  Maple  (Acer  pseudo-platanus)  and  the  Amer- 
ican Elm  (Ulmus  americana)  are  thought  to  fill  all  the  above  require- 
ments when  not  subjected  to  the  attacks  of  insects.  The  Tulip 
Tree  (Liriodendron  tulipfera),  Sugar  Maple  (Acer  saccharinum) , 
Sweet  Gum  (Liquidamber  styraciflua),  and  the  Red  Maple  (Acer 
rubrum)  are  the  most  beautiful  of  trees,  their  only  drawback  being 
that  of  not  growing  freely  after  transplanting.  The  Norway  Maple 
(Acer  platanoides),  the  Negundo  (Acer  negundo),  and  the  American 
Ash  (Fraximus  americana)  are  recommended  for  certain  places.  The 
Button-woods  or  Planes  (Platanus  occidentalis  and  Platanus  orien- 
talis)  are  rapid  growing,  and  for  wide  avenues  are  effective  trees. 

As  a  result  of  twenty-five  years'  experience,  the  trees  are  ranked 
as  follows:  "  Silver  Maple,  Norway  Maple,  and  Eastern  Plane  side 
by  side  in  the  first  rank;  then  the  Ginkgo,  and  Western  Plane; 
and  last  American  Linden,  Oak,  and  Sugar  Maple." 


CHAPTER  XIII 
STREET  DRAINAGE 

701.  The  thorough  drainage  of  a  street  involves  four  elements: 
(1)  the  surface  drainage,  (2)  the  gutters,  (3)  the  catch  basins,  and 
(4)  the  underdrainage.     They  will  be  considered  in  the  reverse  order. 

702.  UNDERDRAINAGE.     The  underdrainage  of  a  street  is  the 
first  step  toward  paving  it.     Without  thorough  subdrainage  a  pave- 
ment is  likely  to  settle  here  and  there,  forming  unsightly  depressions 
on  the  surface,  and  possibly  breaking  through.     The  subsoil  may  be 
drained  by  one  or  more  lines  of  porous  tile  as  described  in  §  114-24; 
but  as  a  rule  the  surface  and  underground  waters  are  both  collected 
in  the  same  drain,  and  therefore  it  is  advisable  to  lay  a  line  of  tile 
at  each  side  of  the  street  or  to  construct  a  larger  conduit  under  the 
center  of  the  street.     Since  the  pavement  is  practically  impervious 
to  water,  a  third  line  of  tile  under  the  middle  of  the  pavement  is 
unnecessary,  however  wet  and  retentive  the  soil  originally. 

If  there  is  a  grass  plat  between  the  pavement  and  sidewalk,  as 
is  usual  on  residence  streets,  the  tile  should  be  laid  under  the  outer 
edge  of  the  parking  or  grass  plat;  and  if  there  is  no  parking,  the  tile 
should  be  laid  under  the  gutter.  The  deeper  the  tile  the  better  the 
drainage  and  the  less  the  liability  of  its  becoming  choked  with  tree 
roots.  The  tile  should  not  be  too  small  since  it  is  to  carry  both  under- 
ground and  surface  water — the  latter  from  a  smooth  and  impervious 
pavement. 

The  formula  for  size  of  tile  for  the  drainage  of  earth  roads 
(§  119)  is  worthless  for  pavements,  since  in  cities  a  large  propor- 
tion of  the  rain  falls  upon  impervious  roofs,  pavements,  sidewalks, 
etc.,  and  nearly  all  speedily  reaches  the  storm- water  sewers.  This 
subject  has  been  very  carefully  studied  in  connection  with  the 
design  of  sewers,  and  the  reader  is  referred  to  treatises  on  that  sub- 
ject, for  further  information  concerning  the  size  of  drains  or  storm- 
water  sewers  required. 

361 


362  STREET   DRAINAGE  [CHAP.    XIII 

703.  CATCH  BASINS.     The  catch  basin  is  a  pit  to  receive  the 
drainage  from  the  surface  of  the  street,  in  which  is  deposited  the 
sand  and  other  solid  matter,  and  from  which  the  water  is  discharged 
into  the  sewer  or  storm-water  drain.     A  catch  basin  should  fulfill 
the  following  conditions:   (1)  The  inlet  should  offer  the  least  possible 
obstruction  to  travel,  should  have  sufficient  capacity  to  pass  speedily 
all  the  water  reaching  it,  and  should  not  easily  be  choked  by  leaves, 
paper,  straw,  etc.     (2)   The  capacity  below  the  outlet  should  be 
sufficient  to  retain  all  sand  and  road  detritus,  and  thus  prevent  it 
from  reaching  the  sewer;  and  will  depend  upon  the  area  drained  and 
the  intervals  between  cleanings.     (3)  The  water  level  should  be  low 
enough  to  prevent  freezing.     (4)  The  construction  should  be  such 
that  the  pit  may  be  easily  cleaned  out.     (5)  The  pipe  connecting 
the  basin  with  the  sewer  should  have  sufficient  capacity,  and  should 
be  so  constructed  as  to  be  easily  freed  of  any  obstruction.     (6) 
It  is  desirable  that  the  outlet  should  be  trapped  so  as  to  prevent 
floating  debris  from  reaching  the  sewer.     (7)  If  the   catch  basin 
discharges  into  a  sewer  which  also  carries  house  sewage,  the  end  of 
the  outlet  pipe  should  be  trapped  to  prevent  the  escape  of  air  from 
the  sewer  to  the  street  through  the  catch  basin. 

704.  The  Construction.     Catch  basins  are  usually  built  of  brick 
masonry,  and  plastered  on  the  inside,  at  least  up  to  the  water  line. 
Fig.  119,  page  363,  the  standard  of  Champaign,  111.,  is  a  good  form. 
The  opening  of  the  inlet  is  protected  by  six  half-inch  iron  rods. 
The  several  parts  of  the  cast  iron  top  are  f  and  \  inch  thick;   and 
the  total  weight  of  the  castings  is  162  pounds.     The  pit  requires 
1,000  brick.     The  total  cost  of  the  catch  basin  when  laid  in  1  to  3 
cement  mortar  is  $17.00  to  $19.00,  including  castings,  excavation, 
and  the  vitrified  elbow. 

Fig.  120,  page  364,  shows  the  standard  catch  basin  of  Providence, 
R.  I.*  This  form  differs  from  that  shown  in  Fig.  119  in  the  form 
of  the  inlet  and  of  the  trap  for  the  outlet.  The  latter  is  made  of 
iron  cast  in  a  single  piece,  and  is  somewhat  complicated  in  form,  but 
a  careful  study  of  the  two  views  shown  in  Fig.  120  will  make  the 
construction  reasonably  clear.  The  seal  in  Fig.  120  is  better  than 
that  in  Fig.  119;  but  the  latter  is  used  only  with  storm-water  sewers 
and  for  such  use  this  trap  is  sufficient.  Not  infrequently,  how- 
ever, the  outlet  of  the  catch  basin  is  left  untrapped;  and  some- 
times an  inlet  is  connected  to  a  sewer  without  the  intervention 

*  By  courtesy  of  Otis  F.  Clapp,  City  Engineer 


CATCH    BASINS 


363 


of  either  a  catch  basin  or  a  trap.     This  practice  is  likely  to  clog  the 
sewer. 

Fig.  121,  page  365,  is  the  standard  for  Milwaukee,  Wis.*     This 
diagram  is  presented  to  show  (1)  the  form  of  the  inlet,  (2)  the  method 


Plcmof  Casting 

Fia.  119. — CHAMPAIGN  CATCH  BASIN. 


of  preventing  floating  debris  from  entering  the  outlet,  and  (3)  the 
method  of  ventilating  the  sewer. 

Fig.  122,  page  365,  shows  the  standard  form  in  St.  Pancras 
Vestry,  London,  England,  f 

In  England  many  earthenware  catch  basins  or  "  gully  pits  " 

*  By  courtesy  of  C.  J.  Poetsch,  City  Engineer. 

t  From  a  special  report  by  William  Nisbet  Blair,  Vestry  Engineer. 


364 


STREET   DRAINAGE 


[CHAP.    XIII 


are  used.     Some  of  these  forms  are  quite  complicated.     American 
engineers  object  to  earthenware  pits  on  account  of  (1)  their  limited 


//////// /X/r/ae  s  fane  /// 
\ — 5'ilO'- — 


R.AN'  WITHOUT  MANHOLE  FRAME 
Fia.  120. — PROVIDENCE  CATCH  BABIN. 

size,  (2)  their  great  cost,  and  (3)  their  liability  to  be  broken  by  the 
weight  and  jar  of  the  street  traffic. 

705.  Location.  The  catch  basin  is  usually  placed  near  the  curb 
with  the  cover  in  the  sidewalk  or  the  parking.  It  is  objectionable 
to  have  the  cover  in  the  sidewalk,  since  (1)  the  cover  itself  is  some- 
thing of  an  obstruction  to  travel  and  is  dangerous  when  it  wears 
smooth  or  is  covered  with  snow,  (2)  the  clearing  of  the  pit  seriously 
interferes  with  the  convenient  use  of  the  footway,  and  (3)  in  empty- 
ing the  pit  the  sludge  is  likely  to  be  spilled  on  the  footway,  and  at 
best  the  odor  is  offensive.  In  some  cities  these  objections  are  elim- 
inated by  placing  the  inlet  at  the  curb  line  and  conducting  the  drain- 
age to  a  catch  basin  near  the  center  of  the  street,  one  basin  serving 


CATCH   BASINS 


365 


for  two  or  more  inlets.     Notice  that  the  catch  basin  shov/n  in  Fig. 
122  cleans  out  in  the  gutter. 

pavement... 


FIG.  121. — MILWAUKEE  CATCH  BASIN. 


It  is  customary  to  place  a  catch  basin  at  the  corner  of  the  curb. 
For  additional  objections  to  this  location,  see  §  712. 

The  number  and  capacity  of  catch  basins  will  depend  upon  the 

Curb 


WaferLine 


Outlet 


P/an 

FIG.  122. — LONDON  CATCH  BASIN. 


area  drained,  the  amount  of  rain,  the  grade  of  the  gutter,  etc.  On 
streets  having  light  or  level  longitudinal  grades,  catch  basins  may 
be  constructed  at  intervals  along  the  gutter  as  the  circumstances 
require. 


366  STREET  DRAINAGE  [CHAP.   XIII 

706.  Form  of  Cover.     When  a  catch  basin  or  sewer  manhole 
is  located  in  a  pavement,  the  shape  and  the  surface  of  the  cover 
require  attention.     The  upper  surface  of  the  cover  and  also  of  its 
frame  should  be  covered  with  projections  to  afford  a  good  foothold 
and  to  prevent  it  from  wearing  slippery.     The  best  form  for  the 
frame  depends  upon  the  material  of  the  pavement.     For  macadam 
and  asphalt  the  round  frame  is  best,  since  it  offers  least  obstruction 
to  travel;  the  next  best  form  is  a  square  frame  set  diagonally  to  the 
line  of  travel.     For  a  pavement  rrade  of  bricks  or  stone  blocks,  the 
frame  set  with  its  sides  parallel  to  the  length  of  the  street  is  beet, 
because  the  bricks  or  blocks  can  be  most  closely  fitted  against  this 
form.     In  Europe  and  in  many  American  cities,  it  is  customary  to 
use  only  a  square  form,  and  to  set  it  diagonally  in  macadam  and 
asphalt  pavements,  and  square  in  stone  block  and  brick. 

Often  water-gate  or  stop-box  covers  are  round  in  plan  and  have 
a  convex  surface,  although  the  convex  surface  is  very  oljectionable. 
The  better  form  is  a  cover  round  in  plan  with  a  flat  recessed  top 
set  flush  with  the  pavement.  Preferably  the  portion  below  the 
ground  should  be  provided  with  a  cast  screw  for  adjusting  the  height., 
This  form  may  be  had  of  dealers  in  street-drainage  goods. 

707.  The  Inlet.     In  a  general  way,  there  are  stone  and  cast-iron 
inlets.     The  former  consist  either  of  an  opening  between  a  store 
cover  and  a  stone  floor,  or  a  slot  through  the  stone  curb  (see  Fig.  120, 
page  364).     This  form  is  usually  entirely  open,  but  it  is  sometin  es 
barred  with  one  or  two  horizontal  iron  rods. 

There  is  a  great  variety  of  cast  iron  inlets  on  the  market,  which 
may  be  classified  as  being  straight  or  curved,  and  also  as  having  a 
vertical  or  a  horizontal  opening.  Fig.  123  shows  an  unprotected 
straight  vertical  inlet.  Sometimes  the  opening  is  protected  by 


FIG.  123.  FIG.  124. 


one  or  more  horizontal  or  vertical  rods.  The  latter  are  the 
better,  as  they  offer  greater  protection  against  the  entrance  of 
debris — particularly  sticks  and  boards.  Fig.  124  shows  a  vertical 
front  curved  for  a  corner,  having  vertical  bars.  Fig.  125  and  126 
are  two  styles  of  a  form  having  both  a  vertical  and  a  horizontal 


CATCH    BASINS 


367 


opening.  Notice  that  Fig.  122,  page  365,  has  only  a  horizontal  open- 
ing. A  horizontal  opening  is  not  so  good  as  a  vertical  one,  since  the 
former  is  easily  stopped  by  a  few  leaves,  and  the  accumulation  of 


FIG.  125. 


FIG.   126. 


water  makes  the  stoppage  more  complete;  while  the  barred  vertical 
opening  is  less  easily  obstructed,  and  as  the  water  rises  it  can  pour 
over  the  obstruction  already  formed. 

708.  Inlet  without  Catch  Basin.     It  is  sometimes  desirable  to 
connect  two  or  more  inlets  to  one  catch  basin — for  example,  see 
§  713.     There  are  various  forms  of  such  inlets  on  the  market  and 
many  cities  have  their  own  special  designs.     Fig.   127,  page  368, 
shows  the  form  of  inlet  used  in  such  a  case  at  Omaha,  Nebraska. 
The  entrance  A  is  reduced  by  cast  ribs  to  three  openings  6X9 
inches  at  the  top  and  4f  X  2  inches  openings  at  the  bottom.     The 
section  B  is  rectangular  in  plan  at  both  top  and  bottom.     The 
section  C  is  rectangular  at  the  top  and  circular  at  the  bottom,  and 
fits  into  the  hub  of  a  vitrified  elbow.     Fig.  128,  page  368,  shows  an- 
other form  of  curb  inlet  without  catch  basin.     Fig.   129,  page  369, 
shows  a  commercial  form  of  inlet,  which  has  an  adjustable  curb. 
It  is  made  to  fit  various  sizes  of  outlet  pipe. 

709.  GUTTERS.    The  Material.     Ordinarily  the  surface  of  the 
pavement  adjacent  to  the  curb  serves  as  a  channel  to  convey  the 
drainage  to  the  nearest  inlet,  i.  e.,  the  gutter  is  formed  of  the  same 
material  as  the  pavement.     With  an  asphalt  or  macadam  pave- 
ment, it  is  customary  to  lay  brick  or  stone  blocks  in  the  gutters — 
with  asphalt  to  prevent  its  deterioration  from  being  continually 
covered  with  mud  and  water,  and  with  water-bound  macadam  to 
prevent  flowing  water  from  disintegrating  it. 

A  combined  concrete  curb  and  gutter  (§  737)  is  frequently  used, 
particularly  with  asphalt,  brick,  or  macadam  on  residence  streets. 
A  concrete  gutter  is  objectionable  on  a  macadamized  street,  on 
account  of  the  crushed  stone's  wearing  below  the  edge  of  the  gutter, 


368 


STREET   DRAINAGE 


[CHAP.    XIII 


a  condition  which  interferes  with  the  drainage;  but  if  the  macadam 
surface  is  reasonably  well  cared  for,  this  objection  is  not  serious.     A 


FIG.  127. — OMAHA  INLET  WITHOUT  CATCH  BASIN. 

concrete  gutter  has  been  objected  to  for  any  pavement  owing  to 
the  liability  of  a  rut  to  form  along  its  outer  edge.  In  practice  neither 
of  these  objections  has  proved  to  be  serious.  A  concrete  gutter  is 
more  efficient  and  looks  better  than  one  of  any  other  available 
material  except  asphalt  (see  last  paragraph  of  §  710). 


FIG.  128. — CURB   INLET,   CHAMPAIGN,  ILL. 

Usually  the  gutter  is  formed  by  continuing  the  ordinary  slope 
of  the  pavement  until  it  intersects  the  curb;    but  occasionally  the 


CATCH   BASINS 


369 


outer  edge  of  the  pavement  is  given  an  upward  inclination,  thus  form- 
ing a  flat  V-shaped  channel  a  little  way  from  the  curb.  This  con- 
struction makes  an  excellent  channel  for  the  water,  but  prevents 
the  driving  of  a  carriage  close  enough  to  the  curb  to  allow  people 
to  step  in  or  out  easily. 

In  some  cases  the  curb  is  set  and  the  gutter  formed  before  the 
pavement  is  laid,  in  which  case  the  curb  and  gutter  are  constructed 


Fio.  129. — COMMERCIAL  INLET  WITHOUT  CATCH  BASIN. 

as  they  would  have  been  if  the  street  were  to  be  paved, — the  gutter 
being  composed  of  stone  blocks,  bricks,  or  concrete  (§  709).  Some- 
times a  street  is  macadamized  or  graveled  when  it  is  not  desired  to 
incur  the  expense  of  setting  a  curb,  in  which  case  the  gutter  is  built 
of  cobble  stones,  or  stone  blocks,  or  bricks,  in  the  form  of  a  very  flat 
V  with  the  side  next  the  property  much  the  steeper. 

710.  Depth.  Where  a  curb  is  used,  the  gutter  should  not  be  so 
deep  as  to  present  a  high  step  for  pedestrians,  nor  so  shallow  as  to 
be  in  danger  of  being  overflowed.  Not  infrequently  gutters  are  made 
needlessly  deep.  It  is  easier  to  keep  a  curb  in  line  with  a  shallow  gut- 
ter than  a  deep  one.  On  streets  having  a  considerable  longitudinal 
grade  the  gutter  can  have  a  uniform  depth,  inlets  being  inserted 
to  draw  off  the  surplus  water;  but  on  streets  having  nearly  level 
grades,  the  gutter  must  increase  in  depth  as  the  inlet  is  approached. 
This  can  be  done  easily  with  a  stone  curb,  but  not  so  easily  with  a 
combination  concrete  curb  and  gutter  (§  737),  since  the  latter  is 
usually  made  in  moulds  having  a  uniform  cross  section;  and  there- 


370  STKEET  DRAINAGE  [CHAP.   XIII 

fore  with  a  concrete  curb  and  gutter,  it  may  be  necessary  to  put  a 
summit  in  the  pavement  to  secure  proper  drainage  of  the  gutters. 
Except  in  extreme  cases,  the  gutter  should  not  be  deeper  than  9  inches 
nor  shallower  than  3  inches;  and  ordinarily  it  should  not  be  more 
than  8  nor  less  than  4  inches — usually  it  is  5  or  6  inches. 

It  may  be  necessary  to  modify  the  preceding  rules  when  one 
side  of  the  street  is  higher  than  the  other  (see  §  693).  In  localities 
where  there  is  a  good  deal  of  snow,  the  gutter  must  be  deeper  than 
stated  above,  for  shallow  gutters  readily  become  clogged  with  snow 
and  slush.  In  some  northern  cities,  the  snow  is  habitually  allowed 
to  pack  upon  the  surface  of  the  street  to  a  depth  of  6  or  more  inches, 
in  which  places  the  depth  of  the  curb  must  be  extremely  deep  to 
prevent  the  melting  snow  and  water  from  filling  the  gutter  and 
flowing  over  the  sidewalk  into  the  basements. 

711.  Grade.     For  most  materials  with  which  gutters  are  paved, 
it  is  improbable  that  the  grade  will  be  so  steep  as  to  do  serious  harm. 
Crushed  stone  and  gravel  are  exceptions  to  this  rule,  however,  and 
these  materials  must  not  be  laid  on  too  steep  a  grade.     They  may  be 
used  on  a  2  per  cent  grade  provided  the  volume  of  water  is  not  too 
great. 

The  minimum  grade  permissible  in  the  gutter  will  depend  chiefly 
upon  the  material  with  which  it  is  paved,  but  somewhat  upon  the 
cost  of  catch  basins.  Almost  any  grade  can  be  obtained  by  estab- 
lishing catch  basins  close  together  and  raising  the  gutter  half  way 
between  them.  In  a  number  of  cities  the  minimum  grade  of  gutters 
paved  with  granite  blocks,  bricks,  rectangular  wood  blocks,  or  mac- 
adam is  1  in  300  or  400.  Except  under  very  favorable  circumstances, 
a  slope  of  1  in  200  (£  of  1  per  cent)  should  be  regarded  as  the  minimum. 

Asphalt  decays  if  continually  wet,  and  therefore  the  condition 
governing  the  minimum  permissible  grade  is  different  for  that  than 
or  Other  materials.  With  a  slope  of  less  than  1  per  cent,  the  gutter 
will  not  keep  itself  clean,  consequently  the  asphalt  will  decay  owing 
to  the  action  of  mud  and  water;  and  hence  asphalt  should  not 
be  laid  in  a  gutter  having  a  fall  of  less  than  1  in  100.  If  this  fall  can 
not  be  obtained,  a  concrete  gutter  should  be  used,  or  the  gutter 
should  be  paved  with  vitrified  brick  or  carefully  dressed  granite 
blocks. 

712.  Drainage  at  Street  Intersection.     In  most  cities  it  is  cus- 
tomary to  construct  catch  basins  at  the  corner  of  the  curb,  using 
an  inlet  with  a  curved  face.     This  practice  is  very  objectionable. 

If  the  walk  across  the  street  is  elevated  above  the  pavement,  it 


GUTTERS 


371 


is  necessary  either  to  carry  the  water  under  the  walk  in  a  pipe,  or  to 
stop  the  cross  walk  within  a  short  distance  of  the  curb  to  leave  a 
channel  for  the  water.  The  latter  method  is  necessary  where  there 
is  much  water.  Frequently  this  channel  is  left  open  at  the  top,  and 
sometimes  it  is  covered  with  a  cast  iron  plate  with  one  edge  resting 
in  a  rabbet  in  the  curb  and  the  opposite  one  in  a  head  stone  or  false 
curb  set  at  the  end  of  the  cross  walk.  The  covered  gutter  is  much 
better  than  the  open  one,  although  the  cast  plates  are  frequently 
struck  by  wheels  and  broken,  and  often  get  displaced.  This  solution 
of  the  problem  is  further  objectionable  since  a  wheel  in  turning  the 
corner  must  surmount  the  first  raised  cross  walk,  then  descend  to 
the  bottom  cf  the  gutter,  and  finally  climb  over  the  second  cross  walk. 
The  face  of  the  inlet  usually  has  a  depth  of  8  to  12  inches  below  the 
top  of  the  curb;  and  hence  if  the  sidewalks  are  wide  or  the  parking 
is  narrow,  the  shock  to  a  vehicle  going  around  such  a  corner  is  con- 
siderable. 

If  the  cross  walk  is  not  elevated,  the  step  from  the  curb  to  the 
bottom  of  the  gutter  is  uncomfortably  high,  and  besides  pedestrians 
are  compelled  to  cross  the  gutter  where  there  is  the  most  water. 

713.  A  much  better  arrangement  than  either  of  the  above  is 
to  place  an  inlet  at  each  side  of  the  corner.  Each  inlet  may  have 
its  own  catch  basin,  or  the  two 


Curh 


,-^Cafch  basin 


Parking 


Wa/H 


Private 
Property 


may  connect  with  a  single  pit 
by  means  of  tile  or  vitrified  pipe 
underground.  Fig.  130,  page 
374,  shows  such  an  arrange- 
ment. Instead  of  this  plan, 
the  two  inlets  at  each  of  the 
four  corners  of  the  street  in- 
tersection may  be  connected 
with  a  single  catch  basin 
placed  in  the  middle  of  the 
intersection  or  in  other  suit- 
able location.  The  inlet  not 
connected  directly  with  a 
catch  basin  can  be  made  by  inserting  the  hub  of  a  curved  vitrified 
pipe  in  the  bottom  of  a  cast  inlet-box  (see  Fig.  127,  128,  and  129). 

The  advantage  of  the  method  shown  in  Fig.  130  is  that  it  allows 
the  intersection  to  be  paved  almost  level  with  the  top  of  the  curb, 
and  hence  there  is  no  obstruction  to  either  pedestrian  or  vehicular 
travel.  The  only  objection  to  it  is  the  expense  for  either  the  extra 


FIG.*  130. — INLETS  AT  STREET  CORNER. 


372 


STREET   DRAINAGE 


[CHAP,  xin 


catch  basin  or  the  extra  inlet  and  connecting  pipe,  but  the  advan- 
tage is  well  worth  this  comparatively  small  cost. 

714.  Where  there  are  no  storrn-water  sewers,  the  gutter  is  some- 
times carried  across  the  street  intersection.     This  is  objectionable 
at  any  season,  and  particularly  so  when  the  gutter  is  filled  with 
snow  or  ice.     If  the  gutter  is  deep  or  the  grade  is  steep,  the  water 
may  be  carried  under  the  intersection  by  a  shallow  culvert  with 
cast  iron  top,  or  better  in  a  cast  iron  pipe;  but  if  the  gutter  is  shallow 
or  the  grade  nearly  level,  the  road  surface  should  be  raised  a  little 
to  give  room  for  a  cast  iron  storm-water  drain  under  the  road- 
way.    The  elevated  intersection  may  be  a  slight  obstruction  to 
travel,  but  it  is  preferable  to  two  open  gutters. 

715.  Elevated  Foot-way  Crossing.     To  aid  pedestrians  in  cross- 
ing the  water  in  the  gutter,  it  was  formerly  the  practice  to  raise  the 
pavement  in  the  line  of  the  crossing  so  that  the  surface  of  the  foot-way 


J.B.. 


SECTION    A-B 


SECTION  C-D 

(ot  Hie  centre) 


SECTION  E-F 

PIG.  131. — ELEVATED  BRICK  CROSSING. 


was  level  from  the  crown  of  the  carriage-way  pavement  to  the 
top  of  the  curb,  and  leave  a  channel  next  to  the  curb  which  was 
either  left  open  or  bridged  with  a  cast  iron  plate.  Fig.  131 
shows  the  details  of  an  elevated  brick  crossing.  Notice  that 
Fig.  131  has  a  limestone  curb  and  a  brick  gutter.  Fig.  132  and  133 
show  the  gutter  at  the  end  of  an  elevated  brick  crossing  when  a  con- 


SURFACE    DRAINAGE 


373 


crete  curb  and  gutter  is  employed.  The  chief  difference  between 
Fig.  132  and  133  is  in  the  form  of  the  false  curb  or  head  stone  on  the 
side  of  the  gutter  toward  the  center  of  the  street.  The  difference 
in  the  merits  of  the  two  methods  is  mainly  in  the  cost,  Fig.  133 
usually  being  slightly  the  cheaper.  In  both  cases  there  is  a  drop  of 
1  inch  in  the  width  of  the  cast  iron  bridge  plate.  Of  course,  the 
crossing  could  be  carried  level  from  gutter  to  gutter,  or  more  drop 
could  be  put  into  the  gutter  plate. 

It  has  always  been  recognized  that  as  far  as  the  use  of  the  car- 
riage-way pavement  is  concerned,  an  elevated  crossing  is  undesir- 


15'xi'CI.  Gutter  Plate 


FIG.  132. — GUTTER  FOR  ELEVATED  BRICK 
CROSSING  WITH  CONCRETE  FALSE  CURB. 


FIG.   133. — GUTTER  FOR  ELEVATED  BRICK 
CROSSING  WITH  LIMESTONE  FALSE  CURB. 


able,  particularly  where  the  pavement  is  used  by  a  large  number  of 
vehicles  or  where  there  is  considerable  rapid  travel;  but  since 
the  introduction  of  the  automobile,  the  elevated  crossing  is  very 
undesirable.  The  elevated  crossing  is  a  serious  obstruction  also  to 
vehicles  rounding  the  corner;  and  besides  the  cast  iron  crossing 
plates  are  easily  displaced  and  are  frequently  broken.  The  elevated 
crossing  should  not  be  used. 

716.  SURFACE  DRAINAGE.  The  drainage  of  the  surface  of  the 
pavement  is  provided  for  by  making  the  center  of  the  pavement 
higher  than  the  sides.  The  principle  governing  the  amount  of  crown 
for  pavements  is  somewhat  different  from  that  of  earth,  gravel,  or 
water-bound  macadam  roads.  First,  a  hard,  smooth  and  practically 
impervious  pavement  needs  no  crown  for  the  drainage  of  the  surface; 
and  on  such  a  pavement,  the  only  advantage  of  a  transverse  slope  is  to 
drain  shallow  depressions  due  to  faulty  construction,  wear,  or  a  set- 
tlement of  the  foundation,  and  to  aid  the  rain  in  washing  the  pave- 
ments. Second,  the  surface  of  the  pavement  has  no  tendency  to 
wash;  and  hence  the  crown  need  not  be  increased  on  a  grade  as  in 


374  STREET   DRAINAGE  [CHAP.    XIII 

the  case  of  earth  roads.  The  less  the  crown  the  better  for  travel, 
and  the  more  uniformly  will  the  travel  be  distributed  over  the  pave- 
ment, although  a  slight  crown  is  inappreciable  in  either  of  these 
respects.  Therefore  pavements  require  only  crown  enough  to  drain 
depressions  of  the  surface  due  to  faulty  construction,  to  wear,  or 
to  settlement  of  the  foundation;  and  the  crown  may  decrease  as 
the  grade  increases. 

717.  Crown.     There  has  been  much  discussion  as  to  the  best  form 
of  the  surface  of  a  pavement.     Some  claim  that  it  should  be  a  con- 
tinuous curve,  while  others  contend  that  it  should  consist  of  two  planes 
meeting  in  the  center.     The  curved  profile  is  defective  in  that  it  gives 
too  little  inclination  near  the  middle,  the  result  being  that  the  pave- 
ment wears  hollow  in  the  center  and  permits  water  to  stand  there. 
To  overcome  this  objection  some  engineers  raise  the  center  of  the 
pavement  ^  or  -f  of  an  inch  above  the  curved  cross  section.     The 
objection  to  the  two  planes  is  that  the  sides  wear  hollow  and  hold 
water.     An  advantage  of  the  curved  profile  is  that  the  center  of  the 
street,  which  is  the  part  especially  devoted  to  travel,  is  nearly  flat; 
while  the  sides,  which  have  the  greater  inclination,  are  occupied  by 
teams  standing  at  the  curb.     Another  advantage  of  the  curved 
profile  is  that  it  gives  a  deeper  gutter,  which  confines  the  storm 
water  to  a  smaller  portion  of  the  street  and  reduces  the  interfer- 
ence with  pedestrian  travel. 

It  is  sometimes  claimed  that  the  curved  form  will  support  the 
greater  load,  because  of  its  arch  action;  but  the  arch  action  of  a 
pavement  is  entirely  inappreciable,  owing  to  the  flatness  of  the  arch, 
to  the  imperfect  fit  of  the  so-called  arch  stones,  and  to  the  insta- 
bility of  the  abutments  or  curbs. 

The  surface  is  usually  a  continuous  curve — generally  a  parabola. 

718.  To  Lay  out  a  Parabolic  Crown.     In  Fig.  134  the  curved  line 
C  B  represents  the  surface  of  the  finished  pavement.     C  is  the  center 
of  the  pavement;    and  C  D  =  A  B  =  the  amount  of  the  crown. 


FIG.  134. — PARABOLIC  CROWNED  PAVEMENT. 


To  find  the  distance  from  the  line  A  C  down  to  the  curved  line  C  B, 
divide  the  half  width  of  roadway,  A  C,  into  any  number  of  equal 


SURFACE   DRAINAGE 


375 


parts,  say  n,  and  designate  the  distance  from  the  point  1  on  A  C 
vertically  down  to  B  C  by  x;  then  by  the  principles  of  the  parabola, 

,  and  the  distance  from  point  2  down  to  the  road  surface 


x  = 


n2 


is  22  x  or  4  x,  and  the  distance  from  3  is  32  x  or  9  x.  In  practice  a 
string  with  knots  in  it  to  represent  the  points  of  division  of  A  C  is 
stretched  from  the  top  of  the  curbs,  and  then  the  ordinates  computed 
as  above  are  measured  with  a  pocket  rule. 

719.  When  construction  begins,  it  is  wise  to  give  the  one  in 
charge  of  the  work  a  drawing  somewhat  like  Fig.    135,  showing 


Sub-grade 


F 


Concrete 


FIG.  135. — METHOD  OF  SHOWING  CROWN  OF  PAVEMENT. 

the  relation  between  the  top  of  the  curbs  and  the  cross  section  of  the 
subgrade,  the  top  of  the  concrete,  and  the  top  of  the  finished  pave- 
ment. Such  a  drawing  prevents  misunderstandings  and  disputes. 
Notice  that  the  curves  in  Fig.  135  are  not  exact  parabolas,  the 
ordinates  at  4  and  12  being  |  inch  too  long;  but  this  is  sufficiently 
exact,  since  it  is  not  possible  to  secure  mathematical  precision  in  this 
class  of  work. 

720.  Rules  for  Amount  of  Crown.    The  practice  of  different 
cities  is  not  at  all  uniform  as  to  the  amount  of  crown.     Numerous 
empirical  formulas  have  been  proposed  for  the  crown  of  pavements; 
but  there  is  not  much  harmony  between  them.* 

721.  Washington  Formula.     Since  1894  the  Engineering  Depart- 
ment of  the    District  of  Columbia    has  employed  the  following 
formula:     C  =  w(100  -  4p)  -5-  (6300  +  50p2),    in    which    C  =  the 


For  a  list  of  many  such  formulas,  see  Engineering  News,  Vol.  63  (1910),  p.  516-18. 


376  STREET   DRAINAGE  [CHAP.    XIII 

crown  in  feet,  w  =  the  distance  between  the  curbs  in  feet,  and  p 
is  the  longitudinal  grade  of  the  pavement  in  percentage.  Notice 
that  the  crown  decreases  as  the  longitudinal  grade  increases. 

722.  Rosewater  Formulas.     Apparently  the  formula  proposed  in 
1902  by  Andrew  Rosewater,  then  City  Engineer  of  Omaha,  is  most 
frequently  used.     The  latest  Rosewater  formula  is  as  follows :   "The 
crown  for  asphalt  is:  C  =  w  (100  —  4p)  -^  5,000,  in  which  the  nomen- 
clature is  as  in  the  section  next  above.     For  brick,  stone  block,  or 
wood  block,  the  crown  is  five  sixths  of  that  for  saphalt."     A  formula 
for  crown  formerly  used  in  Omaha  gave  a  less  crown  than  the  above 
rule  for  brick,  stone  block,  and  wood  block,  and  a  much  less  crown 
for  asphalt.     The  former  formula  is : 

for  brick,  stone  block,  and  wood  block,  C  =  (20  -  p)  ^  1,600 
for  sheet  asphalt  C  =  (9  -  p)  +  600. 

The  latter  formulas  are  still  used  by  many  cities. 

Notice  that  in  the  preceding  rules  the  crown  is  decreased  as  the 
steepness  of  the  longitudinal  grade  increases,  which  is  proper.  Also 
notice  that  according  to  these  rules,  the  crown  of  sheet  asphalt  is 
more  than  that  of  the  other  kinds  of  pavements  mentioned,  which  is 
contrary  to  the  practice  of  many  cities.  Considering  only  the  smooth- 
ness of  the  surface,  it  appears  that  asphalt  should  have  the  least 
crown;  but  considering  only  the  fact  that  asphalt  rots  when  con- 
tinually wet,  it  appears  that  asphalt  should  have  a  large  crown. 

723.  The  above  rules  for  crown  must  be  modified  somewhat 
when  the  two  sides  of  the  street  are  not  at  the  same  elevation — 
see  §  693,  page  357. 

724.  Recommendations  of  A.8.C.E.    Committee.     For  the  crown 
recommended    for    various    road    and    pavement    surfaces    by    a 
committee  of  the  American  Society  of  Civil  Engineers  in  1917,  see 
Table  16,  page  65. 

725.  Dished  Pavements.     The  early  pavements  in  this  country 
and  at  present  those  in  some  cities  in  Europe  and  South  America, 
slope  from  both  sides  towards  the  center.     In  this  form  the  most 
valuable  part  of  the  street  is  devoted  to  drainage  purposes,  and  it  is 
difficult  to  carry  the  water  to  an  intersecting  street.*    The  pave- 
ments of  alleys  usually  slope  to  the  center.     This  form  is  better  for 

*  The  single-gutter  street  pavement  was  ably  advocated  by  W.  G.  Kirchoff  in  a  paper  before 
the  Wisconsin  League  of  Municipalities — See  Engineering  and  Contracting,  Vol.  44  (1915), 
p.  190-91. 


SURFACE    DRAINAGE  377 


alleys  than  a  gutter  at  each  side,  since  it  keeps  the  storm  water  from 
flowing  along  the  side  of  buildings  and  possibly  interfering  with  light 
areas,  cellar  stairways,  etc.,  and  it  also  carries  the  water  over  the 
sidewalk  with  less  annoyance  to  pedestrian  travel. 


CHAPTER    XIV 
CURBS  AND  GUTTERS 

728.  CURB.    A  curb  is  a  plank  or  slab  of  stone  set  at  the  edge 
of  the  roadway  to  protect  the  sidewalk  or  tree  space  and  to  form 
the  side  of  the  gutter.     Curbs  are  not  usually  set  except  where  the 
street  is  paved,  but  they  greatly  improve  the  appearance  of  an 
unpaved  street  and  protect  the  grass  plats  at  the  side  of  the  street, 
particularly  during  the  muddy  season. 

Curbs  were  formerly  made  of  natural  stone,  but  concrete  curbs, 
usually  combined  concrete  curb  and  gutter,  are  increasing  very 
rapidly  in  recent  years — chiefly  because  of  the  decrease  in  the  price 
of  portland  cement.  Natural  stone  is  used  now  only  in  the  vicinity 
of  quarries  of  suitable  stone.  Granite  is  the  best  natural  stone,  but 
it  is  usually  very  expensive.  Limestone  and  sandstone  are  fre- 
quently used,  but  they  are  generally  too  easily  chipped  or  broken. 
Concrete  unless  made  with  unusual  care  or  protected  by  steel  on 
the  edge,  is  too  friable  for  a  business  street  where  heavy  loads  fre- 
quently back  up  against  the  curb. 

729.  Stone  Curb.     Granite  curbs  are  obtained  in  large  quantities 
in  several  states,  notably  Maine,  New  Hampshire,  Massachusetts, 
Connecticut,  New  Jersey,  Pennsylvania,  Georgia,  Wisconsin,  Mis- 
souri, South  Dakota,  California.     Husdon  River  bluestone,  a  variety 
of  sandstone  commercially  known  as  bluestone,  is  much  used  for 
curbs,  on  account  of  its  hardness,  durability,  and  great  transverse 
strength.     It  Is  evenly  bedded,  splits  with  a  smooth  surface,  and  is 
found  in  large  quantities  in  the  counties  of  the  state  of  New  York 
adjoining  the  Hudson  River  from  Albany  to  New  York  City.     The 
sandstones  most  used  for  curbs  are  the  following :  A  gray  stone  from 
Berea,  Ohio;    a  brownish  red  stone,  known  as  Medina  sandstone, 
obtained  in  the  State  of  New  York  on  the  shore  of  Lake  Ontario;  a 
gray,  yellow,  brown  or  red  stone  from  Potsdam,  N.  Y. ;  a  metamor- 
phic  sandstone  from  Sioux  Falls,  South  Dakota,  known  as  Sioux 

378 


STONE   CURBS  379 


Falls  quartzite;    and  a  light-pink  stone  from  Sandstone,   Minn., 
known  as  Kettle  River  sandstone. 

730.  The  thickness  should  be  sufficient  to  give  strength  to  resist 
the  blows  of  wheels  and  to  prevent  the  frost  in  the  earth  back  of  the 
curb  from  breaking  it  off  at  the  top  of  the  gutter.  The  curb  is 
usually  4  to  6  inches,  depending  upon  the  quality  of  the  stone  and 
the  locality.  The  depth  must  be  sufficient  to  prevent  the  thrust  of 
the  earth  behind  the  curb  from  overturning  it,  and  is  usually  18  to 
24  inches.  If  the  sections  are  too  short,  it  is  difficult  to  keep  them 
in  place  and  the  general  appearance  is  not  good;  and  if  they  are  too 
long,  it  is  difficult  to  handle  and  set  them,  and  nearly  impossible  to 
get  a  firm  bearing  on  the  bottom.  They  usually  vary  from  3  to  8 
feet  in  length. 

The  exposed  face  of  the  curb  should  be  bush-hammered  or  axed; 
and  where  the  sidewalk  extends  to  the  curb,  the  back  also  should 
be  smoothly  dressed  so  the  sidewalk  may. fit  closely  against  the  curb. 
The  upper  face  should  be  cut  to  a  slight  bevel  with  the  front  face, 
say  i-inch  to  the  foot,  so  that  when  the  face  of  the  curb  is  set  with  a 
little  inclination  backward,  the  top  face  will  be  level  or  slope  down- 
ward and  to  the  front  a  trifle.  The  pavement  slopes  toward  the 
gutter,  and  therefore  a  wagon  wheel  inclines  toward  the  curb;  hence 
the  curb  is  set  leaning  back  a  little  to  prevent  a  wheel  from  striking 
the  face  when  running  at  the  inner  edge  of  the  gutter  and  also  to 
secure  increased  stability.  The  curb  is  usually  cut  with  a  square 
corner  at  the  outer  upper  edge;  but  it  would  be  better  if  this  corner 
were  rounded  off  slightly,  say  to  a  radius  equal  to  one  third  of  the 
thickness  .of  the  curb,  to  decrease  the  tendency  to  chip.  The  ends 
of  the  sections  should  be  smoothly  dressed  to  the  exposed  depth,  and 
the  part  not  exposed  should  be  knocked  off  so  as  to  permit  the  dressed 
ends  to  come  into  close  contact.  The  ends  should  fit  closely  for 
appearance  and  to  prevent  the  earth,  particularly  if  sand,  from  run- 
ning from  behind  the  curb  between  the  sections  into  the  gutter,  or 
to  prevent  the  sand  cushion  of  a  brick  pavement  from  running  from 
under  the  bricks  into  these  cracks  and  possibly  through  them  into 
holes  behind  the  curb.  In  a  number  of  European  cities,  notably 
Brussels,  the  curb  is  cut  with  a  tongue  in  one  piece  which  fits  into  a 
groove  in  the  next  piece,  to  aid  in  keeping  the  curb  to  line. 

The  curb  should  be  set  with  a  uniform  batter,  in  a  straight  line, 
and  on  a  regular  grade.  To  fulfill  these  conditions  requires  careful 
work  in  the  first  place,  and  to  prevent  the  curb  from  subsequently 
getting  displaced  requires  proper  design  and  thorough  workman- 


380  CURBS   AND   GUTTERS  [CHAP.  XIV 

ship.  The  trench  in  which  the  curb  is  to  be  set  should  be  dug  4  to 
6  inches  below  the  base  of  the  curb  to  allow  for  a  layer  of  gravel  on 
which  to  set  the  stone;  and  the  width  of  the  trench  should  be  at 
least  three  times  the  thickness  of  the  curb  to  allow  room  for  ram- 
ming the  earth  around  the  stone.  The  bottom  of  the  trench  should 
be  made  smooth  and  be  thoroughly  consolidated  by  ramming,  and 
the  gravel  also  should  be  compacted.  Where  gravel  is  expensive, 
it  is  dispensed  with,  the  curb  being  set  upon  brick  or  stone.  In 
filling  the  trench,  the  earth  should  be  thoroughly  rammed  in  layers 
not  more  than  4  inches  thick.  Where  gravel  is  plentiful,  it  is  some- 
times specified  that  the  trench  shall  be  filled  with  gravel  to  8  or  10 
inches  from  the  top. 

In  the  past  there  has  been  so  much  trouble  in  keeping  curbs  in 
line,  that  within  recent  years  there  has  been  a  general  tendency  to 
set  the  curb  in  a  bed  of  concrete — particularly  when  concrete  is 
used  for  the  foundation  of  the  pavement.  A  6-inch  layer  of  con- 
crete is  deposited  in  the  trench  and  the  curb  set  upon  it,  after  which 
the  trench  is  filled  with  concrete  on  the  street  side  up  to  the  base  of 
the  proposed  pavement  and  on  the  back  side  nearly  up  to  the  top  of 
the  curb.  When  set  in  concrete,  the  curb  does  not  need  to  be  as 
deep  as  otherwise,  since  the  concrete  then  practically  becomes  a 
part  of  the  curb. 

731.  Owing  to  the  difficulty  of  keeping  stone  curbs  in  line  or 
rather  owing  to  the  expense  of  setting  them  so  they  will  certainly 
stay  in  line,  stone  curbs  are  becoming  much  less  common  than 
formerly.     They  are  being  replaced  by  concrete  curbs  or  more  fre- 
quently by  combined  concrete  curb  and  gutter  (§  737). 

732.  Cost.     In  most  localities,  split  sandstone  or  limestone  curb- 
ing 4  to  6  inches  thick  can  be  had  for  30  to  40  cents  per  square  foot 
f.o.b.  cars  at  the  destination;   and  often  sawed  stone  can  be  had  at 
about  the  same  price.     The  additional  cost  of  a  bush-hammered  or 
axed  surface  will  vary  with  the  hardness  of  the  stone  and  the  degree 
of  the  finish,  and  curved  sections  will  cost  30  to  50  per  cent  more  than 
straight   pieces.     Hudson   River  bluestone    (sandstone)    curbing   5 
inches  thick  costs  about  30  cents  per  square  foot.     Granite  curb 
costs  from  40  to  50  cents  per  square  foot,  depending  upon  locality 
and  thickness. 

For  more  definite  information,  see  the  price  reports  in  current 
technical  journals. 

733.  Concrete  Curb.     In  some  sections  where  suitable  stone  for 
curbing  is  not  readily  available,  curbs  have  been  made  of  portland 


CONCRETE    CURBS 


381 


cement-  concrete.  Owing  to  the  decreasing  price  of  cement,  this 
form  of  curb  is  coming  into  more  common  use.  It  is  usually  made 
about  6  inches  thick  and  18  or  20  inches  deep.  If  well  made,  it  does 
excellently  for  residence  streets. 

For  suggestions  concerning  the  construction  of  concrete  curb, 
see  §  738-17. 

734.  The  exposed  corner  of  a  concrete  curb,  particularly  on  a 
business  street,  is  sometimes  protected  by  a  steel  angle  or  special 
form  which  is  anchored  to  the  body  of  the  concrete  by  lugs  or  a 
special  stem.     Several  of  these  forms  are  very  efficient;  but  as  they 
are  patented  nothing  more  will  be  said  here.     For  particulars  con- 
sult the  advertising  pages  of  technical  journals. 

735.  Cost.     Table  37,  page  382,  shows  the  cost,  as  determined 
by  38  time  studies,  of  the  concrete  curb  shown  in  Fig.  136.     It  was 
laid  in  sections  6  feet  long  with  thin  metal 

partitions  between.  At  first  a  1  :  2  :  5  mix- 
ture was  used  for  the  body,  but  later  a 
1  :  2i  :  6;  and  the  facing  was  1  :  1|  :  1|. 
The  proportions  were  accurately  measured  by 
volumes.  The  concrete  was  mixed  by  hand 
on  a  wood  platform  in  the  middle  of  the  street. 
As  soon  as  the  concrete  had  set  sufficiently, 
the  front  forms  were  removed  and  the  face 
of  the  concrete  was  scrubbed  with  steel 
brushes  when  the  concrete  had  set  hard 
enough  to  require  them,  but  usually  with 
stiff-bristle  brushes. 

736.  GUTTERS.     Incidentally   the    construction    of    the    gutter 
has  already  been  considered  in  §  709-11,  which  see. 


Concrete* 


FIG.  136. — CROSS  SECTION 
OF  CURB. 


Grave/ 
Macadam 

Cinders* 


Fio.  137. — ST.  Louis  CONCRETE  GUTTER  OR  PARK  DRIVE. 

Fig.  137  shows  a  concrete  gutter  used  in  St.  Louis,  Mo.,  for  park 
drives, 


382 


CURBS   AND    GUTTERS 


[CHAP,  xiv 


737.  COMBINED  CONCRETE  CURB  AND  GUTTER.  In  recent 
years  the  construction  of  combined  concrete  curb  and  gutter  built 
in  place  has  become  very  general  in  the  smaller  cities,  and  in  resi- 

TABLE  37 

COST  OF  CONCRETE  CURB  * 
Superintendence  and  Over-head  Expenses  not  Included 


6 
£ 

1* 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 

11 

12 
13 
14 
15 
16 
17 

18 
19 

Item 

COST  CTS.  PER 
LIN.  FT. 

Aver, 
per 
Amt. 
of 
Total. 

FEET  COMPLETED 
PER  MAN-HOUR. 

Min. 

Max. 

Aver. 

Min. 

Max. 

Aver. 

Labor: 
Foreman               @  37|c.  per  hr. 
Water  boy            @  lOc. 
Trenching             @  18c. 
Placing  forms       @  35c. 
Removing  forms  @  25c. 
Concrete               @  18c.             " 
Facing                   @  18c.             " 
Finishing              @  35c.             " 
Scrubbing             @  35c.             " 
Back-filling           @  18c. 

Total  for  labor 

1.1 

0.4 
2.2 
1.0 
0.5 
2.6 
0.9 
1.8 
0.4 
0.3 

3.9 
1.3 
9.4 
10.4 
5.2 
9.5 
2.9 
10.7 
3.4 
2.0 

2.2 
0.6 
4.9 
4.2 
1.8 
4.3 
1.7 
4.8 
1.7 
1.3 

4.7 

1.3 
10.5 
9.0 
3.8 
9.2 
3.6 
10.3 
3.6 
2.8 

1.9 

2.5 
2.0 
1.2 
6.5 
3.7 
5.0 
5.9 

9.6 
26.4 
38.0 
6.8 
20.4 
20.4 
48.1 
51.0 

'«v 

4.1 
7.2 
7.6 
3.2 
10.6 
8.3 
11.4 
14.1 

11.2 

6.2 
1.2 
6.0 
0.4 
1.0 
1.5 

16.3 

58.7 

8.9 
1.5 
7.7 
0.7 
1.0 
2.0 

21.8 

27.5 

7.6 
1.4 
6.9 
0.6 
1.0 
1.8 

58.8 

16.2 
3.0 
14.7 
1.3 
2.1 
3.9 

Materials: 
Cement  @  $1.20  per  bbl. 

Sand      @    0.75  per  cu.  yd..  .... 
Gravel  @    1.90    "     "  "     
Facing  @    1.75    "     "  "     
Lumber,  1^-inch  spruce  
Water,  waste,  etc. 

Total  for  materials 

19.3 

41.2 

Grand  Total  

27.5 

80.5 

46.8 

100.0 

*  Engineering  and  Contracting,  Vol.  43  (1915),  p.  61. 

dence  districts  of  larger  cities.  Such  construction  is  cheap,  dur- 
able, efficient,  and  good  in  appearance.  It  is  very  popular  with 
brick  and  asphalt  where  the  grades  are  very  flat,  and  is  often  used 
with  a  crushed-stone  pavement.  Fig.  138  shows  the  cross  section 
of  the  usual  form.  Notice  that  the  face  of  the  curb  is  battered. 
This  is  important,  since  the  pavement  is  crowned,  and  therefore  the 
plane  of  a  steel  wagon-tire  is  inclined.  Consequently  if  the  face  of 
the  curb  is  vertical,  the  tire  will  strike  it  at  its  upper  edge;  but  if  the 
face  of  the  curb  makes  an  angle  with  the  pavement  greater  than  90°, 
the  tire  will  strike  the  bottom  of  the  curb  and  do  much  less  damage, 


COMBINED    CURB   AND   GUTTER 


383 


To  prevent  damage  by  the  striking  of  steel  tires,  the  curbs  to  private 
drives,  particularly  narrow  ones,  usually  have  a  convex  face. 

Fig.  139  shows  the  form  of  combined  concrete  curb  and  gutter 
employed  in  St.  Louis,  Mo. 


>sgsys 

S-£-«%^K::^P 

*:;Concrefe^  -• 
•i>a.v.-.-  . ..  -o.-|Vo: 

*'-:\*S?:*\'-\**:*: 


,3to^&''*^WM 


,//Gra\se/ or  Cinders  *' 
'/////////////  //////// 


Fio.  138. — STANDARD  CONCRETE  CURB  AND  GUTTER. 

738.  Foundation.  A  trench  is  excavated  4  to  6  inches  wider 
than  the  base  of  the  concrete,  and  a  layer  of  cinders  or  gravel  4  to 
8  inches  thick  (usually  6  inches)  is  laid,  flooded  with  water,  and 
then  thoroughly  tamped.  Upon  this  foundation  is  erected  the 
forms  in  which  the  concrete  is  to  be  laid. 


3-0" 


FIG.  139. — ST.  Louis  CONCRETE  CURB  AND  GUTTER  FOR  PARK  DRIVE. 

739.  The  Forms.  There  are  two  general  methods  of  constructing 
these  forms:  1.  Some  contractors  lay  alternate  sections  in  boxes 
about  6  feet  long,  and  subsequently  place  boards  against  the  sections 
first  laid  and  construct  the  remaining  sections.  This  plan  is  more 
expensive  and  does  not  secure  as  good  alignment  as  the  method 
described  below.  2.  A  continuous  line  of  plank  is  set  for  the  back  of 
the  curb  and  another  for  the  front  of  the  gutter.  These  planks  are 


384  CURBS   AND   GUTTERS  [CHAP.   XIV 

kept  in  place  by  stakes  on  both  sides.     Partitions  are  inserted  so  as 
to  divide  the  mass  into  sections  6  or  8  feet  long. 

Two  forms  of  partitions  are  in  common  use.  Sometimes  these 
partitions  are  plank  Ij  or  2  inches  thick,  in  which  case  the  sections 
are  laid  alternately,  the  partitions  being  removed  before  the  second 
series  of  blocks  are  formed.  In  other  cases,  the  partitions  are  made 
of  steel  i  or  A  incn  thick,  and  are  left  in  position  until  the  blocks 
are  practically  finished.  There  is  but  little  choice  in  construction 
between  the  two  forms  of  partitions,  except  that  it  is  difficult  to 
withdraw  the  steel  partitions  without  chipping  the  surface — see  §  745. 

740.  The  form  for  the  front  of  the  curb  is  made  by  setting  a 
plank  Ij  or  2  inches  thick  against  the  front  of  the  upper  part  of  the 
partitions  and  clamping  it  to  the  plank  at  the  back  of  the  curb  with 
steel  screw-clamps.     The  lower  edge  of  this  plank  is  rounded  to 
make  the  curve  between  the  face  of  the  curb  and  the  top  of  the 
gutter. 

The  concrete  for  the  base  of  the  gutter  is  deposited  and  tamped, 
and  then  the  mortar  for  the  face  of  the  gutter  is  applied — all  before 
the  form  for  the  front  of  the  curb  is  clamped  into  place. 

741.  Mixing  and  Laying.     For  information  concerning  materials 
proportioning,    and   mixing,    see   Art.    1,    Chapter   VII, — Concrete 
Roads.* 

The  proportions  of  the  concrete  will  depend  upon  the  gradation 
of  the  aggregates  (§  418-24),  and  upon  whether  the  surface  is  to 
consist  of  a  richer  mixture  than  the  body  of  the  concrete  or  whether 
the  surface  is  to  be  finished  integral  with  the  body.  Usually  a 
1:2:4  or  a  1:3:4  mixture  is  used  for  the  body,  and  a  1  :  1^  or 
1  :  2  mixture  for  the  facing  mortar. 

742.  The  upper  face  of  the  gutter  slab  is  finished  by  adding  a 
1-inch  coat  of  rich  mortar  and  tamping  and  troweling  it,  exactly 
as  in  concrete  sidewalk  construction.     It  is  important  that  the  sur- 
face of  the  concrete  be  free  from  mud  or  dust  when  the  topping  is 
deposited;   and  this  is  a  condition  quite  difficult  to  secure,  because 
the  curb  and  gutter  is  built  in  a  narrow  trench,  often  a  considerable 
distance  below  the  surface,  and  usually  with  the  excavated  material 
piled  near.     It  is  also  important  that  the  facing  be  deposited  before 
the  concrete  has  begun  to  set. 

*  For  a  full  discussion  of  the  method  of  testing  the  cement  and  proportioning  the  concrete 
and  of  the  relative  merits  of  gravel  and  broken-stone  concrete,  together  with  tables  of  quanti- 
ties, strength,  cost,  etc.,  see  A  Treatise  on  Masonry  Construction  by  Ira  O.  Baker,  10th  edition, 
pp.  745,  6  X9  inches,  John  Wiley  &  Sons,  New  York  City. 


COMBINED    CURB    AND    GUTTER  385 

743.  There  are  three  distinct  ways  of  finishing  the  exposed  face 
of  the  curb : 

1.  The  surface  is  faced  with  a  rich  mortar  built  integral  with  the 
body.  This  may  be  obtained  in  either  of  three  ways,  of  which  the 
first  is  most  likely  to  secure  a  firm  union  between  the  backing  and 
the  facing,  a.  A  layer  of  rich  mortar  is  deposited  against  the  lower 
third  or  half  of  the  front  form,  and  then  the  concrete  backing  is 
deposited  and  both  are  well  tamped;  and  the  process  is  repeated 
once  or  twice  until  the  form  is  full,  when  the  top  surface  of  the  curb 
is  finished  by  adding  a  1-inch  layer  of  facing  mortar.  6.  A  1- 
inch  plank  is  inserted  inside  of  the  front  form,  and  the  concrete 
for  the  body  is  deposited  and  tamped;  then  the  1-inch  plank  is  care- 
fully removed,  and  a  rich  facing  mortar  is  tamped  into  the  vacant 
space,  c.  Instead  of  the  1-inch  plank  as  above,  use  a  steel  plate 
i  or  -fg  of  an  inch  thick  having  vertical  1-inch  angles  riveted  at 
intervals  along  its  length.  This  plate  is  inserted  behind  the  front 
form,  the  concrete  is  deposited,  and  then  the  facing  mortar  is  depos- 
ited between  the  front  form  and  the  steel  plate;  next  the  steel  plate 
is  removed,  and  the  facing  mortar  and  the  concrete  are  thoroughly 
tamped. 

2.  The  second  method  of  finishing  the  face  of  the  curb  is  to  mix 
the  concrete  rather  wet,  and  spade  the  face,  i.  e.,  after  the  concrete 
is  deposited  force  a  flat  spade  or  its  equivalent  vertically  against  the 
back  of  the  front  form,  and  then  push  the  handle  away  from  the 
form  to  an  angle  of  20°  or  30°  and  withdraw  the  spade.     If  properly 
done,  this  forces  the'  larger  stones  away  from  the  face  and  allows 
enough  mortar  to  flow  out  against  the  form  to  give  a  solid  face  and 
permit  a  good  finish  of  the  surface.     This  makes  the  strongest  face 
of  any  of  the  methods;  but  it  is  not  popular  with  contractors,  since 
to  get  a  solid  face  it  is  necessary  to  mix  the  concrete  so  wet  as  to 
greatly  delay  the  finishing  of  the  face,  which  is  objectionable  for 
several  reasons. 

3.  Sometimes  a  rich  face  is  obtained  by  plastering  the  surface 
after  the  forms  are  removed.     But  this  method  necessitates  using 
dry  concrete  in  the  backing,  so  as  to  remove  the  forms  early,  and 
consequently  is  likely  to  give  a  weak  and  porous  surface  upon  which 
to  apply  the  mortar.     It  is  nearly  impossible  by  this  method  to 
secure  a  firm  union  between  the  plastering  and  the  backing;  and  the 
plastering  is  nearly  certain  to  be  knocked  off  by  passing  wheels 
(particularly  during  the  excavation  of  the  roadway  when  the  passing 
wheels  are  heavy  and  the  concrete  is  weak)  and  to  be  spalled  off  by 


386  CURBS   AND   GUTTERS  [CHAP.   XIV 

the  pressure  due  to  the  temperature  expansion  of  the  curb.    Although 
this  method  is  sometimes  used,  it  should  never  be  permitted. 

744.  Finishing  the  Surface.  There  is  a  difference  of  opinion  as 
to  whether  the  surface  should  be  considered  finished  when  it  has  been 
troweled,  or  whether  it  should  be  afterwards  brushed  with  a  slightly 
wet  brush.  An  ordinary  flat  paint  brush,  with  extra  heavy  bristles, 
cut  off  about  1  inch  below  the  wood  portion,  may  be  used  for  this 
purpose.  The  objections  to  the  trowel-finished  surface  are  that  the 
trowel  marks  show  more  or  less,  and  that  the  surface  has  a  glaze  or 
shine  clearly  indicating  that  the  stone  is  artificial;  while  the  brush 
finish  has  a  uniform  dull  surface  similar  to  a  smoothly  dressed  natural 
stone.  The  objections  to  the  brush-finished  surface  are  that  the 
brush  leaves  a  porous  surface  that  is  not  so  durable  as  a  trowel- 
finished  one,  which  objection  has  considerable  force  if  the  surface  is 
not  first  thoroughly  troweled  and  if  the  brush  is  not  used  lightly. 
The  less  the  troweling  and  the  more  the  brushing,  the  more  rapidly 
the*  surf  ace  can  be  finished;  and  hence  it  is  difficult  when  brushing  is 
permitted  to  prevent  the  slighting  of  the  work.  Both  methods  of 
finishing  are  employed  by  competent  engineers;  but  the  trowel 
finish  is  more  common. 

Recently  a  method  of  finishing  by  drawing  a  template  over  the 
curb  and  gutter  has  been  introduced.  The  few  trials  made  seem  to 
show  that  this  method  is  a  little  less  expensive  than  finishing  with  a 
trowel,  but  that  it  gives  a  better  general  appearance  and  a  better 
alignment,  particularly  at  the  joints. 

745.  Expansion  Joints.  Concrete  curbs  'and  also  combined 
curbs  and  gutters  should  be  built  in  sections  6  or  7  feet  long  with 
open  joints  to  allow  for  expansion.  If  adequate  space  for  expansion 
is  not  provided,  the  compression  due  to  expansion  is  likely  to  crush 
and  splinter  the  curb  at  the  joints,  and  split  off  the  mortar  face  or 
the  plastering.  Many  such  failures  occur.  For  a  discussion  of  a 
similar  problem,  see  Contraction  Joints  in  the  Chapter  on  Concrete 
Roads  (§  465-68).  Sometimes  no  provision  is  made  for  expansion, 
the  curb  or  curb  and  gutter  being  made  continuous  with  false  joints 
marked  at  intervals.  Sometimes  the  curb  or  curb  and  gutter  is 
built  in  alternate  sections  without  any  expansion  space  between 
adjoining  sections,  the  joints  being  simply  a  line  of  weakness  which 
serves  to  prevent  an  unsightly  crack  if  a  section  is  displaced  by  frost 
or  by  the  lateral  pressure  of  the  earth.  The  curb  and  gutter  should 
be  a  practically  permanent  asset,  and  hence  the  prevention  of  its 
destruction  by  temperature  changes  is  an  important  matter.  One  of 


COMBINED    CURB    AND    GUTTER  387 

three  means  may  be  employed  to  prevent  failures  at  the  joints  of 
curbs  and  gutters  due  to  expansion. 

1.  The  gutter  flag  and  the  curb  are  cut  into  short  sections  after 
being  laid,  much  as  a  sidewalk  slab  is  separated  into  short  sections. 
The  expectation  is  that  the  open  joint  will  afford  sufficient  space  for 
expansion;  but  it  is  likely  to  be  filled  on  the  face  during  the  finishing 
of  the  face  of  the  curb  or  gutter,  which  is  particularly  bad  if  a  mortar 
face  is  used  or  if  the  exposed  surface  is  plastered.     Generally  the 
open  joint  is  reasonably  successful  for  a  time;  but  is  likely  to  become 
filled  with  dirt  and  cease  to  be  effective,  and  besides  the  open  joint  in 
the  curb  permits  the  earth  behind  the  curb  to  escape,  or  with  brick 
or  block  pavements  the  open  joint  in  the  gutter  permits  the  sand 
cushion  to  escape. 

2.  Instead  of  cutting  the  curb  and  gutter  as  described  in  the 
preceding  paragraph,  it  is  much  more  common  to  insert  steel  dia- 
phragms or  partitions  in  the  forms  at  intervals  of  6  or  7  feet,  which 
are  withdrawn  as  the  face  of  the  curb  and  gutter  is  finished.     Thfese 
partitions  are  usually  £  or  -£$  inch  thick.     This  joint  is  more  efficient 
than  the  one  described  above,  and  is  easier  to  construct;    but  it 
is  open  to  substantially  all  of  the  objections  to  the  preceding  one. 

3.  Occasionally  partitions  consisting  of  one  or  two  thicknesses 
of  tar  paper  or  one  thickness  of  tar  or  asphalt  felt  are  inserted  in  the 
forms  at  intervals  of  6  or  7  feet;   and  after  the  face  of  the  curb  and 
gutter  has  been  finished  the  paper  or  felt  is  cut  off  with  a  sharp  knife 
a  little  below  the  surface  of  the  concrete. 

746.  Whatever  the  method  of  constructing  the  expansion  joint,  it 
should  be  in  a  vertical  plane  perpendicular  to  the  face  of  the  curb,  or 
one  section  may  push  past  the  other. 

With  any  form  of  expansion  joint,  there  is  danger  that  the  expan- 
sion of  the  straight  curb  will  dislocate  the  curved  curb  at  the  corner 
of  the  block  and  at  the  alley  return.  This  can  be  prevented  by 
making  an  extra  wide  expansion  joint  near  the  corner.  This  joint 
may  be  |  to  1  inch  wide  according  to  the  length  of  the  block  and  the 
temperature  when  the  curb  is  constructed;  and  it  should  be  filled 
with  tar  pitch  or  asphalt.  A  number  of  proprietary  compounds  are 
upon  the  market  for  this  purpose,  made  in  sheets  of  different  thick- 
nesses. 

747.  Curbs  are  frequently  damaged  by  being  pushed  over  or 
broken  by  the  expansion  of  cement  walks.     The  remedy  is  to  insert 
an  expansion  joint  between  the  end  of  the  walk  and  the  back  of  the 
curb,  or  better  in  the  joint  in  the  walk  one  section  back  from  the  curb. 


388  CUEBS   AND    GUTTERS  [CHAP.    XIV 

The  expansion  joint  may  be  filled  with  tar  paper  or  felt;  or  a  thin 
board  may  be  inserted  during  the  construction,  and  after  the  con- 
crete has  set  the  board  is  withdrawn  and  the  space  is  filled  with  tar 
pitch.  The  joints  must  occasionally  be  re-filled — preferably  once 
each  year. 

748.  Cost.  The  cost  will  depend  upon  the  price  of  labor  and 
materials,  and  upon  the  proportions  of  the  mortar  of  the  face  and 
the  concrete  of  the  body.  The  amount  of  cement  required  will  vary 
a  little  with  the  percentage  of  voids,  but  will  depend  chiefly  upon  the 
proportions  of  the  mortar  and  the  concrete. 

The  following  data  are  for  laying  more  than  a  mile  of  combined 
curb  and  gutter  of  the  form  shown  in  Fig.  138,  page  383.  The  pro- 
portions of  the  facing  mortar  was  1:2;  and  that  of  the  concrete 
1:3:4  washed  gravel.  A  barrel  of  portland  cement  made  enough 
1  :  2  mortar  for  the  facing  on  33  linear  feet.  The  length  of  finished 
curb  and  gutter  laid  with  a  barrel  of  cement  was  16  feet,  with  a  varia- 
tion of  1  per  cent  either  way  on  different  days.  A  yard  of  sand  and 
pebbles  laid  18  lineal  feet.  The  loss  of  gravel  in  transportation  and 
handling,  and  the  shrinkage  in  tamping  was  nearly  uniformly  20 
per  cent.  The  average  length  of  curb  and  gutter  completed,  includ- 
ing straight  work  and  curved  returns  at  streets  and  alleys,  and  also 
including  excavation,  per  man  per  hour  was  0.333  foot.  The  trench 
was  excavated  before  the  roadway  was  excavated.  The  concrete 
was  mixed  in  a  batch  mixer  which  discharged  directly  into  the 
trench.  The  two  finishers  received  60  cents  per  hour,  the  three  men 
setting  face  forms  35  cents,  and  the  remainder  25  cents.  The  work- 
ing force  of  18  men  when  constructing  forms  and  laying  curb  and 
gutter  was  divided  as  follows: 

1  foreman  and  finisher, 

1  finisher, 

2  men  setting  face  forms, 

3  men  setting  back  forms, 

2  men  wheeling  and  tamping  cinders, 
2  men  running  concrete  mixer, 
2  men  feeding  concrete  mixer, 
2  men  mixing  face  mortar  by  hand, 
1  man  wheeling  facing  mortar, 
1  man  spreading  facing  mortar, 
1  boy  carrying  water,  etc. 

The  contract  price  in  1916  for  the  curb  and  gutter,  including 
excavation  and  back  filling,  was  50  cents  per  lineal  foot. 


COMBINED    CURB   AND   GUTTER 


389 


749.  Double  Curb  and  Gutter.  Fig.  140  *  shows  the  details  of 
the  form  of  the  concrete  double  curb  and  gutter  referred  to  in  Fig. 
114,  page  357. 


timfe  r  '//I////,///,//////////////, 

^  l///6fortcr6v36«/  ffoc* ''/////, 
v  y.\\\\\\v  .\\\\\\\\\\\\\\\  w^ \ \\\\\\ \ 
Fio.  140. — DOUBLE  CURB  AND  GUTTER. 

750.  Curb  and  Gutter  at  Private  Driveway.     Fig.  141,  page  390, 
shows  the  arrangement  of  the  combined  concrete  curb  and  gutter 
at  a  driveway  to  a  gate  or  a  building.     The  radius  of  the  curve  at 
the  corner  of  the  curb  is  too  small,  as  a  radius  of  4  or  5  feet  would  be 
better. 

751.  Merits   of  Concrete   Curb   and   Gutter.    The  advantages 
of  the  combined  concrete  curb  and  gutter  are:    1.  It  is  usually 
cheaper,  particularly  if  account  be  taken  of  the  fact  that  the  gutter 
occupies  space  that  otherwise  would  be  paved.     2.  The  alignment 
of  the   curb  is  better  and  more   permanent.     3.  The  appearance 
is  better.     4.  Usually  the  concrete  is  more  durable  than  a  natural 
stone  of  equal  cost.     5.  The  gutter  is  smooth,  and  easily  cleaned. 

A  concrete  curb  is  suitable  only  for  residence  streets,  but  is  more 
durable  for  a  business  street  than  soft  sandstone  or  limestone. 

752.  Other    Forms    of    Curbs.     About    1889   there    was    con- 
structed on  two  streets  at  Washington,  D.  C.,  a  concrete  curb  and 
gutter  having  at  the  inner  lower  edge  of  the  curb  a  4X  4-inch  con- 
duit for  telegraph  and  telephone  wires,  with  hand  holes  about  50 
feet  apart.     The  experiment  was  not  considered  successful,  and  the 
conduit  was  never  used  for  wires. 

From  time  to  time  advertisements  appear  of  burned  clay  curbs; 
but  none  have  been  seen  which  are  not  so  thin  as  to  be  easily  broken, 
and  so  constructed  by  sections  fitted  together  as  to  be  unstable. 

753.  RADIUS  OF  CURB  AT  STREET  CORNER.    As  far  as  vehicular 
traffic  is  concerned,  the  larger  the  radius  of  the  curb  the  better; 


*  Trans.  Amer.  Soc.  of  Civil  Engineers,  Vol.  42,  p.  7. 


390 


CURBS  AND   GUTTERS 


[CHAP,  xiv 


but  when  the  gutter  is  carried  to  a  corner  inlet  (§  705),  it  is  incon- 
venient to  construct  or  cover  the  gutter  if  the  curved  curb  intersects 
the  sidewalk,  i.  e.,  if  the  radius  of  the  curved  curb  is  too  great.  If 
the  pavement  has  the  minimum  width,  say  18  or  20  feet,  the  curves 
of  the  corner  curbs  should  be  made  large  so  that  a  vehicle  may  be 
turned  around  at  the  street  intersection. 


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Section  A-B 


Section  C-D- 
Fia.  141. — CONCRETE  CURB  AND  GUTTER  AT  PRIVATE  DRIVEWAY. 

Formerly,  when  curbs  usually  were  natural  stone,  the  cost  of 
curved  sections  was  considerably  more  than  that  of  straight  pieces ; 
and  hence  the  tendency  was  to  keep  the  radius  as  small  as  possible. 
But  now  that  concrete  curb  is  very  common,  curved  curbs  cost  only 
a  little  more  than  straight  ones.  Formerly,  when  most  of  the  vehic- 
ular traffic  was  horse-drawn,  the  chief  objection  to  a  short  radius  was 
the  wear  due  to  wheels  striking  the  curb  at  the  corner;  but  now  on 
streets  having  any  considerable  motor-driven  traffic,  a  corner  curb 


RADIUS  OF  CURB  AT  CORNER  391 

having  a  short  radius  makes  it  nearly  impossible  for  a  motorist  to 
maintain  a  reasonable  rate  of  speed  in  turning  the  corner  and  at  the 
same  time  keep  his  machine  on  the  right  side  of  the  street. 

The  radius  varies  from  2  to  12  feet,  usually  from  6  to  8  feet.  The 
curb  with  a  2-  oot  radius  should  not  be  used  at  a  1,  or  at  least  only  at 
driveways  to  private  grounds.  A  radius  of  10  to  12  feet  is  usually 
satisfactory;  but  on  boulevards  or  streets  where  there  is  consider- 
able automobile  travel,  a  radius  of  16  or  even  20  feet  is  desirable. 
Several  cities  have  recently  spent  considerable  money  to  increase 
the  radius  of  the  corner  curbs,  particularly  on  main  traveled  streets 
and  boulevards,  to  the  satisfaction  and  safety  of  automobilists. 

754.  COMBINED  CURB  AND  WALK.  In  Chicago  a  concrete 
walk  about  1  foot  wide  has  been  constructed  along  the  curb  in  front 
of  a  large  apartment  building,  so  as  to  permit  vehicles  to  stop  any- 
where along  the  curb  to  discharge  or  receive  passengers.  Such  a 
walk  has  been  found  to  be  a  great  convenience. 


CHAPTER  XV 

FOUNDATIONS  FOR  PAVEMENTS  AND   STREET-RAILWAY 

TRACKS 

757.  The  term  foundation  is  sometimes  applied  to  the  natural 
soil  upon  which  an  artificial  structure  rests,  and  sometimes  to  the 
lower  portion  of  the  structure  itself.     The  term  will  be  employed 
here  in  the  latter  sense,  and  the  soil  under  the  foundation  of  the 
pavement  will  be  referred  to  as  the  subgrade. 

The  foundation  of  a  pavement,  as  of  all  other  structures,  is  an 
important  element,  although  it  is  more  frequently  neglected  in  pave- 
ments than  in  other  structures. 

One  of  the  most  perplexing  problems  in  connection  with  pave- 
ments is  the  construction  and  maintenance  of  a  pavement  adjoining 
a  street-railway  track  that  is  durable  and  does  not  interfere  unduly 
with  travel.  Part  of  the  difficulties  are  due  to  the  foundation  and 
part  to  the  construction  of  the  pavement  adjacent  to  the  rails.  The 
former  will  be  considered  in  Art.  3  of  this  Chapter,  and  the  latter  in 
connection  with  the  discussion  of  the  different  kinds  of  pavements. 

ART.  1.    PREPARATION  OF  THE  SUBGRADE 

758.  Whatever  the  form  of  the  pavement  or  of  its  foundation,  it 
must  rest  upon  the  soil;   and  s'nce  the  chief  office  of  the  pavement 
and  of  its  foundation  is  to  distribute  the  concentrated  load  of  the 
wheel  over  an  area  so  great  that  the  natural  soil  will  be  able  to 
support  it,  it  is  important  to   ncrease,  as  much  as  practicable,  the 
bearing  power  of  the  soil  by  drainage  and  by  rolling,  and  thereby  to 
decrease  the  thickness  of  pavement  required. 

759.  DRAINAGE.      The  method  of  draining  the  subgrade  of  a 
pavement  is  substantially  the  same  as  that  of  underdraining  an 
earth  road — see  §  114.     The  subgrade  of  a  pavement  requires  under- 
drainage  fully  as  much  as  does  an  earth  road,   notwithstanding 
the  fact  that  the  former  has  an  impervious  roof.    The  purpose 

392 


ART.  1]  PREPARATION  OF  THE  SUBGRADE  393 

of  the  underdrainage  is  to  prevent  the  surface  of  saturation  from 
rising  so  high  as  to  soften  the  subgrade.  Unless  the  subsoil  is  very 
open  and  porous,  it  is  economical  to  lay  a  tile  under  each  edge  of 
the  pavement,  2  or  3  feet  below  the  surface  of  the  subgrade.  This 
tile  may  empty  into  the  surface-water  catch  basins  (§  703). 

760.  EARTHWORK.  The  machinery  employed  in  making  exca- 
vations and  embankments  for  pavements  is  practically  the  same 
as  that  used  in  constructing  earth  roads — see  §  148-57. 

In  making  embankments  great  care  should  be  taken  to  com- 
pact them  solid — see  Shrinkage  of  Earthwork  (§  140),  Settlement 
of  Embankments  (§  141),  Rol  ing  Embankments  (§  143),  and  Sta- 
bility of  Embankment  (§  146).  For  data  on  the  Cost  of  Earth- 
work, see  §  164-87. 

The  excavation  for  pavements  is  made  by  plowing  and  then 
removing  the  earth  either  with  a  drag  or  a  wheel  scraper  (§  150,  154), 
or  by  loading  it  into  wagons  or  carts  with  hand-shovels.  The 
subgrade,  even  though  on'y  a  comparatively  thin  layer  is  to  be 
removed,  has  recently  been  excavated  and  loaded  into  wagons  with 
a  steam  shovel  usually  of  the  revolving  type;  and  more  recently 
the  excavation  has  been  made  with  the  four-wheel  scraper  (§  154) 
drawn  by  a  steam  engine  while  being  loaded.  It  is  usual  to  specify 
that  no  plowing  shall  be  allowed  within  2  inches  of  the  subgrade,  to 
prevent  the  soil  below  the  subgrade  from  being  loosened.  If  the 
subgrade  is  thoroughly  rolled,  as  described  later,  plowing  a  little 
below  the  finished  surface  is  not  a  serious  matter;  but  if  the  sub- 
grade  is  not  subsequently  well  rolled,  the  loosening  of  the  soil  below 
the  finished  surface  is  very  objectionable,  since  the  foundation  will 
then  have  an  uneven  hardness. 

The  subgrade  is  often  finished 
with  pick  and  shovel,  but  the 
work  can  be  done  much  more 
economically  with  the  scraping 
grader  (§  155)  or  with  the  sur- 
face grader,  Fig.  142.  The  former 
makes  a  more  uniform  surface, 
and  is  usually  more  economical; 

although  the  latter  is  an  effective  FIG.  142.— SURFACE  GRADER. 

implement.     In    either    case    the 

loosened  earth  must  be  hauled  away  with  scrapers  or  wagons. 

762.  A  considerable  part  of  the  excavation  is  often  done  before 
the  curb  is  set,  but  the  curb  is  always  set  before  the  subgrade  is 


394  FOUNDATIONS    FOR    PAVEMENTS  [CHAP.    XV 

finished.  The  exact  position  of  the  subgrade  is  determined  by 
stretching  a  string  transversely  across  the  street  from  curb  to  curb 
and  measuring  ordinates  similar  to  those  shown  in  the  upper  dia- 
gram of  Fig.  135,  page  375.  Some  contractors  pick  narrow  trenches 
down  to  the  subgrade  at  short  intervals  transversely  across  the 
street;  while  others  drive  stakes  with  their  tops  a  specified  dis- 
tance, say  4  or  6  inches,  above  subgrade,  and  provide  the  work- 
men with  a  stick  of  this  length  with  which  to  measure  down  from 
the  top  of  the  stake  to  the  subgrade.  The  former  method  must 
be  employed  when  the  scraping  grader  is  used.  The  passage 
of  the  grader  fills  the  trench  with  loose  earth,  but  it  is  easy 
to  see  the  relative  position  of  the  surface  and  the  bottom  of  the 
trench. 

763.  ROLLING  THE  SUBGRADE.  The  finished  subgrade  should 
be  thoroughly  rolled  to  consolidate  the  surface  and  also  to  discover 
any  soft  places — particularly  over  trenches  that  have  not  been  solidly 
filled.  If  the  roller  reveals  a  low  place,  it  should  be  filled  with  earth 
and  be  rolled  again.  The  roller,  whatever  its  weight,  should  be 
passed  over  the  subgrade  more  than  once,  since  the  successive  pas- 
sages have  something  of  a  kneading  action  and  add  to  the  solidity 
of  the  soil.  Several  passes  with  a  light  roller  give  better  results  than 
a  few  passes  with  a  heavy  one.  It  is  well  to  specify  both  the  weight 
of  the  roller  and  the  number  of  times  it  is  to  pass  over  the  road-bed. 
For  some  hints  applicable  in  rolling  the  subgrade,  see  §  369. 

Formerly  a  horse  roller  was  sometimes  used  for  this  purpose; 
but  a  steam  or  rather  a  self-propelled  roller  (§  378)  is  much  better 
because  it  is  heavier,  and,  still  more  important,  because  with  it  the 
street  can  be  rolled  transversely.  The  street  is  full  of  trenches  made 
often  just  before  the  pavement  is  laid,  in  connecting  the  houses  with 
the  sewer,  the  water,  and  the  gas;  and  as  these  trenches  run  both 
longitudinally  and  transversely,  it  is  necessary  to  run  the  roller  in 
both  directions  if  the  trenches  are  certain  to  be  solidly  filled. 

Unless  the  back-filling  of  a  trench  has  been  unusually  well  tamped, 
a  roller  run  transversely  over  a  trench  will  leave  a  depression.  In 
most  soils,  the  back-filling  will  not  of  itself  settle  into  its  former 
solidity,  however  long  it  is  left  to  the  action  of  traffic  and.  to  the  forces 
of  nature;  and  whatever  the  foundation  of  the  pavement,  the  heavier 
traffic  is  nearly  certain  to  cause  a  settlement  over  these  same  trenches, 
unless  the  subgrade  is  well  rolled.  Traffic  consolidates  only  a  thin 
layer  near  the  surface  which  is  usually  removed  when  the  pave- 
ment is  constructed.  Ordinarily,  if  the  subgrade  is  rolled  both  longi- 


ART.  2]  PREPARATION  OF  THE  SUBGRADE  395 

tudinally  and  transversely  with  a  roller  weighing  10  or  12  tons, 
there  will  be  no  settlement  of  the  pavement. 

In  rolling,  if  a  depression  is  produced  over  a  trench,  it  should 
be  filled  and  then  again  rolled.  If  the  depression  is  of  considerable 
depth,  it  shows  that  the  trench  was  badly  filled  or  was  very  deep, 
or  both;  and  therefore  it  is  wise  to  re-consolidate  the  trench.  One 
way  of  doing  this  is  to  make  numerous  openings  through  the  crust 
and  keep  the  depression  filled  with  water  until  the  earth  in  the 
bottom  of  the  trench  has  become  thoroughly  soaked;  and  then 
after  the  ground  has  dried  out  below,  the  roller  should  again  be  passed 
over  the  surface.  The  surest  way  to  prevent  settlement  over  trenches 
is  to  pack  the  soil  solidly  when  the  trench  is  first  filled.  For-  a  dis- 
cussion of  various  methods  of  back-filling,  see  §  764. 

Insufficient  tamping  in  filling  trenches  or  inefficient  rolling 
of  trenches  is  a  very  common  defect  in  pavement  construction, 
nearly  every  block  presenting  one  or  more  such  depressions.  One 
of  the  purposes  of  a  guarantee  of  the  pavement  (§  652)  is  to  secure 
a  thorough  consolidation  of  the  soil  in  the  trenches. 

764.  FILLING  TRENCHES.     The  back-filling  of  trenches  opened 
to  lay  water  and  gas  pipes,  to  make  house  connection  to  sewers, 
etc.,  so  that  the  road  surface  shall  be  restored  to  its  former  level 
and  remain  so,  is  a  matter  of  importance  on  both  paved  and  un- 
paved   roads— particularly   the  former.     The  failure  to  re-fill  the 
trenches  properly  is  a  source  of  annoyance  to  those  who  use  the 
unpaved  road  and  of  damage  to  the  pavement.     It  is  frequently 
asserted  by  those  having  opportunity  for  knowing,  that  the  dam- 
age to  pavements  through  lack  of  care  in  re-filling  trenches  and 
re-placing  the  pavements  is  greater  than  the  wear  due  to  traffic. 
No  kind  of  municipal  work  should  be  more  rigorously  inspected 
than  the  filling  of  a  trench  over  which  a  pavement  is  to  be  laid. 
The  nearly  universal  result  of  a  neglect  in  this  respect  is  that  a 
pavement  built  at  great  expense  is  disfigured  or  damaged  by  settle- 
ment, the  repair  of  which  will  cost  many  times  as  much  as  it  would 
have  cost  properly  to  fill  the  trench  originally. 

The  principal  cause  of  failure  is  lack  of  care;  but  sometimes  it 
is  due  to  a  mistake  as  to  the  proper  method  to  be  employed.  A 
discriminating  judgment  is  required  to  determine  the  proper  method, 
and  intelligence  and  faithfulness  are  necessary  in  carrying  it  out. 
There  are  several  distinct  methods  used  in  consolidating  the  back- 
filling of  trenches. 

765.  Natural  Settlement.    A  common  practice  of  those  having 


396  FOUNDATIONS    FOR   PAVEMENTS  [CHAP.   XV 

occasion  to  make  excavations  in  unpaved  streets  is  to  cast  back 
loosely  the  material  taken  out,  heaping  it  into  an  unsightly  and 
annoying  ridge  over  the  trench  and  trusting  to  travel  and  the  ele- 
ments to  restore  the  surface  to  its  original  level.  In  nearly  pure 
sand  such  a  ridge  may  in  time  settle  to  the  original  level,  although 
the  damage  due  to  the  temporary  ridge  will  generally  be  much 
more  than  the  cost  of  properly  filling  the  trench  in  the  beginning; 
but  as  a  rule  loam  or  clay  loosely  put  back  will  not  attain  a  sufficient 
degree  of  compactness  to  make  it  a  safe  support  for  a  macadam  or 
other  form  of  pavement.  The  surface  may  become  very  compact 
and  hard;  and  yet  after  the  removal  of  a  foot  or  more  of  soil, 
ordinarily  necessitated  by  the  construction  of  the  pavement,  it  will 
be  found  that  the  earth  in  the  trench  will  settle  considerably  under  a 
roller  run  transversely  over  the  trench.  Even  though  the  surface 
may  support  the  roller,  it  is  highly  probable  that  ultimately  a  trench 
which  has  been  loosely  filled  will  settle  and  cause  a  depression  in  the 
pavement.  This  is  proved  by  the  numerous  depressions  in  pave- 
ments, and  also  by  the  fact  that  when  trenches  loosely  filled  are 
opened  years  afterwards,  it  is  very  common  to  find  open  cavities. 
The  promptness  with  which  natural  settlement  takes  place  depends 
upon  the  climatic  conditions  and  the  underdrainage.  It  is  never 
safe  to  depend  upon  natural  settlement  to  secure  the  proper  com- 
pacting of  the  soil  in  trenches  over  which  a  pavement  is  to  be  laid, 
however  long  the  time  allowed  for  the  settlement,  and  much  less 
the  few  weeks  often  specified. 

766.  Flooding.  Where  the  water  can  be  had  cheaply,  it  is  a 
common  practice  to  attempt  to  consolidate  the  earth  in  the  trench 
by  flooding  or  puddling  it.  If  the  soil  is  sand  or  gravel  and  is  so 
pervious  that  the  trench  will  drain  out  rapidly,  thorough  flushing 
will  compact  the  material  so  that  no  trouble  will  be  experienced 
with  settlement;  but  the  flushing  must  be  done  thoroughly.  It 
is  not  sufficient  to  fill  the  trench  nearly  full  of  loose  material,  and 
then  turn  on  a  gentle  stream  of  water  until  the  trench  is  full;  for 
trenches  thus  filled  are  certain  to  settle  later.  The  sand  or  gravel 
should  be  added  in  layers  not  more  than  8  or  10  inches  thick,  and 
each  layer  should  be  flushed  with  a  stream  of  water  having  force 
enough  to  wash  the  finer  particles  into  the  voids  between  the  larger 
ones.  Substantially  the  same  result  may  be  accomplished  by 
shoveling  the  sand  or  gravel  into  water  8  or  10  inches  deep;  but 
this  method  will  not  be  effective,  if  the  trench  is  filled  with  a  scraper 
or  a  scraping  grader. 


ART.  2]  PREPARATION  OF  THE  SUBGRADE  397 

However,  wherever  flushing  is  effective,  tamping  would  be 
equally  as  good  and  would  probably  be  less  expensive,  if  the  cost 
of  the  water  be  considered.  As  a  rule  attempting  to  consolidate 
trenches  by  flooding  is  bad  practice. 

Neither  of  the  preceding  methods  of  using  water  should  be 
employed  with  clay  or  clayey  soils,  since  flushing  prevents  rather 
than  assists  the  consolidation  of  such  soils.  In  other  words,  flush- 
ing or  puddling  is  useful  only  with  soils  which  water  readily  breaks 
down.  If  clay  is  flooded  or  is  deposited  in  water,  the  trench  is  filled 
with  a  watery  mud  that  will  shrink  very  much  as  it  dries  out  and  will 
always  be  loose  and  porous.  It  is  well  known  that  a  stiff-mud 
brick  which  has  been  moulded  under  exceedingly  heavy  pressure 
will  shrink  in  drying  5  per  cent,  and  with  some  clays  10  per  cent; 
and  of  course  the  thin  clay  mud  in  a  flooded  trench  will  shrink  very 
much  more  than  this. 

767.  Tamping.  Except  in  the  case  of  comparatively  clean 
sand  and  gravel,  back-filling  can  be  thoroughly  done  only  by  tamp- 
ing; and  to  make  this  method  successful  it  is  necessary  (1)  that 
the  material  shall  be  moist  enough  to  be  plastic,  but  neither  too  wet 
nor  too  dry,  (2)  that  it  shall  be  deposited  in  layers  not  more  than 
3  or  4  inches  thick,  and  (3)  that  each  layer  shall  be  thoroughly  tamped. 
To  secure  thorough  tamping  the  relative  numbers  of  tampers  and 
shovelers  is  sometimes  specified;  but  this  alone  is  ineffectual  since 
there  is  a  natural  tendency  for  the  tampers  to  work  less  energetically 
than  the  shovelers,  and  besides  more  labor  is  required  to  tamp  the 
soil  around  the  pipe  than  higher  up. 

The  amount  of  ramming  required  will  vary  with  the  character 
and  condition  of  the  soil.  Clay  and  hard  pan  should  be  moistened 
before  being  tamped,  while  clean  sand  or  clean  gravel  may  be  tamped 
dry.  The  tamping  can  be  most  effectively  done  with  a  compara- 
tively small  light  rammer  or  tamper,  since  the  effect  of  the  blow 
is  transmitted  to  a  greater  depth,  while  a  broad  heavy  rammer 
consolidates  the  surface  only.  A  tamper  weighing  5  or  6  Ib.  is  better 
than  one  weighing  20  or  25  Ib.,  the  lighter  one  being  lifted  higher 
and  giving  less  fatigue  than  the  heavy  one.  It  is  important  to 
remember  that  any  amount  of  ramming  will  affect  only  a  compara- 
tively thin  layer. 

Obviously  back-filling  should  not  be  attempted  when  the  mate- 
rial is  frozen,  since  subsequent  settlement  is  then  sure  to  take  place. 
768.  To  prevent  disturbing  the  surface  of  a  pavement,  plumbers, 
gas  fitters,  etc.,  are  sometimes  given  permission  to  tunnel  under 


398  FOUNDATIONS    FOR    PAVEMENTS  [CHAP.    XV 

the  pavement  to  make  their  connections.  This  practice  is  never 
justifiable  on  account  both  of  the  excessive  cost  and  of  the  impos- 
sibility of  effectively  filling  the  tunnel,  owing  to  the  limited  space 
in  which  the  work  must  be  done.  In  nearly  every  case  a  depression 
occurs  sooner  or  later  over  the  tunnel. 

769.  Replacing  All   the   Material.    The  result  to  be   obtained 
in  filling  a  trench  is  that  the  material  in  the  trench  shall  have  the 
same  compactness  as  the  soil  around  it;    and  therefore  some  con- 
tend that  the  only  proper  way  is  to  put  back  all  the  material  taken 
out.     In  a  majority  of  cases  this  procedure  will   secure  reason- 
ably good  results;    but  under  certain  conditions  it  will  fail.     For 
example,  the  water  pipe  or  sewer  may  occupy  a  large  proportion 
of  the  volume  of  the  trench,  and  consequently  of  necessity  there 
will  be  a  considerable  excess  of  earth.     Again,   putting  back  all 
the  earth  does  not  insure  the  restoration  of  the  original  surface 
nor  certainly  prevent  subsequent  settlement.     It  has  been  shown 
that  soil  when  taken  from  its  natural  place  and  compacted  in  an 
embankment  will  shrink   from  8  to  15  per  cent  (see  §  140),  and 
will  probably  subsequently  settle  2  or  3  per  cent  and  possibly  10  to 
25  per  cent  (see  §  141).     Consequently  with  a  deep  trench  con- 
taining a  small  pipe,  it  is  possible  to  tamp  the  earth  back  so  solidly 
as  not  to  have  enough  to  restore  the  surface;    or  it  is  possible  to 
put  all  the  soil  back  by  tamping  the  lower  portion  of  the  trench 
solidly  and  the  upper  portion  loosely,  and  still  considerable  settle- 
ment take  place.     Therefore  the  specification  to  re-place  all  of  the 
material,  must  have  a  careful  and  intelligent  supervision  to  insure 
good  results. 

In  the  past  it  has  not  been  the  custom  to  fill  trenches  in  such 
a  manner  as  to  prevent  settlement;  and  therefore  if  the  best  results 
are  to  be  insisted  upon,  the  specifications  should  clearly  reveal  that 
fact,  for  contractors  in  bidding  on  work  do  so  on  the  understanding 
that  the  work  is  to  be  done  in  at  least  approximately  the  usual 
manner,  and  any  attempt  to  have  it  done  in  any  better  way,  which 
was  not  clearly  understood  from  the  beginning,  is  likely  to  cause 
friction  and  irritation,  and  possibly  finally  to  result  in  failure. 

770.  Re-filling  with  Sand  or  Concrete.     On  account  of  the  dif- 
ficulty of  getting  trenches  in  clay  or  loam  filled  so  that  there  will 
be  no  settlement,  it  has  been  proposed  to  require  the  trench  to  be 
filled  with  clean  sand  or  gravel.     It  is  not  known  that  this  method 
has  ever  been  tried.     It  would  probably  be  effective,  but  usually 
its  cost  would  be  prohibitive. 


ART.    2]  THE    CONSTRUCTION  399 

In  at  least  a  few  cases  trenches  have  been  filled  with  a  fair  qual- 
ity of  hydraulic  cement  concrete.  The  expense  for  the  concrete 
was  not  justifiable,  since  it  was  much  greater  than  that  required 
thoroughly  to  tamp  the  back-filling. 

Sometimes  municipal  authorities  are  lax  in  inspecting  the  filling 
of  trenches,  owing  to  the  belief  that  the  concrete  foundation  will 
hold  up  the  pavement  even  though  the  material  in  the.  trench  may 
settle;  but  this  is  bad  practice,  since  the  ordinary  thickness  of  con- 
crete is  not  designed  to  act  as  a  bridge,  and  besides  if  it  is  thick 
enough  to  bear  up  over  trenches  it  is  needlessly  thick  elsewhere. 
With  the  usual  thickness  of  concrete  foundations,  a  depression  is 
almost  certain  to  occur  if  the  material  in  the  trench  settles;  and 
hence  the  only  safe  rule  is  to  have  the  trenches  completelv  and 
compactly  filled. 

ART.  2.     THE  CONSTRUCTION 

772.  In  some  cases  a  pavement  has  been  laid  directly  upon  the 
natural  soil;    but  this  is  possible  only  with  brick,  stone-block,  or 
wood-block  pavements  laid  upon  clean  sand  or  gravel.     This  prac- 
tice is  wise  only  with  light  travel.     Formerly  in  Cleveland,  Ohio, 
many  brick  pavements  were  laid  directly  upon  the  native  sand. 

Stone-block  and  brick  pavements  were  formerly  laid  upon  a  layer 
of  gravel  or  broken  stone ;  but  the  decline  in  price  of  hydraulic  cement 
has  made  it  economical  to  substitute  concrete  for  the  layer  of  gravel 
or  broken  stone,  owing  to  the  labor  and  care  required  to  secure  a 
bed  of  uniform  density  and  smooth  surface. 

A  layer  of  portland  cement  concrete  is  now  the  nearly  universal 
foundation  for  street  pavements. 

773.  PORTLAND-CEMENT  CONCRETE  FOUNDATION.     This  is 
by  far  the  most  common  foundation  for  pavements.     The  advan- 
tages of  such  a  foundation  are :  1 .  It  gives  a  smooth  uniform  surface 
upon  which  to  lay  the  pavement.     2.  It  prevents  the  surface  water 
from  percolating  to  the  subgrade.     3.  By  its  thickness  and  resistance 
to  flexure,  it  distributes  the  concentrated  load  over  a  considerable 
area  of  the  subgrade.     4.  Concrete  acts  as  a  bridge  to  support  the 
pavement  in  case  of  a  settlement  of  the  subgrade.     5.  Being  imper- 
vious to  water  and  a  non-conductor  of  heat,  concrete  protects  water 
and  gas  pipes  from  freezing. 

774.  The  Materials.     For  a  discussion  of  the  cement,  the  sand, 
and  the  gravel  or  broken  stone  as  ingredients  for  concrete,  see  Art.  1 
of  Chapter  VII. 


400  FOUNDATIONS    FOR   PAVEMENTS  [CHAP.  XV 

775.  Thickness.  The  thickness  of  the  concrete  varies  from  4  to 
8  inches,  but  is  usually  6  inches.  There  is  considerable  diversity  of 
opinion  as  to  the  sufficiency  of  a  6-inch  concrete  foundation,  par- 
ticularly since  the  introduction  of  motor  trucks.  Examples  are 
frequently  cited  of  the  failure  of  a  concrete  foundation,  particularly 
where  a  motor  truck  or  heavily  loaded  wagon  has  broken  through  a 
pavement;  and  the  conclusion  is  drawn  that  the  concrete  slab  was 
too  thin.  However,  making  the  foundation  thicker  is  not  neces- 
sarily the  economical  remedy.  The  foundation  may  have  failed 
for  one  or  more  of  the  following  reasons :  1.  Insufficient  rolling  of  the 
subgrade.  2.  Insufficient  consolidation  of  back-filling  in  trenches. 
3.  The  use  of  natural  cement  in  the  concrete,  which  is  weaker  than 
Portland  cement  and  lacks  uniformity.  4.  Improper  proportions, 
insufficient  mixing,  or  inadequate  curing  of  the  concrete.  As  a  rule 
insufficient  attention  is  given  to  each  of  these  items.  5.  Passage  of 
loads  over  the  concrete  before  it  had  sufficiently  set.  6.  Vibrations 
due  to  weak  construction  of  street  railway  track,  which  shatter  the 
concrete  and  allow  water  to  get  under  the  foundation  which  upon 
freezing  still  further  cracks  the  concrete. 

On  the  other  hand,  many  examples  can  be  cited  where  a  4-inch 
concrete  base  has  successfully  carried  a  heavy  traffic.  It  is  prob- 
able that  a  well-constructed  slab  4  inches  thick  laid  on  a  well-con- 
solidated subgrade  is  stronger  than  a  foundation  of  poor  concrete 
8  inches  thick  laid  upon  an  insufficiently  rolled  subgrade.* 

776.  In  view  of  the  rapid  introduction  of  the  motor  truck  and  the 
consequent  crushing  of  some  pavement  foundations,  it  is  probable 
that  concrete  pavement  foundations  should  be  improved  in  quality 
or  increased  in  thickness — or  perhaps  both.     The  question  of  im- 
proving the  quality  depends  upon  the  relative  cost  of  materials  and 
labor;  and  the  advisability  of  increasing  the  thickness  can  be  deter- 
mined only  by  a  discriminating  study  of  the  experience  with  a  par- 
ticular thickness. 

777.  The  thickness  of  concrete  roads  (§  447)  gives  some  indica- 
tion as  to  the  required  thickness  of  concrete  pavement  foundations, 
although  the  former  are  ordinarily  more  carefully  constructed  than 
the  latter.     The  thickness  of  concrete  roads  is  usually  about  6  inches, 
and  is  seldom  more  than  6  inches  at  the  sides  and  8  inches  at  the 
center,  the  excess  thickness  at  the  center  being  to  provide  for  reduc- 

*  For  a  discussion  of  this  subject,  pro  and  con,  see  Engineering  News,  VoJ    72  (1914)    p 
176,  367,  558,  and  1033;   and  Vol.  75  (1916),  p.  1097, 


ART.    2]  THE    CONSTRUCTION  401 

tion  by  wear.  The  wearing  coat  adds  thickness  to  the  pavement, 
which  distributes  the  wheel-load  over  a  greater  area  of  the  sub- 
grade,  and  some  kinds  of  wearing  surfaces  also  give  additional  beam 
strength  to  the  pavement  as  a  whole.  For  example,  the  binder  and 
wearing  coat  of  a  sheet  asphalt  pavement  adds  2J  or  3  inches  in 
thickness,  and  gives  considerable  additional  beam  strength.  Again, 
a  portland-cement  grouted  brick  wearing-coat  has  been  found  to 
give  so  much  additional  beam  strength  that  the  total  thickness  of 
the  pavement  has  been  greatly  reduced  in  recent  years  (see  §  1028- 
30). 

778.  It  has  been  proposed  to  limit  motor  trucks  to  certain  streets, 
rather  than  build  all  pavement  foundations  heavy  enough  to  carry 
such  loads.     There  would  be  some  justice  in  such  a  requirement,  but 
the  enforcement  of  it  would  be  difficult. 

779.  The  Proportions.     For  a  discussion  of  the  theory  of  pro- 
portioning concrete,  see  §  417-24,  Chapter  VII  —  Concrete  Roads. 

The  proportions  of  the  concrete  for  a  pavement  foundation  is 
usually  determined  arbitrarily  without  much,  if  any,  reference  to  the 
gradation  of  the  coarse  and  fine  aggregate.  The  proportions  and 
sizes  of  the  aggregate  specified  by  a  number  of  important  cities 
whose  specifications  happened  to  be  at  hand,  are  as  follows  : 

PROPORTIONS  SAND  STONE 


1|  :2J  1"  to  fine  J"  to  1" 

H  :  3  I    to  fine  I    to  l\ 

2:3  I    to  fine  i    to  1  J 

2i  :  4  I    to  fine  I    to  1£ 

3:5  I    to  fine  £    to  2 

3:6  i    to  fine  I    to  2 


The  last  proportions  seem  to  be  much  the  most  used.  It  may  be 
that  with  the  aggregates  ordinarily  employed  in  each  case,  the 
proportions  specified  will  give  a  good  concrete;  but  the  quality 
of  the  concrete  can  not  be  foretold  from  the  above  specifications. 
To  secure  the  best  results  the  proportions  should  be  determined,  or 
at  least  tested,  by  a  sieve  analysis  (see  §  422)  ;  and  to  make  the  spe- 
cifications really  significant  both  the  proportions  and  the  gradation 
of  the  aggregates  should  be  stated. 

For  the  proportions  used  in  concrete  roads  see  §  444.  However, 
it  is  not  customary  to  use  as  rich  a  mixture  in  concrete  pavement- 
foundations  as  in  concrete  wearing-surfaces;  and  under  ordinary 
conditions,  it  is  not  necessary. 


402  FOUNDATIONS    FOR   PAVEMENTS  [CHAP.   XV 

780.  Mixing.     All  that  is  said  in  §  451-58  concerning  the  mixing 
of  concrete  for  concrete  roads,  applies  to  concrete  for  pavement 
foundations. 

781.  Placing.     If  the  subgrade  has  been  rutted  up  by  ordinary 
travel  or  in  the  delivery  of  paving  materials,  the  surface  should 
be  restored;    and  if  the  surface  has  been  much  disturbed,  the  sub- 
grade  should  be  again  rolled.     The  ridges  thrown  up  at  the  sides  of  a 
wheel  track  may  materially  weaken  the  concrete  foundation.     Under 
ordinary  circumstances  the  subgrade  should  be  sprinkled  just  before 
the   concrete  is  laid.     This  will   prevent  the   dry  subgrade  from 
absorbing  moisture  from  the  concrete,  and  will  also  prevent  its  drying 
out  too  fast. 

It  is  important  that  the  surface  of  the  concrete  shall  conform  to 
the  required  grade  and  crown.  The  thickness  may  be  indicated  by 
grade  stakes  set  every  4  or  5  feet.  Some  engineers  require  the  con- 
crete to  be  struck  off  with  a  template  which  may  run  upon  the  curbs 
or  upon  screeds  carefully  placed  for  that  purpose. 

The  edge  of  the  concrete  should  form  a  straight  line  from  curb  to 
to  curb  perpendicular  to  the  line  of  the  street. 

782.  No  contraction  joints  are  provided  in  concrete  pavement 
foundations  as  in  concrete  roads,  since  the  latter  are  not  as  much 
exposed  to  temperature  changes  as  the  former. 

783.  Finishing.     The  concrete  should  be  tamped  to  consolidate 
it.     The  wetter  the  concrete,  the  less  the  tamping  needed;    and 
usually  there  is  very  little  tamping.     Many  engineers  claim  that  the 
concrete  is  ordinarily  unduly  weak  because  it  is  mixed  unduly  wet. 
If  the  concrete  is  not  mixed  too  wet,  the  proper  tamping  or  ramming 
of  the  concrete  will  consolidate  it  and  fill  the  voids,  and  add  mate- 
rially to  its  strength.     Until  recently  it  was  the  usual  custom  to 
finish  the  concrete  foundation  by  light  tamping;   and  often  the  sur- 
face was  unduly  rough.     In  defense  of  this  practice  it  was  claimed 
that  the  rough  concrete  prevented  the  shifting  of  an  asphalt  pave- 
ment; and  that  with  brick,  stone-block,  and  wood-block  pavements 
the  roughness   of   the  concrete  did  no  harm,  as  the  cushion  layer 
gave  a  smooth  surface  upon  which  to  place  the  pavement.     In 
neither  case  is  the  claim  valid.     A  sheet  asphalt  pavement  upon 
such  a  foundation  will  creep  and  form  waves  or  humps  because  of 
the  difference  in  compression  due  to  its  unequal  thickness ;  and  it  has 
been  conclusively  established  that  the  thinner  the  cushion  course  the 
better  for  any  brick  pavement  (see  §  971)  or  block  pavement  (see 
§  1096).     The  utmost  roughening  of  the  surface  of  a  concrete  base  of 


ART.    2]  THE    CONSTRUCTION  403 

sheet  asphalt  pavement  should  be  that  produced  by  a  slight  raking 
while  the  concrete  is  fresh;  and  for  all  other  kinds  of  pavements, 
the  smoother  the  finish  the  better. 

Sometimes  the  surface  of  the  concrete  is  grouted,  that  is,  a  rich 
mortar  is  poured  upon  the  surface  and  swept  over  it  to  level  up  any 
depression  and  to  fill  up  any  honeycombing.  Sometimes  the  surface 
is  broomed  without  the  pouring  on  of  any  grout,  the  surplus  mortar 
being  swept  from  one  part  of  the  surface  to  level  up  depressions  and 
to  fill  up,  or  rather  hide,  honeycombing.  The  term  slushing  is 
sometimes  applied  to  each  of  these  processes.  No  such  method  of 
finishing  the  surface  should  ever  be  permitted;  although  in  extreme 
cases  concrete  made  of  fine  stone  in  the  stated  proportions  may  be 
used  to  level  up  depressions. 

Some  engineers  claim  that  the  surface  of  a  concrete  base  should  be 
floated  to  secure  a  uniform  smooth  surface  upon  which  to  lay  asphalt- 
block  or  wood-block  pavements.  With  a  brick  pavement  the  same 
result  is  accomplished  in  another  way  (see  §  982). 

Not  infrequently  loose  stones  are  left  on  the  upper  surface  of  the 
concrete  foundation  while  laying  the  binder  course  of  a  sheet  asphalt 
pavement  or  the  cushion  course  of  a  brick  or  block  pavement.  Such 
a  practice  is  inexcusable,  since  the  labor  to  remove  such  stones  is 
slight,  and  since  they  have  a  seriously  destructive  effect  upon  the 
wearing  coat. 

784.  Curing.     In  building  concrete  roads,  it  is  nearly  universal 
after  the  concrete  is  laid  to  protect  it  during  curing  by  covering  it 
with  canvas,  or  damp  earth,  or  a  sheet  of  water  (see  §  464);  but  in 
constructing   concrete   pavement-foundations,    it   is   quite   unusual 
to  take  any  such  precautions,   and   consequently  the   concrete  is 
frequently  seriously  damaged  by  drying  out  too  rapidly  in  hot  or 
windy  weather,  or  by  exposure  to  low  temperature. 

The  period  during  which  the  concrete  base  should  be  allowed 
to  harden  will  depend  upon  the  weather  conditions  and  the  kind 
of  pavement  to  be  laid  upon  it,  or  rather  upon  the  method  of  delivering 
the  subsequent  paving  materials.  Teaming  over  the  concrete  in 
building  the  remainder  of  the  pavement  should  never  be  permitted 
in  less  than  10  to  15  days,  depending  upon  the  weather;  and  the 
pavement  should  not  be  open  to  heavy  loads  in  less  than  15  to  21 
days  from  the  time  the  concrete  foundation  was  laid.  The  pressure 
to  shorten  this  time  is  often  very  great,  particularly  on  a  business 
street. 

785.  Cost   of   Concrete   Foundations.    Materials.    The   cost   of 


404  FOUNDATIONS    FOR    PAVEMENTS  [CHAP.    XV. 

materials  varies  with  the  locality  and  the  conditions  of  the  markets 
(see  §  425-27) ;  and  hence  it  is  unwise  to  cite  examples  except  as  in 
§  790.  When  the  prices  are  known,  estimates  may  be  easily  pre- 
pared by  the  use  of  Table  28,  page  237. 

786.  Labor.     Formerly  concrete  for  pavement  foundations  was 
mixed  by  hand;   but  in  recent  years  it  is  almost  always  mixed  by 
machine. 

787.  Hand  Mixing:    The  following  data  on  the  labor-cost  of 
hand-mixing  are  out  of  date  as  to  the  method  of  mixing  and  also  as 
to  the  cost  of  labor;  but  as  the  price  of  labor  is  stated,  these  data  may 
be  useful  in  making  estimates  when  hand  mixing  is  to  be  employed. 

In  a  small  western  city  the  average  cost  to  the  contractor  of 
mixing  and  laying  a  thickness  of  6  inches  of  concrete  during  two 
years  was  about  7  cents  per  square  yard,  for  1  part  cement,  2  parts 
sand,  and  4  parts  broken  stone,  turned  six  times  exclusive  of  casting 
into  place.  With  gravel  instead  of  broken  stone  the  cost  was  about 
6  cents  per  square  yard;  and  with  four  turnings  instead  of  six,  the 
cost  was  about  half  a  cent  less  than  the  prices  above.  All  the  mixing 
was  done  with  shovels.  The  wages  of  common  laborers  was  $1.50 
for  10  hours. 

In  a  large  western  city  the  average  cost  to  various  contractors 
of  mixing  and  laying  a  thickness  of  6  inches  of  concrete  was  5J 
cents  per  square  yard.  The  mixing  was  done  with  hoes,  the  specifi- 
cations requiring  that  the  concrete  should  be  mixed  until  each 
particle  of  the  stone  was  completely  covered  with  mortar.  The 
wages  of  common  laborers  was  $1.50  for  10  hours. 

778.  The  following  example  *  gives  the  distribution  of  the  labor 
of  laying  a  6-inch  concrete  pavement  foundation,  in  hours  per  square 
yard: 

ITEMS.  HOURS 

PER  SQ.  YD. 

4  men  filling  barrows  with  sand  and  stone 0.15 

10  men  wheeling,  mixing,  and  shoveling  to  place  (3  or  4  steps) 0 . 37 

2  men  ramming 0 . 07 

1  water  boy,  equivalent  in  common  labor 0.01 

1  foreman,  equivalent  in  common  labor .  0 . 06 


Total  hours  per  square  yard 0  67 

The  sand  and  stone  were  dumped  in  the  street  upon  boards,  and 
were  hauled  in  wheel-barrows  about  40  feet  to  the  mixing  boards. 

*  Engineering  News,  Vol.  46  (1901),  p.  424. 


ART.    2]  THE    CONSTRUCTION  405 

The  mortar  was  turned  three,  and  the  stone  three  or  four  times. 
Two  gangs  under  separate  foremen  worked  side  by  side  in  the  same 
street. 

The  same  correspondent  gives  another  example  which  required 
0.56  hour  per  sq.  yd.,  in  which  case  the  mortar  was  turned  only 
once  and  the  stone  twice,  water  being  used  in  abundance. 

The  cost  of  labor  in  mixing  and  laying  concrete  is  often  8  or  9 
cents  a  square  yard.  For  the  most  economical  work  the  'sand  and 
stone  should  be  deposited  in  ridges  on  the  subgrade  near  the  middle 
of  the  street;  and  if  they  are  piled  on  the  parking,  the  cost  will  be 
considerably  greater  than  above. 

789.  For  data  on  cost  of  hand-mixed  concrete  for  concrete  roads, 
see  §  374. 

790.  Total  Cost.     The  total  cost  of  a  concrete  pavement-foun- 
dation laid  in  a  city  in  the  Central  States  in  1916  was  as  follows: 

ITEMS  Cu.  YD.          SQ.  YD. 
SUBGRADE : 

Rough  grading,— 1,504  cu.  yd $0.298 

Surfacing  and  rolling  3,380  sq.  yd.,  and  cleaning  up 0 . 090            $0 . 040 

Miscellaneous  expense 0.036              0.009 

Total,  exclusive  of  superintendence,   depreciation  on  

machinery,  administration $0.424 

CONCRETE  BASE: 

Cement  at  $1.48  per  bbl.  on  job,  net $1 . 365  $0. 227 

Sand  at  $1.40  per  cu.  yd.  on  job 0 . 731  0 . 122 

Gravel  at  $1.50  per  cu.  yd.  on  job 0 . 934  0 . 156 

Coal  and  water 0.035  0.006 

Labor 0.336  0.056 

Miscellaneous  expense 0.038  0.017 

Total,  exclusive  of  superintendence,  'depreciation  on 
machinery,  administration $3 . 439  $0 . 584 

The  pavement  was  34  feet  wide.  The  proportions  of  the  con- 
crete were  1:3:5;  and  the  thickness  was  6  inches.  The  concrete 
was  mixed  in  a  one-bag  mixer.  The  loss  on  bags  was  1  per  cent. 
The  wages  of  common  labor  was  20  cents  per  hour;  the  engine  runner 
on  the  concrete  mixer,  30  cents  per  hour;  and  a  team,  wagon,  and 
driver,  50  cents  per  hour. 

791.  OLD  MACADAM  FOUNDATION.    It  not  infrequently  hap- 
pens that  a  high-class  pavement  is  to  replace  a  water-bound  gravel  or 
macadam  surface,  in  which  case  it  may  be  economical  to  use  the  old 
pavement  as  a  foundation  for  the  new,    This  form  of  foundation  has 


406  FOUNDATIONS    FOR    PAVEMENTS  [CHAP.  XV 

been  discussed  in  connection  with  the  construction  of  concrete  roads. 
The  possibility  of  utilizing  an  old  gravel  or  macadam  road  as  a  foun- 
dation occurs  more  frequently  with  a  narrow  concrete  or  brick  rural 
road  than  with  a  comparatively  wide  street  pavement.  For  a  con- 
sideration of  the  difficulties  encountered  and  of  the  methods  to  be 
emplo3red,  see  §  437. 

792.  BITUMINOUS    CONCRETE    FOUNDATION.     Bituminous- 
cement  concrete  has  some  advantages  over  hydraulic-cement  con- 
crete for  pavement  foundations.     1.  The  bituminous  concrete  does 
not  require  any  time  for  curing  and  hardening;    and  consequently 
the  wearing  coat  may  be  laid  as  soon  as  the  foundation  is  completed, 
which  may  be  a  decided  advantage  on  a  busy  thoroughfare.     2. 
The  bituminous  concrete  is  more  flexible  than  hydraulic  concrete, 
and  hence  is  not  so  likely  to  crack.     3.  If  the  wearing  surface  of  a 
pavement  is  made  with  a  bituminous  cement,  a  bituminous  concrete 
foundation  is  advantageous,  since  then  the  whole  pavement  can  be 
made  with  one  kind  of  equipment  and  organization.     4.  A  bitumin- 
ous wearing  coat  will  adhere  better  to  a  bituminous  concrete  base 
than  to  a  hydraulic  concrete  base.     5.  The  use  of  a  bituminous 
concrete  base  makes  unnecessary  the  binder  course  of  a  sheet  asphalt 
pavement;  but  on  the  other  hand,  with  a  bituminous  concrete  base 
it  is  practically  impossible  to  remove  the  bituminous  wearing  coat 
without  materially  damaging  the  foundation.     However,  it  is  claimed 
that  by  the  use  of  a  surface  heater  (Fig.  161,  page  450),  repairs  can 
be  made  in  the  asphalt  wearing  coat  without  damage  to  the  bitumin- 
ous foundation.     For  a  further  discussion  of  a  bituminous  concrete 
foundation  for  sheet  or  monolithic  asphalt  pavements,  see  §  806-08. 

793.  The  question  of  economy  depends  upon  the  local  prices  of 
bituminous  and  hydraulic  cement.     At  present  the  price  of  bitumi- 
nous cement  is  substantially  1  cent  per  pound,  while  that  of  portland 
cement  is  about  $2.40  for  376  pounds  or  about  0.66  cent  per  pound. 
The  specific  gravity  of  bituminous  cement  (a  paste)  is  about  1,  while 
that  of  hydraulic  cement  paste  is  about  2;  and  hence  the  prices  per 
unit  of  volume  are  about  1  to  1.3,  or  in  other  words,  at  present  prices 
the  hydraulic  cement  concrete  is  about  30  per  cent  the  more  expen- 
sive.    Or  to  put  it  another  way,  when  portland  cement  costs  more 
than  $1.88  per  barrel,  there  is  a  possibility  that  bituminous  concrete 
may  be  the  cheaper.     There  has  not  been  sufficient  experience  with 
bituminous  concrete  to  determine  with  any  considerable  accuracy  the 
cost  of  mixing  and  laying  it. 

794.  Bituminous  concrete  pavement  foundations  were  used  in  a 


ART.    3]  FOUNDATIONS    OF   TRACKS  407 

number  of  cities  in  this  country  from  about  1880  to  1895,  owing  prob- 
ably to  the  high  price  of  hydraulic  cement,  particularly  portland 
cement;  and  some  of  these  foundations  are  still  giving  satisfaction. 
Tar  concrete  has  been  used  in  England  for  pavement  foundations 
for  many  years. 

795.  The   strength   of   a   bituminous   concrete   foundation   will 
depend  upon  the  kind  and  quality  of  the  bituminous  cement  used; 
and  no  such  foundation  is  as  strong  as  one  made  with  equal  care  of 
Portland  cement. 

ART.  3.     FOUNDATIONS  OF  STREET-RAILWAY  TRACKS 

796.  One  of  the  most  common  failures  of  pavements  is  adjacent 
to  the  rails  of  a  street-car  track;   and  is  often  due  to  the  defective 
foundation  of  the  track.     In  a  general  way  these  failures  are  due  to 
the  vertical  vibrations  of  the  rails,  which  pounds  the  foundation 
to  pieces  and  also  breaks  the  bond  between  the  rail  and  the  pave- 
ment, thus  permitting  water  to  enter  which  on  freezing  heaves  the 
pavement.     The  vibrations  of  the  rail  may  be  due  to  the  deflection  of 
the  rail  or  to  the  compression  of  the  ties  and  foundation  or  to  both. 

The  design  and  usually  also  the  construction  of  the  foundation 
of  the  track  is  a  function  of  the  engineer  of  the  street  railway,  while 
the  construction  and  maintenance  of  the  pavement  adjoining  the 
track  is  under  the  direction  of  the  city  engineer;  but  to  secure 
reasonably  satisfactory  results  requires  the  co-operation  of  both 
interests.* 

797.  FOUNDATION.     The    foundation    may    be    of    gravel,    or 
broken   stone   or   concrete.     With   gravel   ballast  it  is   practically 
impossible  to  prevent  the  rail  from  working  up  and  down  owing  to 
the  movement  of  the  ballast  and  the  difficulty  of  tamping  the  ties 
uniformly.     Broken  stone  gives  better  results  than  gravel,  but  is  far 
from    satisfactory.     Gravel  or  broken-stone  ballast  is   reasonably 
satisfactory  for  railways  in  the  open  country  upon  a  private  right- 
of-way;  but  the  case  is  very  different  on  a  city  street  where  all  have 
equal  rights  and  which  must  be  paved  and  maintained  for  general 
public  use.     It  is  generally  conceded  that  the  track,  at  least  in  streets 
having  any  considerable  amount  of  heavy  traffic,  should  rest  upon  a 
concrete  foundation. 

*  For  an  instructive  article  on  the  relations  of  the  two  interests  involved  and  a  discussion 
thereon  by  several  engineers,  see  "A  Suggested  Change  of  Policy  for  Maintaining  the  Pave- 
ment adjoining  the  Railway,"  by  N.  S.  Sprague,  Chief  Engineer  of  Bureau  of  Engineering, 
Pittsburgh,  Pa.,  in  Proc.  Amer.  Soc.  Municipal  Improvements,  1915,  p.  271-77  and  277-82. 


408 


FOUNDATIONS   FOK   PAVEMENTS 


[CHAP,  xv 


There  is  great  variety  in  the  form  of  the  concrete  foundation.  It 
may  be  a  slab  of  uniform  thickness  extending  under  the  track,  or  the 
thickness  may  be  increased  longitudinally  under  each  rail  or  trans- 
versely under  each  tie.  An  important  advantage  of  the  first  form  of 
construction  over  either  of  the  others  is  that  it  eliminates  trenches  in 
the  subgrade,  which  can  not  be  rolled  and  which  are  likely  to  be  par- 
tially filled  with  loose  earth  by  the  breaking  down  of  the  edges  of  the 
trench  by  workmen.  With  the  trench  construction  the  depth  of  the 
concrete  is  not  likely  to  be  uniform,  and  is  likely  to  be  laid  on  loose 
earth  instead  of  a  compacted  subgrade. 

798.  Examples.  Fig.  143  shows  the  two  types  of  track  founda- 
tions recommended  by  the  Committee  on  Way  of  the  American 


?to9"of/.3:G  concrete 


i^vf-^-y—  -' 

bottom  of  tie- depend/ng    //X 
a?  tffcjcv^  erflowng  base. 

>-"-H  A/fistoaffieVhsnto  an 

pro**  surfcce  atow,  end  under  draw  anf  in  &*%%&  **^*** 

Tree  &- for  a//  so/te  except  very  neavy  cfay,  fyr  dense  traffic, 
and  for  cars  a/}  /o  J&  fans  we/gbf: 


.  \ 

-°^^L  v  -  J-  -  f^^-t  ^tT,^ 


Pedt/ey  or  tinders- 

center //ne  of  douto/e  f/vc/t 
Always  provide  yurface  and  under  dnr/ns. 

~YPeC:~  ror  fmwwfo-Fefatn>/WfOfy0nrfvrtcerfa/n  rt?0d?  ground, 
fbr  densest  traffic,  and  for  fiear/esf  cars. 

Fia.  143. — TRACK  FOUNDATIONS  FOR  PAVED  STREETS. 

Electric  Railway  Engineering  Association.*  Notice  that  in  both 
examples  the  rail  is  the  T  pattern;  but  as  far  as  shown  in  Fig. 
143,  there  is  no  difference  between  the  T  and  the  grooved  rail. 
For  track  foundations  with  a  grooved  rail,  see  Fig.  159,  page  442, 
Fig.  160,  page,  443,  and  Fig.  196,  page  540;  and  for  two  track- 


*  Proceedings  Amer.  Elect.  Ry.  Eng'g  Assoc.,  1915,  p.  471. 


ART.  3] 


FOUNDATIONS   OF   TRACKS 


409 


foundations  with  T  rails,  see  Fig.  197,  page  540.  Notice  that  the 
lower  half  of  the  last  shows  a  concrete  beam  under  each  rail. 

For  a  cross  section  of  a  street-railway  track  in  a  sheet  asphalt 
pavement,  see  Fig.  159  and  160,  page  442-43;  in  a  brick  pavement, 
see  Fig.  196  and  197,  page  540;  and  in  a  stone-block  pavement,  see 
Fig.  218,  page  587. 

799.  THE  TIES.  The  ties  are  either  wood  or  steel,  the  former 
being  the  more  common.  Wood  ties  have  a  long  life  when  embedded 
in  concrete,  especially  if  treated  by  some  preservative  process; 


.V*.  ft  :-  •  ;'«  °'  :'  •'  '  '  ':  »  •'  •  r  .'•  r  *  '.  :  Q./.'  >.'••  •  *  •  '  •'  j 


Tie 


f 


I  • 

FIG.   144. — STEEL-TIE  STREET-RAILWAY  TRACK. 

and  there  is  an  increasing' use  of  treated  wood  ties.  For  illustrations 
showing  track  with  wood  ties,  see  Fig.  143. 

Steel  ties  have  been  used  to  only  a  comparatively  small  extent. 
For  a  cross  section  of  the  track  with  steel  tie  used  in  Chicago,  see 
Fig.  144.  The  construction  shown  in  Fig.  144  has  been  found 
to  be  objectionable  on  account  of  the  difficulty  of  paving  around 
and  over  the  tie-rod  between  the  rails.  Concrete  ties  have  been 
used  in  street-railway  track  only  a  little,  if  at  all. 

800.  THE  RAILS.  The  rails  are  one  of  three  types,  viz.:  the 
flat-top,  the  grooved  top,  and  the  T  section.  The  flat-top  rail,  which 
has  been  almost  abandoned,  was  very  destructive  of  the  pavement, 
as  steel-tired  vehicles  frequently  ran  with  one  wheel  on  one  of  the 
railway  rails,  the  result  being  that  the  wheel  on  the  pavement  wore  a 
rut.  Where  the  flat-top  rail  is  in  use,  it  is  not  uncommon  to  find  at 
least  two  ruts  on  each  side  of  each  track — one  made  by  a  broad-gage 
wagon  running  with  its  left  wheels  on  the  left  rail  and  its  right  wheel 
to  the  right  of  the  right  rail,  one  made  by  any  wagon  running  with  its 
left  wheel  on  the  right  rail ;  and  if  the  wagons  are  of  two  different  gages, 
the  last  position  will  cause  two  narrow  ruts  close  together  or  one  wide 
one.  For  illustrations  of  track  construction  using  the  T  section,  see 
Fig.  143,  page  408,  and  Fig.  197,  page  540;  and  for  illustrations 


410  FOUNDATIONS    FOR    PAVEMENTS  [CHAP.  XV 

showing  a  grooved  top,  see  Fig.  159  and  160,  page  442-43,  and  Fig. 
196,  page  540. 

801.  THE  PAVING.  In  selecting  the  material  for  the  pavement 
adjacent  to  the  rails,  consideration  should  be  given  to  the  amount 
and  character  of  the  vehicular  traffic  and  to  the  relative  cost  and  life 
of  the  several  classes  of  pavements.  The  preference  of  the  members 
of  the  American  Electric  Railway  P]ngineering  Association  is  as 
follows:  granite-block,  Medina  sandstone-block,  creosoted  wood- 
block, vitrified  brick,  asphalt-block,  sheet  asphalt,  bituminous 
concrete,  bituminous  macadam,  water-bound  macadam.  The  rail- 
way company  usually  prefers  granite  block  because  of  its  durability, 
and  possibly  also  because  its  roughness  deters  vehicles  from  using 
the  railway  area. 

The  precautions  to  be  taken  in  laying  the  different  paving  mate- 
rials adjacent  to  a  street-railway  track  will  be  considered  in  the  sub- 
sequent chapter  treating  the  respective  kind  of  pavement. 

For  cross  sections  of  a  sheet  asphalt  pavement  adjacent  to  a 
street  railway  track,  see  Fig.  159  and  160,  page  442-43;  for  a  cross 
section  of  a  brick  pavement  and  a  street-railway  track,  see  Fig. 
196  and  197,  page  540. 


CHAPTER  XVI 


ASPHALT  PAVEMENTS 

802.  For  a  discussion  of  asphalt,  its  sources,  its  characteristics, 
the  methods  of  testing  it,  and  specifications  for  asphalt  for  different 
purposes,  see  Art.  1  of  Chapter  VIII — Bituminous  Road  Materials. 

Asphalt  is  used  in  three  forms  of  pavements,  viz. :  sheet  asphalt, 
asphalt  concrete,  and  asphalt  block.  The  wearing  coat  of  the  first 
consists  essentially  of  an  asphalt  mortar,  i.  e.,  a  mixture  of  sand  and 
asphalt  cement;  and  that  of  the  second  is  an  asphalt  concrete,  i.  e., 
a  mixture  of  broken  stone  and  asphalt  cement.  Both  of  these  forms 
of  asphalt  pavements  are  mixed  and  laid  hot.  The  wearing  coat  of 
an  asphalt  block  pavement  consists  of  blocks  of  asphalt  concrete 
moulded  hot  and  laid  when  cold.  The  first  form  is  by  far  the  most 
common;  but  the  second,  of  which  bitulithic  pavement  is  a  form, 
although  a  comparatively  recent  development,  is  quite  widely 
used  (§  636). 

Each  of  these  forms  will  be  treated  in  a  separate  article. 

ART.  1.     SHEET  ASPHALT  PAVEMENTS 

803.  A  sheet  or  monolithic  asphalt  pavement  consists  primarily 
of  (1)  a  wearing  coat   I?  to  2  inches  thick  composed  of  asphalt 
paving  cement  mixed  with  sand;   (2)  a  binder  course  1  to  1J  inches 


FIG.   145. — Two  FORMS  OF  SHEET  ASPHALT  PAVEMENTS. 

thick,  composed  of  broken  stone  and  asphalt  cement;    and  (3)  a 
foundation  of  hydraulic-cement  concrete — see  Fig.  145. 

In  this  country  when  the  term  asphalt  pavement  is  used  the  above 
form  is  usually  intended.     The  term  sheet  or  monolithic  pavement  is 

411 


412  ASPHALT   PAVEMENTS  [CHAP.   XVI 

not  distinctive,  since  rock  asphalt  also  is  laid  as  a  continuous  sheet; 
but  no  confusion  is  likely  to  result,  since  in  this  country  the  term 
sheet  is  commonly  used  to  distinguish  the  monolithic  form  from  the 
asphalt-block  pavement,  and  since  in  Europe  only  one  form  of  asphalt 
pavement  is  used,  monolithic  natural  rock.  In  centra-distinction 
to  a  pavement  made  of  natural  asphalt  limestone  or  sandstone,  the 
above  pavement  could  with  some  propriety  be  called  an  artificial 
asphalt  pavement,  or  the  wearing  coat  could  with  still  more  propriety 
be  called  an  artificial  asphalt  paving  compound;  but  the  dis- 
tinction is  not  important,  since  the  sheet  asphalt  pavement  is  laid 
almost  exclusively  in  this  country  and  the  rock  asphalt  almost 
exclusively  in  Europe. 

804.  HISTORICAL.     The  first  artificial  sheet  asphalt   pavement 
in  this  country  was  laid  in  Newark,  N.  J.,  in  front  of  the  city  hall  in 
1870.     In  1873  a  small  piece  was  laid  on  Fifth  Avenue,  New  York 
City,  opposite  the  Worth  Monument.       A  few  other  experimental 
sections  were  laid;  but  the  first  test  on  a  large  scale  was  in  1876  on 
Pennsylvania  Avenue,  in  Washington,  D.  C.     Preceding  1882,  out- 
side of  Washington,  D.  C.,  there  were  not  more  than  half  a  dozen 
streets  in  this  country  paved  with  any  form  of  asphalt;    but  since 
that  date,  asphalt  pavements  have  increased  rapidly,  and  now  hun- 
dreds of  miles  of  it  are  in  use  on  the  streets  of  American  cities.     The 
following  statistics  show  the  rapid  growth  of  this  industry:   In  this 
country  in  1880  there  were  300,000  square  yards  of  sheet  asphalt 
pavements;  in  1885,  1,800,000;  in  1890,  8,100,000;  in  1895,  21,500,- 
000;     in    1900,    38,000,000;    in    1909,  83,227,000.     In    Europe  in 
1900,  the  latest  data  available,  there  were  only  about  3,000,000 
square  yards  of  asphalt  pavements  of  all  kinds. 

In  1909  in  the  United  States  according  to  the  data  on  page  320, 
about  one  fifth  of  all  pavements  were  sheet  asphalt;  and  if  water- 
bound  gravel  and  macadam  be  excluded,  about  one  third  of  all 
durable  pavements  were  sheet  asphalt. 

Asphalt  pavements  can  be  adapted  to  a  wide  range  of  tempera- 
ture, and  are  in  extensive  use  from  Winnepeg  to  Panama — from  the 
far  north  to  the  tropics. 

805.  THE  FOUNDATION.     For  a  description  of  the  method  of 
preparing  the  subgrade,   see  Art.    1   of  Chapter  XV,— Pavement 
Foundations. 

Since  the  sheet-asphalt  wearing  surface  has  little  or  no  power  in 
itself  to  act  as  a  bridge,  it  is  essential  that  it  be  placed  upon  a  firm 
foundation;  and  consequently  it  is  nearly  always  placed  upon  a  bed 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  413 

of  hydraulic-cement  concrete,  which  formerly  was  sometimes  made 
with  natural  cement  but  is  now  always  made  with  portland  cement. 
For  heavy  city  traffic,  the  concrete  is  usually  6  inches  thick;  but 
for  light  traffic,  it  is  sometimes  only  4  inches  thick.  For  a  discussion 
of  the  proper  thickness  of  a  portland-cement  concrete  foundation 
and  the  method  of  laying  it,  see  §  773-84. 

It  is  necessary  that  the  concrete  be  thoroughly  dry  before  the 
asphalt  mixture  is  laid  upon  it,  as  the  generation  of  steam  caused 
by  placing  the  hot  material  upon  a  damp  foundation  will  produce 
blistering  and  possibly  disintegration  of  the  wearing  coat.  This  is  a 
matter  that  needs  close  attention  in  laying  an  asphalt  pavement. 
To  dry  the  foundation  after  a  rain  or  during  damp  weather,  fine  hot 
sand  is  sometimes  spread  over  the  concrete  and  then  swept  off;  but 
this  method  is  expensive  and  not  very  effective,  and  besides  there  is 
liability  that  enough  sand  will  be  left  upon  the  foundation  to  inter- 
fere with  the  adhesion  of  the  asphalt. 

806.  Bituminous    Concrete    Foundation.     It    is    claimed,    with 
apparent  justification,  that  asphalt  pavements  usually  fail  because 
of  a  defective  foundation  rather  than  because  of  inherent  defects 
in  the  surface  coat  or  of  the  wearing  away  of  its  materials;  and  some 
claim  that  better  results  would  be  obtained  by  the  use  of  a  bitumi- 
nous concrete  foundation  instead  of  a  hydraulic  concrete  base. 

For  a  general  discussion  of  a  bituminous  concrete  base,  see 
§  792-95. 

807.  It  is  claimed  that  the  bituminous  concrete  base  is  superior 
to  the  portland-cement  concrete  foundation  in  four  particulars,  as 
follows : 

1.  There  is  a  lack  of  frictional  resistance  between  the  wearing  coat 
and  the  portland-cement  foundation,  which  gives  rise  to  one  of  the 
most  common  failures  of  sheet  asphalt  pavements.  The  pressure  and 
impact  of  wheels  upon  the  surface  of  the  pavement  produce  a  horizon- 
tal component  which  causes  the  asphalt  to  creep  and  form  waves  or 
humps,  which  make  the  pavement  uncomfortable  in  use  particularly 
by  automobiles,  and  these  humps  are  difficult  to  remove.  A  slight 
roughening  of  the  top  of  the  concrete  base  is  insufficient  to  resist  this 
lateral  movement;  and  an  excessive  roughening  is  harmful  rather 
than  otherwise,  since  a  difference  in  thickness  of  the  asphalt  sheet 
causes  a  difference  in  compression  and  a  consequent  lateral  move- 
ment. It  is  claimed  that  the  asphalt  wearing  coat  will  unite  more 
firmly  with  a  bituminous  concrete  foundation  than  with  a  hydraulic 
concrete  base. 


414  ASPHALT   PAVEMENTS  [CHAP.  XVI 

2.  There  is  a  lack  of  adhesion  between  the  asphalt  mixture 
and  the  portland-cement  concrete,  which  gives  rise  to  a  second  com- 
mon form  of  failure,  since  there  is  insufficient  adhesion  between  the 
asphalt  and  the  concrete  to  resist  the  action  of  water  at  the  surface 
of  contact  of  the  two  materials.     If  the  concrete  foundation  is  not 
very  dense,  water  will  be  drawn  to  the  top  of  it  by  capillary  action ; 
and  the  effect  of  water  on  the  under  side  of  the  asphalt  tends  to  dis- 
integrate the  bond  between  the  asphalt  and  the  hydraulic  concrete. 
Further,  since  the  asphalt  sheet   prevents  evaporation,   the  upper 
surface  of  a  hydraulic   concrete  foundation  is  usually  moist;   and 
hence  any  frost  action  tends   still  further  to  destroy  the  bond  be- 
tween the  two  materials.     It  is  claimed   that  with  equal    care  and 
equally    suitable   proportions,  bituminous    concrete  will    be    more 
waterproof    than    hydraulic    concrete;     but    the    experience    with 
bituminous   concrete  under  modern  methods   of  preparing,  mixing 
and  laying  has  been  insufficient  to  establish  such  a  conclusion. 

3.  A  third  objection  urged  against  a  hydraulic-concrete  founda- 
tion is  that  cracks  in  the  asphalt  sheet  are  caused  by  temperature 
and  setting  cracks  in  the  foundation.     This  is  probably  true;    but 
the  wise  remedy  is  to  properly  cure  the  concrete  (§  784). 

4.  Another  argument  for  the  superiority  of  a  bituminous  base  over 
a  hydraulic  one  is  that  the  former  is  more  elastic ;  and  hence  absorbs 
the  effect  of  the  impact  of  traffic,  and  prevents  the  lateral  flow  of 
the  wearing  coat.     This  effect  can  not  be  very  great;    and  it  is  of 
doubtful  value,  since  the  chief  advantage  of  a  monolithic  asphalt 
wearing  surface  is  its  inherent  stability. 

808.  On   the   other   hand,    the   portland-cement   concrete   base 
possesses  the  following   points   of  superiority  over  a   bituminous 
concrete  base. 

1.  A  portland-cement  concrete  base  is  stronger,  and  hence  will 
better  distribute  concentrated  loads,  will  better  bridge  over  soft 
spots  in  the  subgrade,  and  will  better  resist  the  tendency  to  crack 
due  to  unequal  settlement. 

2.  Ordinarily  it  is  cheaper. 

3.  There  is  less  difficulty  in  making  repairs  in  the  asphalt  sur- 
face. 

809.  Other  Foundations.    A  sheet  asphalt  wearing  surface  has 
been  laid  on  old  cobble-stone,  brick,  and  stone-block  pavements;   but 
with  varying  success.     The  conditions  necessary  for  success  seem 
to  be:    1.  The  old  pavement  must  be  firm  and  solid.     2.  The  old 
pavement  must  have  a  fairly  uniform  surface  so  that  the  asphalt 


ART.    1]  SHEET   ASPHALT    PAVEMENTS  4l5 

coat  will  have  a  nearly  uniform  thickness,  say  not  less  than  1  or  Ij 
inches  nor  more  than  2  or  2J  inches.  3.  The  old  pavement  must  be 
perfectly  clean  and  absolutely  dry.  4.  Of  course,  the  asphalt  wearing 
course  must  be  made  of  suitable  material,  have  appropriate  con- 
sistency, and  be  properly  applied. 

Apparently,  failures  in  laying  sheet  asphalt  on  an  old  pavement 
have  been  more  common  than  successes. 

810.  BINDER  COURSE.     In  the  past  there  has  been  considerable 
trouble  in  getting  an  asphalt  wearing  coat  to  adhere  to  a  hydraulic- 
cement  foundation,  and  various  expedients  have  been  tried;   but 
at  present  one  of  two  methods  is  always  employed,  viz.:   either 
apply  an   asphalt  paint  coat  to  the  foundation,   or  lay  a  binder 
course. 

811.  Paint  Coat.     A  paint  coat  consists  of  an  asphalt  cement 
fluxed  with  naphtha  or  benzine,  which  is  applied  with  a  brush  or 
squeegee  to  the  portland-cement  concrete.     It  is  essential  that  the 
concrete  shall  have  a  fairly  smooth  surface;   that  the  base  shall  be 
perfectly  dry;  that  the  paint  coat  shall  be  bright  and  glossy,  but  not 
sticky;    and  that  it  shall  be  kept  clean  until  the  wearing  coat  is 
applied. 

This  method  of  construction  is  applicable  only  to  a  light-traffic 
street  or  road  where  low  first  cost  is  necessary.  The  method  has 
been  used  to  a  considerable  extent  by  cities  in  California.* 

812.  Kinds   of  Binder  Course.     The  binder  course  is  a  layer 
about  1^  inches  thick  of  broken  stone  cemented  together  with  asphal- 
tic  paving  cement  (§  818)  and  rolled  in  place  while  hot.     It  is  often 
called  simply  the  binder;   but  this  is  likely  to  cause  confusion,  since 
the  term  binder  usually  refers  to  the  cementing  material  in  a  water- 
bound  gravel  or  macadam  road,  or  in  a  bituminous  macadam  or 
concrete,  etc. 

There  are  two  forms  of  binder  course,  the  open  and  the  closed. 
The  former  does  not  contain  as  much  cement,  i.  e.,  is  not  as  rich, 
as  the  latter;  and  its  aggregate  is  not  as  carefully  graded.  The 
open  binder  is  used  to  secure  a  cheaper  pavement;  and  will  not 
endure  under  medium  or  heavy  traffic.  The  closed  binder  has 
greater  inherent  stability,  and  hence  is  preferable  for  medium  or 
heavy  traffic. 

813.  Specifications  for  Open  Binder.     Broken  Stone.    The  broken 
stone  should  be  clean,  and  have  a  compact  texture  and  uniform  grain. 

*  California    Highways,    January,    1915. 


416  ASPHALT   PAVEMENTS  [CHAP.    XVI 

For  medium  or  heavy  traffic  the  stone  should  be  strong  and  break 
with  sharp  edges  and  corners. 

The  broken  stone  should  have  the  following  gradation  on  screens 
having  circular  opening:  "  All  of  the  material  shall  pass  a  If -inch 
screen;  and  not  more  than  10  per  cent,  nor  less  than  1  per  cent, 
shall  be  retained  on  a  1-inch  screen;  and  not  more  than  10  per  cent, 
nor  less  than  3  per  cent,  shall  pass  a  j-inch  screen."4 

814.  Sand.    It  is  not  customary  to  use  sand  in  an  open  binder; 
and  it  is  often  specified  that  the  stone  shall  not  contain  more  than  a 
certain  amount  of  fine  material.     However,  the  more  fine  material 
(sand  or  screenings)   in  the  binder  the  more  compact  and  more 
desirable  it  is,  provided  the  fine  material  is  not  more  than  enough 
to  fill  the  voids.     But  the  greater  the  amount  of  fine  material,  the 
more  the  bitumen  required  to  coat  all  the  particles;    and  conse- 
quently the  more  expensive  the  binder.     For  the  latter  reason,  it  is 
not  customary  to  use  much  fine  material  in  an  open  binder;    and 
usually  no  fine  material  is  used  except  that  in  the  stone. 

815.  Asphalt    Cement.     The    asphalt    cement    is    of    the    grade 
stated  in  §  542.     The  amount  of  cement  should  be  sufficient  to  coat 
the  fragments  of  stone  and  bind  them  together  reasonably  well ;  and 
will  depend  upon  the  kind  and  gradation  of  the  stone  and  the  rich- 
ness desired.     With  trap  or  hard  limestone  graded  as  above,  only 

3  or  4  per  cent  of  pure  bitumen  is  used;   and  with  a  soft  limestone 

4  or  5  per  cent  is  common.     There  should  not  be  so  much  cement 
that  it  will  run  off  the  stone,  but  there  should  be  enough  to  give  a 
bright  glossy  coat  to  the  stone. 

After  being  rolled  the  surface  of  the  binder  course  will  be  porous 
or  open,  and  hence  the  name  given  to  it. 

816.  Specifications    for    Closed    Binder.     Broken    Stone.     The 
quality  of  the  broken  stone  should  be  the  same  as  for  open  binder 
(§  815). 

The  gradation  of  the  broken  stone  should  be  as  follows,  as  deter- 
mined with  screens  having  circular  openings:  "  95  per  cent  of  the 
binder  aggregate  shall  pass  a  screen  having  circular  openings  equal 
to  three  quarters  of  the  thickness  of  the  binder  course  to  be  laid; 
and  the  smallest  dimensions  of  the  remaining  5  per  cent  shall  not 
exceed  the  thickness  of  the  binder  course.  The  aggregate  shall  have 
the  following  composition:  20  to  50  per  cent  shall  pass  a  |-inch 

*  Report  of  Committee  of  the  Amer.  Soc.  of  C.  E.,  Proc.  Vol.  42  (1916),  p.  1626;  or  page  5 
of  Specifications  for  Sheet  Asphalt  Pavements  of  Amer.  Soc.  of  Municipal  Improvements, 
adopted  October  14,  1915. 


ART.    1]  SHEET    ASPHALT   PAVEMENTS  417 

screen  and  be  retained  on  a  10-mesh  screen;  and  15  to  35  per  cent 
shall  pass  a  10-mesh  screen."* 

817.  Sand.     The  chief  difference  between  an  open  and  a  closed 
binder  is  the  greater  density  and  stability  of  the  latter.     A  closed 
binder  should  contain  enough  screenings  or  sand  to  fill  the  voids  in 
the  coarse  material;  and  the  gradation  of  the  coarse  and  fine  aggre- 
gate should  be  such  as  to  give  a  minimum  percentage  of  voids,  and 
consequently  require  a  minimum  amount  of  asphalt  cement.     Ap- 
parently not  much  attention  has  been  given  to  the  quantity  or 
gradation  of   the  fine   material,  i.  e.,  sand  and   stone  screenings, 
for  the  binder  course;    and  certainly  no  specifications  for  the  fine 
material  have  been  published.     The  quantities  in    Table    38,  page 
418,  are  from  actual  practice  in  one  of  the  largest  cities  in  this 
country. 

818.  Asphalt    Cement.     The    object    in    using    a    closed    binder 
course  is  to  secure  maximum  stability,  and  hence  enough  asphalt 
cement  must  be  used  to  coat  all  the  fragments  of  the  aggregate 
and  fill  all  the  voids.     If  the  aggregate  contains  considerable  fine 
sand  and  dust,  the  stability  will  be  greater  but  more  asphalt  cement 
will  be  required.     With  very  carefully  graded  aggregate  3.5  per  cent 
of  asphalt  cement  will  give  a  very  stable  mixture;   but  if  the  gra- 
dation is  not  so  good  7  per  cent  may  be  required.     There  should  be 
enough  asphalt  cement  to  give  stability;  but  an  excess  may  be  very 
harmful,  as  it  will  likely  collect  in  pools  in  the  truck  while  being  taken 
to  the  street  and  appear  in  spots  in  the  binder  course,  from  which  it 
will  be  drawn  up  on  a  hot  day  into  the  wearing  coat  and  soften 
it.     A  uniformly  distributed  excess  is  less  dangerous  on  a  light- 
traffic  street  than  on  a  heavy-traffic  one,  since  in  the  former  the 
wearing  surface  is  likely  to  lose  its  volatile  matter  and  crack,  while  a 
rich  binder  will  slowly  enrich  the  wearing  surface. 

The  asphalt  cement  should  be  softer  than  that  in  the  wearing 
coat,  because  the  binder  is  more  open  than  the  wearing  coat,  and 
hence  more  of  the  lighter  oils  are  volatilized  in  the  mixing,  and  also 
because  the  softer  cement  makes  a  mixture  less  liable  to  rupture. 
In  ordinary  practice  the  cement  for  the  binder  has  a  penetration 
20  or  more  greater  than  that  for  the  wearing  coat. 

819.  Amount  of  Bitumen  in  Binder.     To  illustrate  the  method 
of  determining  the  per  cent  of  bitumen  in  a  particular  mixture, 

*  Report  of  Committee  of  the  Amer.  Soc.  of  C.  E.,  Proc.  Vol.  42  (1916),  p.  1629;  or  Speci- 
fications for  Sheet  Asphalt  Pavement*  of  Amer.  Soc.  of  Municipal  Improvements,  adopted 
October  14,  1915. 


418 


ASPHALT  PAVEMENTS 


CHAP.  XVI 


assume  that  the  composition  of  the  asphalt  cement  and  the  binder 
course  are  as  stated  in  Table  38.     The  pure  bitumen  in  the  bitumi- 

TABLE  38 
COMPOSITION  OF  BINDER  COURSE 


ASPHALT  CEMENT 

MATERIAL  FOR  BINDER  COURSE 

BITU- 
MEN IN 
BINDER 

Ingredients 

Per 

Cent 

Ingredients 

Lb. 

Per 
Cent 

Mexican  asphalt.  .  . 
Trinidad  asphalt.  .  . 
Indian  flux  

Total    

40 
40 
20 

Asphalt  cement  
Sand  

122 
438 
1  190 

7 
25 
68 

5.8% 

Broken  stone  .  

One  batch  or  boxful  

100 

1  750 

100 

nous  materials  is  as  follows:  Mexican  asphalt,  99.6  per  cent; 
Trinidad  asphalt,  56.0  per  cent;  and  Indian  flux,  99.6  per  cent. 
Then  the  bitumen  in  the  asphalt  cement  is : 

Mexican  asphalt 40  X  99.6  =  39.84% 

Trinidad  asphalt 40  X  56.0  =  22.40% 

Indian  flux 20  X  99.6  =  19.92% 

Total  Bitumen  in  Asphalt  Cement =82 . 16% 

Total  Bitumen  in  Binder  Course =82. 16  X  7=  5.8  % 

It  is  impossible  from  the  above  computations  to  determine 
whether  or  not  the  stated  amount  of  bitumen  will  fill  the  voids  in 
the  mineral  matter.  That  could  be  determined  accurately  only  by 
direct  test,  but  could  be  determined  approximately  from  a  knowl- 
edge of  the  gradation  of  the  mineral  matter  and  a  comparison  of  it 
with  the  composition  of  standard  binder  mixtures.  The  method  of 
determining  the  amount  of  bitumen  and  the  gradation  of  the  mineral 
matter  necessary  to  give  a  binder  course  of  maximum  stability  is 
exactly  the  same  as  for  the  wearing  course  (§  825  et  seq.),  and  hence 
will  not  be  discussed  here. 

820.  Mixing  Binder  Course.  The  aggregate  and  the  asphalt 
cement  should  be  heated  separately,  the  exact  temperature  of  each 
depending  mainly  upon  the  character  of  the  asphalt  cement. 
The  cement  is  usually  heated  to  a  temperature  between  120°  C. 
(250°  F.)  and  177°  C.  (350°  F.) ;  and  the  aggregate  is  heated  between 
107°  C.  (225°  F.)  and  177°  C.  (350°  F.). 

The  aggregate  and  the  cement  should  be  thoroughly  mixed  by 


ART.    1]  SHEET   ASPHALT    PAVEMENTS  /419 

machinery  until  a  homogeneous  mixture  is  obtained  in  which  all 
the  particles  of  the  aggregate  are  covered  with  cement. 

The  mixing  is  done  in  a  box  in  which  revolves  two  axles  each 
carrying  a  series  of   oblique   paddles   set   spirally — see  Fig.    146. 


FIG.  146. — MACHINE  FOB  MIXING  ASPHALT  BINDER. 

The  mixture  is  discharged  through  a  sliding  door  in  the  bot- 
tom of  the  box.  The  capacity  of  mixers  vary  from  1000  to  2000 
pounds. 

Fig.  147,  page  420,  shows  a  complete  semi-portable  asphalt  mix- 
ing plant.  On  the  left  is  the  boiler;  and  on  the  right  is  the  elevator 
for  the  fine  and  coarse  aggregate,  and  the  drum  for  drying  them. 
Behind  the  boiler  is  the  asphalt-heating  tank  or  kettle;  and  behind 
the  sand-drying  drum  is  the  mixer  with  sand-storage  bins  above  it. 

Fig.  148,  page  421,  shows  a  portable  asphalt-mixing  plant.  Sim- 
ilar plants  are  mounted  upon  one  and  sometimes  upon  two  steam- 
railroad  cars.  In  the  larger  cities  are  fixed  plants  which  consist  of 
quite  a  group  of  buildings. 

821.  Laying  Binder  Course.  The  binder  course  should  be  trans- 
ported to  the  street  in  wagons  or  trucks  covered  with  canvas  or  tar- 
paulin; and  when  delivered  should  have  a  temperature  between  93° 
and  163°  C.  (200°  and  325°  F.),  the  temperature  between  these 
limits  being  regulated  according  to  the  temperature  of  the  atmosphere 
and  the  ease  with  which  the  binder  course  can  be  spread.  The 
temperature  of  the  binder  on  the  street  should  be  no  greater  than  is 
necessary  to  permit  the  mixture  to  be  easily  spread. 

The  stone  should  be  covered  with  a  bright  glossy  coat  of  asphalt 


420 


ASPHALT    PAVEMENTS 


[CHAP,  xvi 


FIG.  147. — SEMI-PORTABLE  ASPHALT  MIXING  PLANT. 


ART.    1] 


SHEET   ASPHALT    PAVEMENTS 


421 


cement,  as  otherwise  it  will  have  no  coherence.  On  the  other  hand, 
there  should  be  no  excess  of  cement,  as  is  shown  by  its  running 
from  the  bottom  of  the  truck  or  by  too  great  richness  of  the  bottom 
of  the  load.  If  the  stone  was  heated  too  hot,  the  cement  may  run 
off  the  stone  at  the  top  of  the  load  and  accumulate  at  the  bottom,  in 
which  case  the  surface  of  the  load  will  appear  dull  and  dead. 

On  arrival  on  the  street  the  mixture  should  be  dumped  upon  the 
foundation  at  such  distance  from  where  it  is  to  be  spread  as  to 


FIG.  148. — PORTABLE  ASPHALT  MIXING  PLANT. 

require  that  all  of  the  material  shall  be  moved  from  where  dumped. 
This  is  necessary  to  secure  a  uniform  thickness  after  rolling,  since 
the  portion  at  the  bottom  of  the  pile  is  considerably  compressed  by 
the  fall  and  the  weight  of  the  incumbent  mass,  and  hence  if  it  is  not 
moved  the  binder  course  after  being  rolled  will  be  thicker  at  this 
point  than  elsewhere. 

The  binder  is  first  roughly  shoveled  into  place,  and  then  is  leveled 
off  with  rakes  or  shovels.  An  open  binder  may  be  leveled  with  hot 
iron  rakes  having  long  tines;  but  if  a  closed  binder  is  employed,  the 
spreading  should  be  done  with  a  shovel  or  the  back  of  a  rake,  as  the 
use  of  the  tines  will  bring  the  larger  stones  to  the  surface  and  produce 


422 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


segregation.  Fig.  149  shows  the  spreading  of  the  binder  course. 
Notice  that  the  binder  is  dumped  on  a  steel  plate  from  which 
it  is  shoveled  into  place. 

The  binder  course  may  be  allowed  to  cool  somewhat  before  being 
rolled,  for  if  it  is  too  hot  when  rolled  it  is  likely  to  stick  to  the  roller 
and  also  be  pushed  out  of  place.  The  rolling  should  be  done  with  a 
self-propelled  tandem  roller  weighing  5  to  7  tons,-  giving  a  pressure 


Fia.  149. — SPREADING  THE  BINDER  COURSE. 

of  not  less  tnan  20U  pounds  per  lineal  inch  of  tread.  The  object  of 
the  rolling  is  a  kneading  action  as  well  as  a  compression,  and  hence 
many  passes  of  a  light  roller  are  better  than  a  few  passes  of  a  heavy 
roller. 

After  rolling  the  surface  should  be  of  uniform  density;  and  par- 
ticularly there  should  be  no  spots  containing  an  excess  of  asphalt 
cement,  since  on  a  hot  day  the  excess  is  likely  to  be  drawn  up  into 
the  wearing  coat  and  soften  it. 

822.  After  the  rolling  of  the  binder  course  is  completed,  the 
wearing  coat  (§  825)  should  be  applied  at  once,  while  the  binder  is 
clean  and  hot;  or  it  should  at  least  be  added  during  the  same  day  for 
fear  the  binder  may  become  dirty  and  dusty.     This  is  necessary  to 
secure  the  maximum  bond  between  the  binder  and  the  wearing  coat; 
and  if  this  is  not  done,  the  binder  course  should  be  protected  from 
mud  and  excessive  dust. 

823.  Thickness  of  Binder  Course.    The  proper  thickness  of  the 


ART.    1]  SHEET   ASPHALT    PAVEMENTS  423 

binder  course  depends  upon  the  amount  of  the  traffic.  For  light 
traffic  the  thickness  is  usually  1  inch  after  being  rolled  and  for  medium 
traffic  is  li  inches,  while  for  very  heavy  traffic  it  is  sometimes  2 
inches.  For  data  on  the  thickness  of  binder  course  in  various  cities, 
see  Table  46,  page  453. 

The  compression  by  rolling  is  usually  about  40  per  cent.  The 
area  that  a  given  weight  of  the  binder  course  will  cover  will  depend 
upon  the  hardness  and  gradation  of  the  stone,  upon  the  consistency 
and  temperature  of  the  asphaltic  cement,  and  upon  the  degree  of 
compression. 

824.  The  thickness  to  which  the  compressed  mixture  should  be 
spread  to  give  the  specified  thickness  can  be  determined  either  by 
computation  or  by  trial. 

1.  To  compute  the  area  to  be  covered  by  a  given  weight,  first 
determine  the  specific  gravity  of  the  compressed  binder  course,  and 
then  compute  the  area  that  a  given  weight  should  cover  to  give  the 
required  thickness  after  rolling. 

2.  Lay  a  given  weight  on  a  known  area,  roll  it,  and  then  measure 
the  thickness  by  probing  at  several  points  with  a  dull  pointed  awl, 
being  careful  that  the  awl  penetrates  to  the  foundation.     After  a 
few  trials  the  exact  area  to  be  covered  by  a  stated  weight  to  give  the 
desired  thickness,  can  be  accurately  found. 

825.  WEARING  COAT.     The  wearing  coat  consists  of  sand,  and 
fine  mineral  dust  or  filler,  and  asphalt  cement.     The  sand  and  filler 
are  often  referred  to  as  the  mineral  aggregate. 

826.  The  Sand.     The  sand  is  a  very  important  element  in  a 
sheet  asphalt  pavement,  since  it  constitutes  at  least  three  fourths 
of  the  wearing  coat.     The  sand  should  consist  of  hard  and  durable 
grains.     It  should  be  free  from  vegetable  matter,  and  should  not 
contain  much  clay  or  loam;  although  a  small  amount  of  these,  if  not 
closely  adhering  to  the  grains,  will  act  as  a  filler  and  do  no  harm. 
However,  a  small  amount  of  clayey  material  adhering  to  the  grains 
is  highly  objectionable,  since  in  passing  through  the  heating  drum  or 
dryer  it  is  likely  to  be  burned   onto  the  grains  to  such  an  extent  as 
not  to  be  removed  in  the  mixer  and  consequently  the  bitumen  will 
not  adhere  well  to  the  sand. 

The  gradation  of  the  sand,  i.  e.,  the  relative  proportions  of  grains 
of  different  sizes,  is  the  chief  characteristic  to  be  considered;  occa- 
sionally, however,  a  sand  is  found  which  for  some  unknown  reason, 
perhaps  the  shape  of  the  grains  or  character  of  the  surface,  proves 
to  be  unsuitable  for  sheet  asphalt  pavements. 


424 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


827.  Gradation  of  Sand.  Until  comparatively  recently  but  little 
attention  was  given  to  the  gradation  of  the  sand;  but  it  is  now 
known  that  this  is  one  of  the  most  important  elements  in  the 
construction  of  a  sheet  asphalt  pavement.  Table  39  shows  the 
grading  of  the. sand  used  for  the  wearing  coat  of  sheet  asphalt  pave- 
ments in  a  number  of  cities  before  the  importance  of  this  element  was 
appreciated.  The  last  line  of  the  table  shows  the  grading  now  be- 
lieved to  be  the  best  attainable.  The  data  in  this  table  are  mainly 
interesting  as  showing  a  possible  reason  for  the  unsatisfactory  service 
of  some  sheet  asphalt  pavements. 

TABLE  39 
FORMER  GRADING  OP  SAND  FOR  SHEET  ASPHALT  PAVEMENTS* 


o* 

* 

*S 
tf 

City. 

PER  CENT  PASSING  SIEVE  No. 

Total 
Per 
Cent. 

200 

100 

80 

50 

40 

30 

20 

10 

1 
2 

3 
4 
5 

6 

7 

8 

Boston,  1899:         

6 

31 
1 

10 
2 

2 

32 

0 
2 
2 

0 
14 
2 
17 

14.5 
0.0 

13 

39 
6 

68 
15 

1 
33 

1 
25 
19 

1 

26 
4 
40 

14.5 
17.0 

14 

21 
10 

15 
17 

4 
13 

2 
29 
19 

1 
14 
22 
30 

14.5 
17.0 

31 

8 
41 

5 

52 

53 

18 

36 
36 
41 

48 
38 
28 
10 

26.2 
30.0 

20 

1 
19 

2 
9 

25 
3 

32 
4 
12 

46 
6 
19 

1 

12.3 
13.0 

10 
15' 

2 

10 
1 

17 
3 
3 

3 
2 
10 
1 

9.0 
10.0 

4 
5' 

"2" 
3 

2 
3 

Y 
2 

100 

100 
100 

100 
100 

100 
100 

100 
100 
100 

100 
100 
100 
100 

100 
100 

Buffalo: 
bank,  fine  

lake  coarse 

Chicago,  1896: 
fine 

medium 

Louisville  : 
river. 

bar               

Milwaukee  : 
coarse  beach  

9 
1 
2 

1 

l6" 
1 

5.6 

8.0 

3 
2 
0 
5 

3.4 
5.0 

White  Fish  Bay  

Omaha  

St.  Louis,  1897: 
coarse. 

fine. 

river,  coarse 

fine.        .    . 

Richardson's  Ideal  :f 
mineral  aggregate  

sand  proper  

*  Richardson's  Modern  Sheet  Asphalt  Pavement,  p.  85. 
t  Ibid.,  p.  332. 

The  ideal  grading  for  sand  for  a  sheet  asphalt  was  obtained  by 
analyzing  pavements  that  had  given  the  best  service;  and  has  been 
abundantly  tested  in  practice  for  more  than  twenty  years.  Unfor- 
tunately it  is  not  often  that  a  natural  sand  can  be  found  which 
approximates  the  ideal  grading;  and  therefore  an  artificial  mixture 
must  be  prepared  by  screening  the  sand  into  several  lots  excluding 
one  or  more  of  the  lots,  and  then  remixing  the  remainder,  or  by  com- 


ART.    1] 


SHEET   ASPHALT   PAVEMENTS 


425 


bining  portions  of  different  sands.  Unless  the  available  natural 
sand  is  nearly  ideal,  the  securing  of  the  best  grading  will  entail  con- 
siderable expense;  and  in  this  case  it  may  be  wiser  to  accept  a  grading 
only  approximating  the  ideal.  Table  40  shows  the  ideal  grading  for 
heavy  traffic  and  also  two  permissible  gradings. 

TABLE  40 

STANDARD  GRADINGS  FOR  SAND  FOR  SHEET  ASPHALT  PAVEMENTS 


Grade  of 
of  Sand. 

Passing 
Sieve  No. 

RICHARDSON'S  GRADINGS.* 

Forrest's  Per- 
missible Grad- 
ing, t 

Ideal  for  Heavy  Traffic. 

Permissible  for 
Light  Traffic. 

Dust  

200 

100 
80 

50 
40 

30 
20 
10 

8 

00.0 

17.0% 
17.0 

00.0 

26.0% 

30.0 
13.0 

43.0% 

30.0% 
0.0 

00.0 
20  to  30% 
not  over  40% 

20  to  30% 
not  over  10% 

Fine   » 

Medium 

Total                34% 

30.0% 
13.0 

Coarse 

Total                43% 

10.0% 
8.0 
5.0 

Very  coarse  

Total                23% 
0.0 
Total       100     100 

100 

100 

*  Richardson's  Asphalt  Construction  for  Pavements  and  Highways,  1913,  p.  29 
t  C.  N.  Forrest,  Chief  Chemist,  Barber  Asphalt  Paving  Co.,  in  private  letter  to  the  author 
dated  August  17,  1917. 

Notice  that  the  aggregate  for  heavy  traffic  is  finer  than  that  for 
light  traffic.  The  reason  for  this  is  as  follows:  Large  pieces  of 
aggregate  will  be  fractured  sooner  or  later  by  the  passage  over  them 
of  heavy  loads;  and  when  this  occurs  there  are  two  surfaces  which 
are  not  cemented  together.  This  condition  permits  a  movement  and 
a  grinding  action,  and  also  allows  the  entrance  of  water;  and  thus 
two  extremely  destructive  agents  are  set  to  work. 

Table  41,  page  426,  shows  the  grading  of  sands  used  recently  by 
the  Barber  Asphalt  Paving  Co.,  in  pavements  in  various  cities. 
These  data  are  instructive  as  showing  the  degree  of  agreement  of  the 
gradings  of  the  best  available  sands  with  the  standards  stated  in 


426 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


Table  40,  and  are  also  a  valuable  guide  in  selecting  a  sand  for  use 
in  a  pavement. 

TABLE  41 
AVERAGE  GRADING  OF  SANDS  RECENTLY  USED  IN  SHEET  ASPHALT  PAVEMENTS  * 


City. 

PER  CENT  PASSING  SIEVE  No. 

.23 
1  o 

PH£ 

Fine  Sand. 

Medium  Sand. 

Coarse  Sand. 

100 

80 

Total 

50 

40 

Total 

30 

20 

4.0 
4.2 
2.5 
8.9 
5.5 
5.3 
5.3 
5.5 

8.0 

10 

Total 

Boston         

13.2 
16.7 
20.6 
28.3 
16.4 
14.6 
9.2 
20.5 

17.0 

15.8 
12.5 
21.9 
19.2 
11.0 
13.3 
17.2 
19.2 

17.0 

29.0 
29.3 
42.5 
45.5 
27.4 
27.9 
26.4 
39.7 

34.0 

42.0 

34.7 
43.6 
23.4 
35.6 
37.5 
51.1 
37.1 

30.0 

15.8 
9.7 
6.4 
8.9 
17.8 
17.3 
7.9 
8.2 

13.0 

57.8 
44.4 
50.0 
32.3 
53.4 
54.8 
59.0 
45.3 

43.0 

6.6 

5.5 
2.5 
8.9 
9.6 
9.3 
5.3 
6.8 

10.0 

2.6 
9.7 
2.5 
4.4 
4.1 
2.7 
4.0 
2.7 

5.0 

13.2 
19.4 
7.5 
22.2 
19.2 
17.3 
14.6 
15.0 

23.0 

7 

Buffalo                          •  •  •  • 

Chicago                        .... 

Kansas  City             • 

Louisvills                  

NGW  York             

Omaha                  

St  Louis 

Richardson's  Ideal  

*  Compiled  from  Richardson's  Modern  Sheet  Asphalt  Pavement,  p.  331. 

828.  The  Filler.    The  filler  is  fine  mineral  matter  mixed  with  the 
sand  to  fill  the  voids,  and  thus  reduce  the  amount  of  asphalt  required, 
and  also  to  make  the  wearing  coat  more  waterproof  and  less  plastic. 
Further,  the  use  of  a  filler  permits  the  use  of  a  softer  asphalt  cement 
and  thus  makes  the  wearing  coat  less  liable  to  internal  displacement 
in  summer  and  less  brittle  in  winter. 

The  filler  is  usually  either  pulverized  limestone  or  portland  cement, 
generally  the  former.  Portland  cement  is  preferable  for  heavy  traffic 
or  where  the  asphalt  surface  is  subject  to  the  action  of  water.  The 
heavier  the  filler  per  unit  of  volume  the  better,  since  this  usually 
indicates  greater  density;  and  the  denser  the  filler,  the  denser  and 
more  stable  the  wearing  coat.  The  valuable  part  of  the  filler  is 
the  impalpable  dust  which  is  much  finer  than  the  particles  just 
passing  a  200-mesh  sieve.  "  A  good  filler  should  contain  at  least 
60  per  cent  by  weight  of  actual  dust,  and  preferably  70  per  cent." 

The  amount  of  material  in  the  wearing  coat  passing  a  200-mesh 
sieve  varies  from  10  to  20  per  cent,  according  to  the  grading  of  the 
sand,  but  usually  from  12  to  16 — see  §  835.  Less  filler  is  required 
with  Trinidad  asphalt  than  other  kinds,  since  it  naturally  contains 
about  44  per  cent  of  finely  divided  mineral  matter. 

829.  ASPHALT  CEMENT.     The  method  of  preparing  the  asphalt 
cement  has  been  described  in  §  530;    and  specifications  for  it  are 
found  in  §  542. 


AET.    1]  SHEET   ASPHALT   PAVEMENTS  427 

The  amount  of  asphalt  cement  in  the  wearing  coat  should  be 
sufficient  to  coat  every  particle  of  mineral  matter  and  fill  all  the  voids, 
but  should  not  be  enough  to  make  the  mixture  too  susceptible  to 
pressure  and  temperature  changes.  If  too  much  asphalt  cement  is 
used,  the  sand  grains  will  be  readily  displaced  among  themselves,  and 
the  wearing  coat  will  push  out  of  place.  If  too  little  cement  is  used, 
the  surface  will  crack  and  is  liable  to  be  displaced  because  of  lack 
of  solidity.  The  finer  the  mineral  aggregate,  the  greater  the  amount 
of  cement  required  for  stability.  Too  much  dust  in  the  aggregate 
causes  the  mixture  to  be  mushy. 

830.  Amount  of  Cement.     The  amount  of  asphalt  cement  to  be 
used  in  any  particular  case  depends  upon  the  gradation  of  the  sand 
and  the  filler,  and  to  some  extent  is  a  matter  of  judgment  and  experi- 
ence;   but  there  are  four  tests  that  are  guides  in  determining  the 
best  proportions  of  sand,  filler,  and  cement  for  the  wearing  coat. 
These  tests  are  the  paper-pat  test,  the  impact  test,  and  the  deter- 
mination of  the  density  and  the  absorptive  power  of  the  compressed 
mixture. 

831.  Paper-pat   Test.     This  test  is  made  as  follows:    Secure  a 
sheet  of  manila  paper  having  a  smooth  surface,  crease  it  down  the 
middle,  and  lay  it  opened  out  on  a  smooth  firm  wood  surface,  not 
stone  or  metal,  which  would  cool  the  mixture  too  rapidly.     With  a 
wooden  paddle  having  a  blade  3  or  4  inches  wide  and  about  J  inch 
thick,  tapering  to  an  edge,  take  a  paddleful  of  the  hot  mixture,  being 
careful  to  get  a  representative  sample.     Note  the  temperature  of 
the  mixture.     Drop  the  mixture  sidewise  from  the  paddle  on  to  the 
paper,  and  fold  the  paper  over  the  mixture.     With  a  block  of  wood 
press  the  surface  of  the  mixture  down  until  it  is  flat,  and  then  strike 
it  five  or  six  blows  with  the  block  until  the  pat  is  about  half  an  inch 
thick.     Open  the  paper,  and  the  stain  upon  the  paper  indicates  the 
amount  of  bitumen  in  the  mixture.     Fig.  150-53,  page  428-31,  show 
four  progressive  characteristic  stains.*     Fig.  150  indicates  a  mixture 
in  which  there  is  a  considerable  deficiency  of  bitumen,  Fig.  151  a 
slight  deficiency  of  bitumen,  Fig.  152  a  mixture  having  the  proper 
amount,  and  Fig.  153  an  excess  of  bitumen. 

In  interpreting  the  character  of  the  stain  consideration  must  be 
given  to  the  temperature  of  the.  sample  and  to  the  kind  of  asphalt. 
The  sample  must  be  taken  and  the  stain  made  when  the  temperature 
of  the  mixture  is  such  that  the  asphalt  is  quite  liquid.  If  the  mix- 

*  By  courtesy  of  D.  T.  Pierce,  Executive  Assistant,  Barber  Asphalt  Paving  Co. 


428 


ASPHALT    PAVEMENTS 


[CHAP,  xvi 


FIG.  150. — LIGHT  STAIN.  , 


ART.    1] 


SHEET   ASPHALT    PAVEMENTS 


429 


Fio.  151. — MEDIUM  STAIN. 


430 


ASPHALT   PAVEMENTS 


[CHAP  xvi 


FIG.  152. — STRONG  STAIN, 


ART.    1] 


SHEET   ASPHALT   PAVEMENTS 


431 


Fio.  153. — HEAVY  STAIN. 


432  ASPHALT   PAVEMENTS  [CHAP.    XVI 

ture  is  too  cold,  the  test  is  of  no  value;  and  if  the  mixture  is  too  hot, 
the  stain  will  be  stronger  than  for  the  proper  temperature.  Further, 
the  test  is  more  valuable  for  Trinidad  asphalt  than  for  other  kinds  of 
asphalt,  since  the  latter  are  more  susceptible  to  temperature  changes. 

The  appearance  of  the  surface  of  the  hot  pat  is  nearly  as  instruc- 
tive as  that  of  the  stain  on  the  paper.  If  the  mixture  is  unbalanced 
in  any  way,  the  surface  will  have  a  greasy  appearance,  which  may 
be  due  to  an  excess  of  either  bitumen  or  filler;  but  the  cause  of  the 
greasiness  can  be  determined  only  by  trial. 

832.  Density.  Another  method  of  testing  the  correctness  of  the 
proportions  of  the  wearing  coat  is  to  determine  the  density,  or  spe- 
cific gravity,  of  the  compressed  mixture.  A  cylindrical  test  speci- 
men 1J  inch  in  diameter  and  about  1  inch  high,  is  moulded  while  hot 
under  standard  pressure.  The  specific  gravity  of  the  specimen  is 
then  determined  either  by  weighing  it  in  air  and  in  water,  or  by 
weighing  it  in  air  and  measuring  its  volume. 

The  following  example  illustrates  the  method  of  making  this  test.* 

The  customary  mixture  in  parts  by  weight  is : 

Sand 75  per  cent 

Dust  or  filler 10  "     " 

Trinidad  asphalt  cement 15  "     " 

Total 100  "     " 

The  specific  gravity  of  the  sand  is  2.65,  the  limestone  dust  2.60, 
and  the  asphalt  cement  1.25.  The  volumes  of  the  materials  in  the 
mixture  are : 

Sand 75  -f-  2.65=28.30  units  of  volume  =  64.10% 

Limestone  dust 10-^-2.60=3.85     "     "        "      =     8.72 

Asphalt  cement.. 15 -r  1 .25  =  12.00     "     "         "       =27.18 

Total 44.15     "     "        "      =100.00% 

If  the  mass  is  compressed  so  as  to  exclude  all  the  entrained  air, 
i.  e.,  so  that  all  the  voids  in  the  mineral  aggregate  are  filled  with 
asphalt  cement,  then  the  specific  gravity  of  the  mixture  would  be: 

Sand 64. 1%  X  2.66  =  1.699 

Limestone  dust 8 . 7%  X  2 . 60  =    .226 

Asphalt  cement 27.2%  X  1.25=    .340 

Ultimate  specific  gravity .  .  .  . ' =  2 . 265 

The  specific  gravity  of  the  specimen  should  be  at  least  2.20,  and 
is  usually  not  over  2.22.  If  portland  cement  is  used  as  a  filler  instead 
of  limestone  dust,  the  specific  gravity  will  be  about  0.02  higher, 

*  Richardaon's  Modern  Asphalt  Pavement,  p.  581, 


ART.    1] 


SHEET  ASPHALT   PAVEMENTS 


433 


since  the  specific  gravity  of  portland  cement  is  about  3.10.  The 
greater  the  proportion  of  asphalt,  the  less  the  specific  gravity. 

If  the  specific  gravity  of  the  specimen  is  2.22,  the  voids  are: 
(2.265  -  2.22) -T- 2.265  =  2  per  cent. 

Pavements  having  the  standard  grading,  after  being  rolled,  have 
within  1  or  2  per  cent  of  the  ultimate  specific  gravity;  but  some 
pavements  laid  without  regard  to  the  best  gradation,  have  specific 
gravities  as  low  as  1.90,  and  have  not  given  reasonably  satisfactory 
service. 

833.  Absorptive  Power.  The  denser  the  mixture  and  the  larger 
the  percentage  of  bitumen  it  contains,  the  more  resistant  it  will  be 
to  the  action  of  water;  hence  a  determination  of  the  absorptive  power 
of  the  compressed  mixture  gives  valuable  information  concerning 
the  correctness  of  the  proposed  proportions,  particularly  if  the  pave- 
ment is  to  be  laid  in  a  humid  atmosphere. 

The  absorptive  power  is  determined  by  moulding  a  cylindrical 
specimen  as  in  determining  the  density  (§  832),  weighing  it,  suspend- 
ing it  in  water,  and  then  weighing  it  at  intervals.  The  gain  in 
weight  is  the  water  absorbed;  and  the  absorption  per  unit  of  area 
may  then  be  computed.  Table  42  shows  the  results  with  two  mix- 
tures, the  first  containing  coarse  sand  and  too  little  filler,  and  the 
second  being  Richardson's  ideal  mixture. 

TABLE  42 

ABSORPTION  OP  CYLINDERS  OP  WEARING  COAT  * 
Pounds  per  Square  Yard 


Interval 

WASHINGTON  MIXTURE, 
1893 

IDEAL  MIXTURE,  1904. 

Trinidad 
Asphalt. 

Bermudez 
Asphalt. 

Trinidad 
Asphalt. 

Bermudez 
Asphalt. 

Seven  days      

0.314 
0.434 
0.502 

0.063 
0.194 

0.306 

0.080 

0.093 
0.107 

0.094 
0.093 
0.104 

Fourteen  days    

Twenty-eicht  days 

834.  Impact  Test.  A  cylindrical  test  piece  is  moulded  as  in 
making  the  density  test  (§  832),  and  then  it  is  subjected  to  suc- 
cessive blows  of  the  dropping  weight  of  the  impact  machine,  f  The 
dropping  weight  or  hammer  weighs  2  kilograms  (4.40  lb.);  and  the 
height  of  fall  is  1  centimeter  for  the  first  blow,  and  an  increase  of  1 
centimeter  for  each  successive  blow  until  the  test  piece  fails.  The 

*  Richardson's  Modern  Asphalt  Pavements,  p.  468. 

t  Bulletin  No.  44,  Office  of  Public  Roads,  U.  S.  Department  of  Agriculture,  June  10,  1912, 
p.  9-11. 


434 


ASPHALT   PAVEMENTS 


[CHAP  xvi 


number  of  blows  required  to  produce  rupture  is  assumed  to  repre- 
sent the  toughness  of  the  specimen.  The  number  of  blows  required 
to  produce  failure  will  depend  upon  the  consistency  of  the  asphalt 
cement  and  upon  the  temperature  of  the  specimen  at  the  time  of 
testing.  The  best  mixtures  at  a  temperature  of  78°  F.  require  from 
20  to  30  blows  to  produce  rupture. 

835.  Proportion  from  Practice.  Table  43  shows  the  grading  of 
the  wearing  coat  of  sheet  asphalt  pavements  laid  in  1916  and  1917 
by  different  contractors  in  a  number  of  cities,*  and  also  Richard- 
son's ideal  proportions  f  and  Forrest's  permissible  composition.  J 

TABLE  43 
PROPORTIONS  FOR  SHEET  ASPHALT  PAVEMENTS 

Laid  in  1916  and  1917 


Ref. 

No. 

LOCALITY. 

Bitu- 
men, 
per 
Cent. 

PER  CENT  PASSING  SIEVE  No. 

State. 

City. 

200 

100 

80 

50 

40 

10.0 
11.0 
.0 
10.1 
12.1 
4.3 
12.0 

10.0 

ast 
sible 

30 

8.0 
11.0 
5.3 
13.1 
2.9 
10.0 

8.0 
15- 

20 

5.0 
9.0 
12.3 
5.9 
10.4 
3.1 
7.0 

5.0 
25 

1* 

6.0 
10.0 

7.6 
5.8 
5.1 
2.0 

3.0 

4 

2~0 

1.0 
1.5 
1.5 
1.8 
2.9 

1 
2 
3 

4 
5 

? 

8 
9 

Massachusetts.  .  .  . 
New  York 

Boston  
Roxbury.  .  .  . 
New  York  .  . 
Kingston.  .  . 
Hamilton.  .  . 
Newark  .... 
Sumter 

11.4 
10.1 
10.4 
10.5 
10.2 
10.1 
10.0 

10.0 
9-11 

12.6 
12.9 
17.1 
11.6 
16.1 
10.3 
11.0 

10.0 
10-12 

17.0 
12.0 
13 
9.4 
7.7 
14.2 
13.0 

10.0 
15 

7.0 
6.0 
.6 
7.4 
3.9 
16.2 
10.0 

20.0 
-25 

21.0 
17.0 
47 
30.7 
18.9 
30.9 
25.0 

24.0 
le 
pos 

North  Carolina.  .  .  . 
Ohio 

South  Carolina.  .  .  . 

Richardson's  Ideal, 
Forrest's  Permissib! 

it  least  
e  

836.  The   specifications   for   sheet   asphalt   pavements   adopted 
by  the  American  Society  of  Municipal  Improvements  on  October  14, 
1915,  contain  the  following  requirements  for  the  composition  of  the 
wearing  coat: 
Bitumen I 9.5  to  13.5% 


Passing  200  mesh .  .  .not  less  than  10% 


total  not  less  than  25% 


total  15  to  50% 


total  10  to  35% 


Passing  80  mesh 10  to  35% 

Passing  50  mesh 4  to  35% 

Passing  40  mesh 4  to  25% 

Passing  30  mesh 4  to  20% 

Passing  20  mesh 4  to  12% 

Passing  10  mesh 2  to    8%  J 

Passing  8  mesh 0  to    5% 

"  The  minimum  amount  of  bitumen  shall  be  used  only  in  mixtures  con- 
taining the  minimum  passing  the  80  mesh;  and  the  percentage  of  bitumen 
must  increase  as  the  amount  passing  the  80  mesh  increases." 

*  By  courtesy  of  D.  T.  Pierce,  Executive  Assistant,  Barber  Asphalt  Paving  Co.,  in  letter  to 
the  author  under  date  of  Sept.  14,  1917. 

t  Richardson's  Moderu  Sheet  Asphalt  Pavement,  p.  326. 

J  C.  N.  Forrest,  Chief  Chemist,  Barber  Asphalt  Paving  Co.,  in  letter  to  [the  author  dated 
August  17,  1917. 


ART.    1] 


SHEET   ASPHALT   PAVEMENTS 


435 


837.  Percentage  of  Bitumen.  Notice  that  Table  43  shows  the 
percentage  of  bitumen;  while  the  wearing  coat  is  a  mixture  of  asphalt 
cement,  filler  and  sand.  To  compute  the  percentage  of  bitumen 
in  the  wearing  coat  proceed  as  follows:  Assume  that  the  asphalt 
cement  and  the  wearing  coat  have  the  compositions  stated  in 
Table  44. 

TABLE  44 
COMPOSITION  OF  WEARING  COAT 


ASPHALT  CEMENT 

WEARING  ( 

?OAT 

Ingredients 

Per 
Cent. 

Ingredients 

Lb. 

Per  Cent. 

Mexican  asphalt  
Trinidad  asphalt 

40 
40 

Asphalt  cement  
Limestone  dust. 

285 
300 

14.2 
15  0 

Indian  flux 

20 

Sand. 

1  415 

70  8 

Total 

100 

Total      .    . 

2000 

100  0 

The  bitumen  in  the  bituminous  materials  is  as  follows:  Mexican 
asphalt,  99.6  per  cent;  Trinidad  asphalt,  56.0  per  cent;  and  Indian 
flux,  99.6  per  cent.  Then  the  pure  bitumen  in  the  asphaltic  cement 
may  be  computed  as  follows: 

Mexican  asphalt 40  X  99.6  =  39.84  per  cent 

Trinidad  asphalt 40  X  56.0  =  22.40     "     " 

Indian  flux.  .  .  .20  X  99.6  =  19.92     "     " 


Total  bitumen  in  asphalt  cement =82 . 16 

Hence  the  total  bitumen  in  the  wearing  coat  is  82.16X14.02  =  11.7 
per  cent. 

838.  Mixing  the  Wearing  Coat.  The  sand  and  the  asphalt 
cement  should  be  heated  separately.  The  sand  should  have  a  tem- 
perature between  275  and  400°  F.  (135-205°  C.),  and  the  asphalt 
cement  from  250  to  350°  F.  (121-177°  C.).  The  exact  tempera- 
ture in  any  case  depends  upon  the  asphalt  used;  and  the  temper- 
ature between  these  limits  is  to  be  adopted  to  suit  the  particular 
asphalt.  A  temperature  which  is  appropriate  for  one  asphalt 
may  harden  another  too  much;  or  a  temperature  which  makes  one 
asphalt  so  fluid  that  it  separates  from  the  aggregate,  may  make  an- 
other asphalt  so  stiff  that  it  can  not  be  properly  spread  and  rolled. 
The  sand,  filler  and  asphalt  cement  for  each  batch  or  mixerful  should 
be  carefully  and  separately  weighed,  and  then  be  dumped  into  the 
mixer. 


436  ASPHALT   PAVEMENTS  [CHAP.   XVI 

The  mixing  is  done  in  a  machine  like  that  shown  in  Fig.  146, 
page  419.  The  mixing  should  be  very  thorough,  and  be  continued 
until  the  mass  is  homogeneous  and  each  particle  of  aggregate  is 
covered  with  asphalt.  The  mixing  usually  requires  1  to  1J  minutes. 

839.  Laying  the  Wearing  Coat.  The  mixture  for  the  wearing 
coat  should  be  brought  to  the  street  in  wagons  or  trucks  covered 
with  canvas,  at  a  temperature  of  230  to  350°  F.  (110-177°  C.). 
The  temperature  within  the  above  limits  is  regulated  according  to 
the  kind  of  asphalt,  the  temperature  of  the  air,  and  the  ease  with 
which  the  mixture  is  spread. 

The  top  of  the  binding  course  should  be  perfectly  dry  when 
the  wearing  coat  is  laid,  to  prevent  the  top  course  from  being  sep- 
arated from  the  course  below  by  the  formation  of  steam.  Asphalt 
should  not  be  laid  in  cold  weather,  since  the  paving  mixture  may 
become  chilled  between  the  mixing  plant  and  the  street,  and  par- 
ticularly when  it  comes  in  contact  with  the  cold  foundation. 

The  mixture  should  be  dumped  outside  of  the  area  on  which 
it  is  to  be  spread,  so  that  it  shall  all  be  moved  in  being  put  into 
place  and  thus  secure  an  even  distribution  of  the  material.  This  is 
very  important,  since  great  care  must  be  exercised  to  prevent  de- 
pressions or  elevations  in  the  finished  surfaces,  as  the  impact  due  to 
such  spots  is  likely  to  cause  the  wearing  coat  to  be  pushed  out  of 
place.  The  mixture  is  thrown  into  place  with  hot  shovels  after 
which  it  is  uniformly  spread  with  hot  rakes.  The  depth  to  which 
the  mixture  is  to  be  spread  is  regulated  by  chalk  lines  on  the  curb, 
by  the  length  of  the  teeth  of  the  rake,  and  sometimes  by  rods  sup- 
ported on  feet  of  a  length  sufficient  to  bring  the  top  of  the  rod  to 
the  level  of  the  uncompacted  mixture.  The  compression  in  rolling 
varies  with  the  richness  of  the  mixture,  the  leaner  mixtures  compress- 
ing most;  and  is  usually  from  three  tenths  to  four  tenths. 

Fig.  154  shows  the  spreading  of  the  wearing  course  of  a  pave- 
ment on  Fifth  Avenue,  New  York  City. 

840.  Immediately  after  being  spread  the  wearing  coat  should 
be  composed  by  rolling  or  tamping.  Tamping  irons  are  used 
around  man-hole  covers,  near  curbs,  etc.,  where  the  roller  can 
not  conveniently  be  used.  Fig.  155  shows  two  forms  of  asphalt 
tampers.  The  left-hand  one  is  8  inches  in  diameter  and  weighs 
about  30  lb.;  and  the  right-hand  one  has  a  face  2JX5  inches  and 
weighs  about  18  lb.  Hot  smoothing  irons,  Fig.  156,  page  438,  are 
employed  to  finish  the  gutters,  angles,  edges,  and  all  joints  or  junc- 
tures where  one  day's  work  joins  that  of  another.  The  tampers 


ART.    1] 


SHEET  ASPHALT   PAVEMENTS 


437 


and  smoothing  irons  are  heated  in  a  metal  basket  which  is  moved 
forward  on  wheels. 

Formerly  the  first  rolling  was.  done  with  a  light  hand  roller  with 


FIG.  154. — SPREADING  THE  WEARING  COAT  OP  AN  ASPHALT  PAVEMENT. 

a  very  long  handle.  Fig.  157,  page  438,  shows  a  form  of  hand  roller 
formerly  used.  The  hand  roller  has  been  abandoned  in  favor  of  a 
light  self-propelling  tandem  roller  (Fig.  71,  page  213).  The  use  of 


Fio.  155. — TAMPERS  FOR  ASPHALT  PAVEMENTS. 

hot  smoothing  irons  and  hot  rollers  are  objectionable  since  it  is 
impossible  always  to  have  them  of  such  a  temperature  as  not  to  injure 
the  pavement;  and  since,  if  the  mixture  is  delivered  at  the  proper 


438 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


temperature,  and  the  raking  and  spreading  is  done  expeditiously, 
they  are  unnecessary.  Experience  shows  that  the  surface  of  pave- 
ments upon  which  hot  smoothing  irons  were  used  scales  and  flakes 
ofi  more  than  a  surface  laid  without  hot  tools. 


Fio.  156. — ASPHALT  SMOOTHING  IBONS. 


Fia.  157. — HAND  ASPHALT-ROLLER  WITH  FIRE  POT. 

841.  Rolling.  Immediately  after  being  spread  the  wearing  coat 
should  be  rolled.  It  is  important  that  the  rolling  should  closely 
follow  the  spreading,  so  that  the  material  shall  not  cool  before  the 
final  compression  is  obtained.  The  state  of  the  weather  is  an  ele- 
ment to  be  considered;  for  if  a  strong  wind  be  blowing,  the  material, 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  439 

spread  over  a  broad  surface  only  2  or  3  inches  thick,  will  cool  much 
more  rapidly  than  on  a  calm  day  when  the  temperature  is  consid- 
erably lower. 

It  is  usually  specified  that  the  rolling  shall  be  finished  with  a 
roller  giving  a  compression  of  at  least  200  Ib.  per  lineal  inch.  But 
to  secure  the  best  results  the  rolling  should  be  begun  with  a  2J-ton 
tandem  roller,  and  be  followed  with  a  5-ton  roller,  and  be  com- 
pleted with  an  8-ton  roller.  The  first  gives  a  compression  of  about 
60  Ib.  per  linear  inch  of  face  under  the  front  roll  and  about  125  under 
the  driving  drum;  the  second  200,  and  the  third  280,  under  the 
driving  drum.  Usually  only  a  5-ton  and  an  8-ton  roller  are  used, 
and  sometimes  only  an  8-  or  10-ton.  The  attempts  to  do  all  the  roll- 
ing with  a  single  8-  or  10-ton  roller  is  very  objectionable,  since  the 
roller  is  too  heavy  for  the  hot  material,  and  hence  the  rolling  is 
delayed  until  the  mixture  is  too  cold  to  compact  well.  Further, 
the  maximum  compression  can  not  be  produced  by  pressure  alone, 
but  requires  somewhat  of  a  kneading  action;  and  hence  several 
passages  of  a  light  roller  are  better  than  fewer  passages  of  a  heavier 
roller.  On  the  other  hand,  the  heavier  roller  is  needed  at  the  end 
of  the  rolling  to  secure  the  greatest  possible  compression.  Many 
loaded-wagon  tires  give  a  greater  pressure  per  inch  of  face  than  the 
heaviest  asphalt  roller;  and^therefore  if  the  rolling  is  not  done  with 
a  reasonably  heavy  roller  and  is  not  long  continued,  the  traffic  may 
make  indentations  on  the  surface  and  possibly  seriously  push  the 
wearing  coat  out  of  place. 

The  lubricating  effect  of  the  warm  asphalt  aids  in  the  com- 
pression, so  that  under  the  roller  the  grains  of  sand  are  wedged 
together  and  the  finer  particles  worked  into  the  voids,  until  the  mix- 
ture occupies  less  space  than  the  mineral  aggregate  alone  could  pos- 
sibly be  made  to  occupy.  This  is  proved  "by  the  fact  that  if  all  the 
bitumen  be  extracted  from  a  fragment  of  good  pavement  of  known 
volume,  it  is  found  to  be  quite  impossible  to  reduce  the  dry  sand 
obtained  to  as  small  a  volume  'as  it  occupied  in  the  pavement. 

If  the  asphalt  mixture  adheres  to"the  roller,  the  face  of  the  roller 
may  be  slightly  moistened  with  a  mixture  of  kerosene  and  water. 
Sometimes  water  is  sprayed  on  the  roller;  but  the  use  of  an  excessive 
amount  of  water  should  not  be  allowed.  The  sticking  can  be  pre- 
vented by  sprinkling  or  dusting  portland  cement  on  the  pavement. 

If  the  street  is  wide  enough,  the  pavement  should  be  rolled 
transversely  as  well  as  longitudinally;  and  if  this  is  not  possible,  the 
roller  should  run  as  obliquely  as  possible,  so  that  any  little  inequality 


440 


ASPHALT  PAVEMENTS 


CHAP.  XVI 


which  might  be  caused  by  the  roller's  moving  lengthwise  may  be 
taken  out  by  the  cross  action.  The  rolling  should  be  kept  up  until 
the  heaviest  roller  leaves  no  mark,  a  result  which  usually  requires 
at  least  5  hours  for  each  one  thousand  square  yards  of  surface. 

New  York  City  specifies  that  the  rolling  shall  be  continued  until 
the  wearing  coat  has  a  stated  specific  gravity,  viz.,  2.10  for  a  pave- 
ment laid  between  April  1st  and  December  1st,  and  not  less  than 
2.05  for  a  pavement  laid  between  December  1  and  April  1.  For 
data  on  laboratory  tests  of  density,  see  §  832. 

As  soon  as  the  rolling  is  completed,  the  pavement  may  be  thrown 
open.  Traffic,  if  not  of  too  heavy  vehicles,  is  an  advantage  to  a  newly 
laid  asphalt  pavement,  since  the  pressure  of  the  wheels  aids  in  con- 
solidating the  wearing  coat  and  in  closing  the  surface,  a  result  which 
helps  to  retain  the  volatile  oils  and  prevents  the  entrance  of  water. 
Asphalt  pavements  in  unfrequented  streets  do  not  wear  so  well  as 
those  under  a  moderately  heavy  traffic. 

Fig.  158  shows  two  rollers  rolling  the  wearing  surface  of  an 
asphalt  pavement  on  Fifth  Avenue,  New  York  City. 


Fio.  158. — ROLLING  TEE  WEAKING  COAT  OF  AN  ASPHALT  PAVEMENT. 

842.  In  spreading  and  raking  the  wearing  coat  there  is  a  ten- 
dency for  the  workmen  to  step  on  the  uncompressed  mixture.  If 
the  foot-print  is  filled  by  raking  material  into  it,  this  part  will  con- 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  441 

tain  more  material,  and  hence  when  rolled  it  will  not  be  brought  down 
even  with  the  adjoining  portion  and  a  hump  in  the  finished  pavement 
will  result.  The  bump  of  passing  wheels  against  this  hump  and  the 
impact  due  to  the  drop  of  the  wheel  after  having  passed  the  hump 
are  nearly  certain  to  cause  a  gradually  increasing  movement  of  the 
wearing  coat.  Therefore  stepping  on  the  uncompressed  wearing 
coat  should  be  absolutely  prohibited;  and  if  it  does  take  place,  the 
foot-print  should  be  thoroughly  obliterated  by  raking  or  at  least  the 
track  should  not  be  filled  with  loose  material. 

For  much  the  same  reasons  as  in  the  preceding  paragraph,  all 
compressed  lumps  of  the  wearing  coat  should  be  broken  up  with  the 
rake.  Lumps  in  the  uncompressed  material  make  humps  in  the 
finished  surface;  and  the  rebound  of  wheels  in  passing  such  humps 
causes  displacements  of  the  wearing  coat  and  starts  waves. 

843.  Thickness  of  Wearing  Coat.    The  wearing  coat  is  usually 
1J  or  2  inches  thick,  the  former  for  light  traffic  and  the  latter  for 
heavy.     It  has  been  established  that  if  the  wearing  coat  is  more  than 
2  inches  thick,  there  is  danger  of  its  flowing  under  travel,  i.  e.,  work- 
ing into  humps  and  waves. 

For  data  on  the  thickness  employed  in  various  cities,  see  Table  46, 
page  453. 

844.  The  area  that  should  be  covered  by  a  given  weight  of  mate- 
rial can  be  determined  in  either  of  the  two  methods  described  for 
laying  the  binder  course — see  §  824.     The  method  of  determining 
the  area  by  computation  is  more  appropriate  for  the  wearing  coat 
than  for  the  binder  course,  since  often  the  specific  gravity  of  the 
former  is  determined  in  fixing  proper  proportions  of  the  ingredients, 
and  also  since  the  specific  gravity  of  the  wearing  coat  is  not  likely  to 
vary  as  much  as  that  of  the  binder  course. 

If  the  thickness  of  the  wearing  coat  is  determined  by  probing, 
the  test  should  be  made  with  a  putty  knife  rather  than  an  awl,  as  in 
probing  the  binder  course,  so  that  the  knife  edge  will  be  arrested  by 
the  stones  of  the  binder  course. 

845.  Some  engineers  specify  that  when  completed  the  top  of 
the  wearing  coat  shall  be  J  inch  above  the  top  surface  of  the  gutter 
flag,  to  allow  for  further  compression  by  traffic  without  bringing  the 
surface  of  the  asphalt  below  the  top  of  the  gutter  flag.     This  is  of 
doubtful  wisdom,  since  it  constructs  a  shoulder  to  eliminate  the  pos- 
sibility of  one  being  formed  by  travel. 

846.  ASPHALT  ADJACENT  TO  TRACK.    It  is  difficult  to  lay  and 
maintain  sheet  asphalt  next  to  the  rails  of  a  street-car  track.     It  is 


442  ASPHALT   PAVEMENTS  [CHAP.    XVI 

well  known  that  many  more  failures  of  pavements  occur  on  streets 
having  car  tracks  than  on  those  without  tracks,  and  that  most  of 
these  failures  are  adjacent  and  parallel  to  the  rails.  Part  of  the  dif- 
ficulties is  due  to  the  foundation,  the  ties,  and  the  rails;  and  these 
have  already  been  considered  in  Art.  3  of  Chapter  XV — foundations 
of  Street-Railway  Tracks.  Part  of  the  difficulties  with  sheet  asphalt 
is  in  getting  a  good  union  between  the  asphalt  and  the  rail.  The 
hot  asphalt  should  be  compressed  thoroughly  under  and  around  the 
head  and  flange  of  the  rail;  and  a  good  union  can  not  be  obtained  if 
the  rail  is  cold,  since  the  asphalt  will  become  chilled,  and  then  can 
not  be  compressed  and  will  not  adhere  to  the  rail.  The  surface  of 
the  asphalt  should  be  laid  even  with  the  top  of  the  rail.  If  it  is  laid 
lower,  the  rail  will  be  an  obstruction  to  vehicular  travel,  and  vehicle 
wheels  will  follow  the  rail  and  make  a  rut  next  to  the  rail;  and  if 
the  surface  of  the  asphalt  is  laid  higher  than  the  top  of  the  rail, 
steel-tired  wheels  will  break  down  the  edge  of  the  pavement. 

Fig.  159  shows  the  method  of  laying  sheet  asphalt  adjacent 
to  railroad  rails  adopted  in  Hartford,  Conn.* 

Notice  in  Fig.  159  that  the  asphalt  is  in  contact  with  the  rail. 
It  is  troublesome  to  maintain  the  connection  between  the  rail  and 
the  asphalt  because  the  deflection  of  the  rail  will  break  the  bond 


-tt 


r/4^*w&^/r':£?**^W'^ 

FIG.  159.  —  STANDARD  PRACTICE  IN  HARTFORD,  CONN. 

and  permit  water  to  penetrate  to  the  open  binder  where  it  freezes 
and  lifts  the  wearing  coat,  and  this  allows  the  process  to  be  repeated 
upon  a  larger  scale.  The  only  preventive  is  to  apply  a  thick 
coat  of  soft  or  rather  elastic  asphaltic  cement  to  the  sides  of  the  rail 
before  laying  either  the  binder  course  or  the  wearing  coat;  and  after 
the  pavement  is  in  service,  the  only  remedy  is  to  fill  the  crack  adja- 
cent to  the  rail  frequently  during  freezing  weather. 

Notice  that  Fig.  159  is  for  a  grooved  rail;  but  the  same  form  of 
construction  would  apply  equally  well  with  a  T  rail. 

*  Engineering  News,  Vol.  73  (1915),  p.  888. 


ART.    1]  SHEET  ASPHALT  PAVEMENTS  443 

The  Baltimore  (Md.)  Pavement  Commission  has  recently 
adopted  the  method  of  laying  sheet  asphalt  pavements  adjacent 
to  street-railway  tracks  shown  in  Fig.  160.*  Notice  that  a  vitri- 
fied paving  block  intervenes  between  the  asphalt  and  the  rail. 
The  advantage  of  this  construction  is  that  the  vitrified  blocks  can 
be  put  into  place  before  the  laying  of  the  asphalt  is  begun.  The 
extreme  form  of  the  construction  shown  in  Fig.  160  is  that  in  which 
the  whole  area  between  the  ends  of  the  ties  is  paved  with  vitrified 
blocks  —  see  Fig.  196,  page  540,  which  also  is  a  standard  in  Balti- 
more. 


3"Rail 


Fia.  160.— STANDARD  PRACTICE  IN  BALTIMORE,  MD. 

847.  CAUSES   OF   FAILURE.     The    construction   of    an   asphalt 
pavement  involves  greater  care  in  selecting  and  combining  the  in- 
gredients than  most  other  kinds  of  pavements.     Most  other  forms 
of  pavements  are  constructed  of  a  natural  or  artificial  surfacing 
material  which  is  prepared  and  inspected  (at  least  in  part)  before 
being  brought  on  to  the  street  and  which  needs  only  to  be  laid,  while 
the  important  parts  of  a  sheet  asphalt  pavement  must  be  fabricated 
in  place  on  the  street;    and  hence  greater  care  is  required  in  the 
laying. 

Unfortunately  the  custom  has  been  to  contract  with  asphalt 
paving  companies  to  lay  asphalt  pavements  and  to  guarantee  them 
for  a  term  of  years,  and  consequently  the  municipalities  have  as  a 
rule  made  little  or  no  investigation  of  the  materials  used  nor  of  the 
methods  employed  in  laying  the  pavement.  The  result  is  that  there 
are  but  few,  if  any,  public  records  showing  the  history  of  the  pave- 
ment; and  therefore  it  is  often  impossible  to  determine  the  cause 
of  either  failure  or  success.  The  causes  of  failure,  exclusive  of  those 
due  to  faulty  foundation  and  street-railway  track,  may  be  grouped 
under  the  following  heads.  Unsuitable  material,  improper  manip- 
ulation, and  deterioration  in  use. 

848.  Unsuitable   Material.    The   sand   should   be   clean,   hard, 
and  properly  graded;  the  filler  should  be  hard  and  properly  graded; 

*  Engineering  News,  Vol.  73  (1915),  p.  884. 


444  ASPHALT   PAVEMENTS  [CHAP.   XVI 

the  asphalt  cement  should  meet  the  usual  specifications,  particularly 
as  to  consistency  or  penetration.  Each  of  these  items  has  already 
been  discussed;  and  the  best  means  of  preventing  failure  under 
this  head  is  to  observe  the  standard  specifications. 

849.  Improper  Manipulation.    Even  though  the  materials  may 
be  the  best,  there  is  an  abundant  opportunity  for  failure  through 
improper  manipulation  in  heating  and  mixing  the  materials. 

850.  Burned  Asphalt.     The  asphalt  may  have  been  damaged  by 
over-heating  or  "  burning."     The  burning  of  the  asphalt  causes  the 
surface  of  the  pavement  to  disintegrate  in  spots  during  cold  weather; 
and  may  be  revealed  by  a  brittleness  and  a  tendency  to  crack  while 
being  rolled.     Excessive  heat  converts  the  petroline,   or  cementi- 
tious   constituent  of  asphalt,   into  asphaltine,  which  is  devoid  of 
cementing  properties,  and  by  so  much  reduces  the  cementing  quality 
— the  vital  element — of  the  asphalt.     This  over-heating  may  take 
place  during  the  refining  (§  492),  or  during  the  fluxing  (§  530),  or 
in  mixing  the  asphaltic  cement  and  the  sand  (§  838). 

Sometimes  the  kettle  is  mounted  within  brick  walls  directly 
over  a  fire  which  comes  in  contact  with  only  a  comparatively  small 
part  of  the  heating  surface,  in  which  case  it  is  highly  improbable  that 
the  firing  will  be  done  so  evenly  and  slowly  as  not  to  burn  at  least 
part  of  the  asphalt.  The  fire  should  not  be  allowed  to  come  in 
direct  contact  with  the  melting  kettle  or  tank,  thereby  guaranteeing 
that  no  portion  of  the  asphalt  can  be  burned.  When  the  asphalt  has 
been  badly  burned,  it  will  be  revealed  by  a  brittleness  during  rolling; 
but  there  is  no  way  of  determining  a  lesser  degree  of  burning,  although 
it  still  may  be  sufficient  to  cause  a  serious  defect  which  will  finally 
develop  into  cracks  and  rotten  spots.  Therefore  the  inspector 
should  insist  upon  a  method  of  melting  that  will  insure  an  unburned 
product.  It  is  usually  specified  that  the  asphalt  shall  be  heated 
by  steam. 

The  over-heating  of  the  asphalt  may  be  produced  also  by  over- 
heating the  sand  (§  838).  Every  precaution  should  be  used  to 
have  each  batch  of  sand  heated  uniformly  throughout,  and  its 
temperature  should  be  taken  before  mixing  it  with  the  asphalt. 
As  a  further  check,  the  temperature  of  each  load  of  paving 
compound  sent  to  the  street  should  be  taken  and  recorded  at  the 
mixing  plant. 

851.  Improper  Consistency.    The  paving  cement  may  have  been 
mixed  too  hard  or  too  soft.     If  the  cement  is  too  hard,  the  pave- 
ment will  have,  a  tendency  to  crack  during  cold  weather;   and  if  it 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  445 

is  too  soft,  it  will  push  out  of  place  and  form  rolls  or  waves  under 
traffic. 

852.  Insufficient    Bitumen.    The   wearing   coat   may  not   have 
contained  sufficient  cementing  material  (§  830).     Within  me  limits 
imposed  by  the  proper  softness  and  haraness  of  the  pavement,  the 
greater  the  per  cent  of  asphalt  the  greater  the  life  of  the  pavement; 
and  consequently  contractors  in  laying  a  pavement  under  a  long- 
time guarantee  generally  use  the  maximum  amount  of  asphaltic 
cement,  but  when  the  maintenance  period  is  short  they  generally  use 
the  minimum.     In  fluxing,  the  tendency  is  for  the  bitumen  to  rise 
and  the  mineral  impurities  to  settle;  and  consequently  if  the  tank  is 
worked  too  low,  there  is  a  likelihood  that  the  last  material  taken 
from  the  tank  will  contain  too  small  a  proportion  of  bitumen  and 
too  large  a  proportion  of  sediment  or  mineral  matter.     This  can  be 
prevented  by  careful  inspection  and  by  frequently  taking  samples 
and  analyzing  them. 

853.  Inadequate  Mixing.    The  ingredients  of  the  wearing  coat 
may  not  have  been  sufficiently  mixed.     It  is  important  that  each 
grain  of  sand  shall  be  entirely  surrounded  by  the  cementing  mate- 
rial, so  that  no  two  pieces  shall  come  into  actual  contact.     If  the 
mixing  is  not  well  done,  the  pavement  will  disintegrate  in  spots. 

854.  Rich  Binder.     If  an  excess  of  asphalt  is  used  in  the  binder 
course,  it  is  likely  to  work  to  the  surface  of  that  course  and  then 
being  absorbed  by  the  wearing  coat  cause  it  to  disintegrate.     This 
cause  of  failure  manifests  itself  by  irregular  blotches  on  the  surface 
of  the  pavement. 

855.  Cement  Chilled.    The  mixture  for  the  wearing  coat  may 
become  chilled  while  being  transported  from  the  mixing  plant  to  the 
street.     To  prevent  this  possibility,  the  temperature  of  each  load 
should  be  taken  just  before  it  is  laid.     The  material  may  also  become 
chilled  by  a  delay  in  tamping  and  rolling,  or  by  attempting  to  work 
during  too  cold  weather  or  during  the  prevalence  of  a  high  wind. 
A  batch  of  chilled  mixture  will  cause  a  weak  spot  in  the  pavement. 

856.  Separation  of  Cement  and  Sand.     If  the  distance  from  the 
plant  to  the  street  is  long  or  there  is  unusual  delay,  some  of  the 
asphaltic  cement  may  work  down  to  the  bottom  of  the  load,  and 
when  the  material  is  dumped  there  will  be  both  rich  and  lean  spots 
— both  of  which  are  equally  objectionable.     The  rich  spots  will 
have  a  tendency  to  roll  or  crowd  toward  the  gutter;   and  the  lean 
spots  will  have  a  tendency  to  disintegrate  under  traffic. 

857.  Damp  or  Dirty  Foundation.    The  wearing  coat  may  have 


446  ASPHALT   PAVEMENTS  [CHAP.    XVI 

been  laid  on  a  dirty  or  damp  foundation,  and  therefore  have  been 
prevented  from  uniting  firmly  with  the  foundation.  This  con- 
dition will  be  revealed  by  a  tendency  of  the  pavement  to  roll  or  push 
out  of  place  while  sound  and  firm  on  the  surface. 

858.  Inadequate  Compression.     The  wearing  coat  may  not  have 
received  sufficient  compression.     The  surface  must  be  thoroughly 
compacted — particularly  in  the  gutters — to  keep   out  rain   water 
and  the  acids  and  oxygen  dissolved  in  it.     The  effect  of  oxidation 
is  gradually  to  destroy  the  cementing  power  of  the  bitumen. 

859.  Deterioration  in  Service.    All  materials  in  nature  are  under- 
going changes  due  to  the  action  of  the  elements,  and  asphalt  pave- 
ments are  no  exception.     The  following  are  some  of  the  principal 
causes  leading  to  the  gradual  deterioration  of  such  pavements. 

860.  Ordinary  Wear.     The  pavement  may  decrease  in  thickness 
due  to  loss  of  material  by  the  abrasion  of  hoofs  and  wheels;    but 
since  the  surface  is  smooth  and  somewhat  elastic,  the  loss  by  wear 
is  almost  imperceptible.     In  some  cases  the  pavement  decreases  in 
thickness  with  use,  but  the  decrease  is  due  to  consolidation  rather 
than  to  loss  of  material. 

861.  Natural  Decay.    All  asphalts  gradually  lose  their  cement- 
ing power  with  age  by  volatilization,  evaporation,  and  oxidation. 
The  pavement  is  peculiarly  exposed  to  the  action  of  the  sun's  heat, 
and  to  the  combined  action  of  rain  water,  acids,  oxygen,  and  frost. 
The  greater  the  cementing  power  of  the  asphalt  originally  and  the 
softer  the  cement,  the  longer  the  pavement  will  resist  the  influence 
of  volatilization  and  evaporation;  and  the  more  nearly  the  voids  of 
the  sand  are  filled  with  cement  and  the  more  firmly  the  pavement 
is  consolidated,  the  longer  it  will  resist  the  action  of  water,  acids, 
oxygen,  and  frost.     The  general  decay  of  the  asphalt  will  be  indi- 
cated by  a  tendency  of  cracks  to  form  during  cold  weather  (§  866), 
particularly  during  a  sudden  and  extreme  drop  in  the  temperature. 

862.  Weak  Foundation.     A  weak  or  improperly  prepared  founda- 
tion by  unequal  settlement  or  settlement  in  spots  will  cause  cracks 
and  depressions  in  the  surface  which  under  traffic  will  speedily  enlarge 
and  cause  the  pavement  soon  to  break  up. 

863.  Porous    Foundation.    A    porous    foundation    permits    the 
ground  water  to  rise,  by  capillary  action  and  possibly  also  by  hydro- 
static pressure,  to  the  underside  of  the  wearing  coat,  where  by 
freezing  it  may  break  the  bond  between  the  top  layer  and  the  base, 
and  thus  permit  the  wearing  coat  to  be  pushed  out  of  place  and 
broken.     This  effect  has  been  known  to  occur  with  a  concrete  foun- 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  447 

dation ;  but  it  is  not  likely  to  occur  with  good  concrete.  If  a  section 
of  pavement  disintegrating  from  this  cause  be  examined,  there  will 
be  found  a  layer  of  perfectly  sound  and  good  material  at  the  surface, 
while  the  lower  side  of  the  wearing  coat  will  show  evidence  of  being 
disintegrated  by  water — that  is,  the  sand  will  appear  clean  and 
white  in  spots  as  though  there  had  been  insufficient  asphalt  cement 
to  cover  it.  The  concrete  base  under  the  affected  spot  will  generally 
be  found  to  be  damp  or  even  wet.  This  defect  may  be  prevented 
by  underdraining  the  soil. 

864.  Leaky  Joints.     Lack  of  a  water-tight  joint  between   the 
asphalt  surface  and  the  curb,  the  gutter,  man-hole  covers,  crossings, 
street-car  rails,  etc.,  may  permit  the  water  to  enter  the  lower  and 
less  compact  part  of  the  wearing  coat,  where  by  its  solvent  action 
and  also  by  freezing  it  may  do  material  damage.     It  is  nearly  impos- 
sible to  keep  these  joints  tight,  particularly  adjacent  to  the  street- 
car rails.     The  damage  often  extends  a  considerable  distance  from 
the  place  where  the  water  enters. 

865.  Illuminating    Gas.     Ordinary    illuminating    gas,    escaping 
from  leaky  pipes  under  the  pavement,  is  absorbed  by  the  pavement, 
and  causes  the  disintegration  of  the  asphalt.     There  is  but  one  way 
to  stop  the  disintegration  of  a  pavement  from  this  cause,  and  that  is 
to  stop  the  leak  of  gas. 

Pavements  affected  by  illuminating  gas  first  give  signs  of  their 
disintegration  by  a  slight  depression  over  the  affected  spot,  later 
fine  cracks  appear  parallel  to  the  line  of  the  street,  and  finally  the 
surface  coat  begins  to  crown. 

866.  Cracks.     Long    irregular    cracks    in    the    wearing    surface 
frequently  occur  during  cold  weather.     They  usually  start  at  the 
gutter  or  man-hole  frame,  and  gradually  extend  across  the  street. 
They  are  often  found  at  the  joint  between  an  old  and  a  new  pave- 
ment or  at  the  joint  made  between  one  day's  work  and  another. 
These  cracks  are  due  to  the  contraction  of  the  wearing  surface,  and 
should  not  be  confounded  with  cracks  due  to  the  failure  of  the 
foundation.     Usually  these  cracks  do  not  occur  until  the  pavement 
is  two  or  three  years  old;   at  least  they  are  most  likely  to  occur  in 
an  old  pavement — one  in  which  the  asphalt  has  lost  part  of  its 
cementing  power  by  age.     These  cracks  appear  sooner  and  increase 
more  rapidly  on  a  street  having  only  a  light  traffic.     When  the 
pavement  is  subjected  to  a  continuous  traffic,  the  asphalt  surface 
which  is  more  or  less  plastic  at  all  temperatures,  is  kept  from  crack- 
ing by  the  constant  kneading  action  of  the  traffic.     Again,  when  an 


448  ASPHALT    PAVEMENTS  [CHAP.    XVI 

asphalt  surface  has  but  little  or  no  traffic,  it  becomes  more  porous 
owing  to  expansion  and  contraction  from  heat  and  cold  without  the 
compression  due  to  traffic,  and  as  a  consequence  is  materially  weak- 
ened. If  cracks  occur  on  a  street  having  a  fair  amount  of  traffic, 
it  is  evident  that  the  paving  mixture  is  at  fault — either  there  was 
not  enough  bitumen  or  the  asphalt  cement  was  too  hard. 

Some  engineers  leave  expansion  joints,  i.  e.,  cut  the  wearing 
coat  through,  at  intervals  to  prevent  these  irregular  contraction 
cracks.  Such  a  procedure  is  of  doubtful  propriety,  since  the  pave- 
ment if  properly  constructed  will  not  crack  in  several  years  under 
the  most  adverse  conditions,  and  then  only  at  long  intervals  and 
generally  at  some  old  joint;  and  if  the  pavement  is  improperly 
made,  the  expansion  joint  will  have  only  a  slight  tendency  to  pre- 
vent these  irregular  cracks.  The  principle  of  the  expansion  joint  is 
not  applicable  to  materials  with  no  structural  strength,  like  asphalt 
mixtures.  These  joints  are  not  only  useless,  but  really  detrimental 
to  a  pavement.  They  are  only  another  form  of  the  defect  they  are 
intended  to  remedy,  for  they  are  crevices  which  retain  mud  and 
water  which  tend  to  rot  the  asphalt,  and  the  edges  of  the  joints  are 
easily  broken  down  by  traffic  which  also  widens  the  crack. 

867.  Shifting  under  Traffic.     The  surface  coat  sometimes  flows 
under  traffic,  i.  e.,  pushes  lengthwise  of  the  street  into  waves  or 
crowds  toward  the  gutter.     This  defect  occurs  in  pavements  having 
too  soft  a  wearing  surface,  or  where  there  is  a  defective  bond  either 
between  the  base  and  the  binder,  or  between  the  binder  and  the 
wearing  surface.     This  is  a  defect  that  is  impossible  to  guard  against 
entirely  on  a  street  having  very  heavy  traffic,  and  especially  where 
the  traffic  is  confined  to  a  narrow  section  of  the  street;    but  this 
defect  is  inexcusable  on  streets  having  only  moderately  heavy  traffic. 
This  flowing  is  commonly  caused  by  the  surface  of  the  hydraulic 
concrete  base  under  the  pavement  being  too  smooth,  which  is  the 
case  where  gravel  concrete  is  used  or  where  a  stone-and-gravel  con- 
crete is  so  rich  that  its  surface  is  covered  with  mortar  that  was 
brought  to  the  top  by  ramming.     Unless  the  binder  and  the  surface 
mixtures  are  made  very  hard,  a  condition  which  makes  the  pave- 
ment likely  to  crack,  the  wearing  coat  will  slide  on  such  a  base  if 
there  is  much  traffic.     Pavements  often  roll  from  a  defect  in  the 
binder — either  because  it  was  too  rich  in  asphaltic  cement,  or  because 
it  was  dirty  when  the  wearing  surface  was  laid. 

868.  Damage  by  Bonfires.     Another  cause  of  damage  to  asphalt 
pavements  is  the  building  of  fires  upon  them,     Of  course  this  ouglrc 


ABT.    1]  SHEET  ASPHALT   PAVEMENTS  449 

never  to  occur,  but  even  in  the  best  regulated  municipalities  it  does 
sometimes  happen. 

869.  METHODS    OF    REPAIRING.     The  repairs  necessitated  in 
the  maintenance  of  an  asphalt  pavement  may  be  classified  as  follows : 
(1)  those  due  to  a  settlement  of  the  subgrade;    (2)  those  due  to  a 
disintegration  of  the  pavement  in  spots;   (3)  those  due  to  the  forma- 
tion of  waves;    (4)  those  due  to  the  formation  of  cracks;    (5)  the 
painting  of  the  gutter;  and  (6)  the  remedying  of  defects  next  to  the 
street-car  rails,  crossing  stones,  man-hole  covers,  etc. 

870.  Settlement  of  Subgrade.     The  majority  of  repairs  are  neces- 
sitated by  the  settlement  of  the  foundation  over  trenches.     To  repair 
these  defects,  it  is  necessary  to  remove  the  wearing  coat,  the  binder, 
and  the  foundation;  and  then,  after  having  consolidated  the  material 
in  the  trench  (see  §  764),  to  re-lay  the  pavement  much  as  in  the  original 
construction.     The  edges  of  the  binder  course  and  also  of  the  wearing 
coat  should  be  thoroughly  covered  with  a  thin  coat  of  asphaltic 
cement  to  secure  a  perfect  union  of  the  old  and  the  new  material. 
Both  the  binder  course  and  the  wearing  coat  should  be  thoroughly 
tamped  or  rolled.     Owing  to  the  difficulty  of  fully  consolidating  the 
patch,  it  is  left  a  trifle  high  to  prevent  a  possible  depression. 

871.  Disintegration.     If  the  wearing  coat  disintegrates  in  spots, 
or  forms  "  macaroons/7  from  any  of  the  causes  described  in  §  848-65, 
the  affected  part  must  generally  be  cut  out,  since  it  is  usually  affected 
to  its  full  depth.     If  the  binder  course  is  the  cause  of  the  deteriora- 
tion (see  §  812),  it  also  must  be  cut  out.     The  new  material  is  to  be 
laid  as  described  in  the  preceding  paragraph.     If  the  disintegration 
does  not  extend  to  the  full  depth  of  the  wearing  coat,  the  repair  may 
be  made  by  "  skimming,"  as  described  in  the  succeeding  paragraph. 

872.  Formation  of  Waves  or  Humps.    If  the  wearing  coat  has 
shifted  under  the  traffic  so  as  to  form  waves,  i.  e.,  until  it  is  thicker 
in  some  parts  than  others,  or  if  the  wearing  coat  has  crowded  towards 
the  gutter,  it  may  be  necessary  to  melt  off  a  portion  of  the  high  part, 
and  also  to  re-surface  the  thin  part.     This  is  called  skimming.     The 
asphalt  is  melted  off  either  with  an  open  grate  on  low  wheels  in  which 
coke  is  burned;  or  with  a  special  heater  having  a  tank  for  gasoline, 
a  hood  over  the  burner,  and  an  asbestos  mat  to  protect  the  adjacent 
pavement.     Fig.  161  shows  one  of  several  forms  of  surface  heaters 
in  common  use.     The  surface  is  heated  until  the  affected  portion 
can  be  raked  off;  and  then  new  material  is  added  to  bring  the  pave- 
ment to  its  proper  thickness. 

"  Whenever  the  surface-heater  or  skimming  method  is  employed, 


450 


ASPHALT  PAVEMENTS 


[CHAP,  xvi 


all  defective  surface  shall  be  removed  before  replacing  it  with  new 
material.  In  all  cases  the  old  surface  shall  be  removed  to  a  depth 
of  not  less  than  one  quarter  inch;  and  the  new  surface  must,  when 
compressed,  be  not  less  than  one  half  inch  in  thickness.  The  heat 
shall  be  applied  in  such  a  manner  as  not  to  injure  the  remaining 


FIG.  161. — SURFACE  HEATER  FOR  REPAIRING  ASPHALT  PAVEMENTS. 

pavement.  All  burnt  and  loose  material  shall  at  once  be  com- 
pletely removed;  and,  while  the  remaining  portion  of  the  old  pave- 
ment is  still  warm,  the  new  material  shall  be  placed.  The  new 
and  freshly  prepared  wearing  coat  shall  be  laid  in  strict  accordance 
with  the  specifications  for  the  original  pavement."  * 

873.  Cracks.     When  cracks  have  formed  in  the  wearing  coat, 
all  the  loose  material  is  cut  off,  the  crack  is  cleaned  out,  and  hot 
asphaltic  cement  is  poured  in. 

874.  Painting  Gutters.     Owing  to  the  disintegrating  effect  of 
water,    asphalt    gutters    usually    require    comparatively    frequent 
repairs  either  by  painting  with  asphalt  rich  in  bitumen,  or  by  skim- 
ming (§  872),  or  by  removing  the  wearing  coat  and  re-laying  it, 
using  an  asphalt  richer  in  bitumen  than  that  in  the  remainder  of 
the  pavement. 

875.  Recording  Repairs.     The  present  practice  is  to  make  the 
repairs  to  asphalt  pavements  by  contract  with  a  guarantee  of  the 
work  for  a  number  of  years;   therefore  it  is  important  that  a  record 
should  be  kept  of  the  area  and  location  of  the  several  patches  and 
also  of  the  date  when  each  was  made.     This  is  done  by  dividing 
the  pavement  into  imaginary  squares,  say,  10  feet  on  a  side;    and 

*  Specifications  of  Amer.   Soc.  jol  Municipal    Improvements   for    Sheet    Asphalt   Paving, 
approved  Oct.  14,  1915,  p.  12. 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  451 

then  when  a  patch  is  to  be  made,  one  or  more  of  these  squares  should 
be  located  by  chalk  marks  on  the  pavement,  and  the  boundary 
of  the  patch  should  be  sketched  in  a  cross-ruled  note-book.  The 
records  of  the  individual  patches  are  afterwards  platted  upon  a 
single  sheet  to  see  that  a  subsequent  patch  does  not  overlap  one  for 
which  the  guarantee  has  not  expired. 

876.  Using  Old  Materials.     In  some  cities  it  is  customary  to 
permit  the  re-use  of  the  old  asphalt;  but  this  is  of  doubtful  widsom, 
since  usually  the  repair  is  required  by  the  inferiority  of  the  old 
material,  and  since  it  is  likely  to  be  over-heated  in  being  removed. 
If  the  asphalt  is  not  damaged,  and  is  cut  out  with  an  axe,  it  may  be 
used  again,  provided  (1)  the  pieces  are  kept  clean,  (2)  it  is  re-heated 
slowly  and  carefully,  and  (3)  new  asphalt  is  added  to  flux  the  old. 
It  is  difficult  to  melt  old  material  without  burning  it,  and  it  is  also 
difficult  to  secure  a  uniform  mixture  with  it. 

877.  COST  OF  CONSTRUCTION  OF  SHEET  ASPHALT  PAVEMENTS. 
Asphalt   pavements  are   comparatively  expensive,   since  the  tools 
and  machinery  employed  in  mixing  and  laying  the  asphalt  are 
costly  and  subject  to  large  depreciation  whether  idle  or  in  use, 
and  also  since  the  business  requires  a  considerable  proportion  of 
skilled  labor.     One  of  the  peculiarities  of  the  business  is  the  dis- 
proportionate amount  of  capital  invested  in  the  plant  compared 
with  the  business  done,  often  an  expensive  plant  being  maintained 
in  a  city  for  one  or  more  years  without  laying  any  pavement  or  at 
most  only  a  small  amount.     Or  a  portable  plant  is  moved  to  a  small 
city   for   a   comparatively   small   amount   of   pavement.     Another 
peculiarity  is  that  the  working  season  is  short,  extending  only  from, 
say,  the  first  of  May  to  the  first  of  November;  and  as  expert  superin- 
tendents and  foremen  are  indispensable,  it  is  necessary  to  employ 
this  skilled  labor  by  the  year. 

878.  In  connection  with  data  on  the  cost  of  construction  of  a 
pavement,  it  should  not  be  overlooked  that  the  cost  of  the  pavement 
proper  is  not  usually  the  total  cost  which  the  property  holder  must 
pay  for  the  improvement  of  the  street  when  it  is  paved.     Usually 
the  improvement  of  the  street  includes  four  items,  viz. :   (1)  excava- 
tion for  the  pavement,  (2)  the  construction  of  curbs  and  gutters, 
(3)  laying  drains  and  building  catch  basins,  man-holes,  etc.,  and  (4) 
the  pavement  itself.     1.  Under  ordinary  conditions  the  excavation, 
exclusive  of  surfacing  and  rolling  the  subgrade,  will  cost  10  to  15 
cents  per  square  yard.     2.  Combined  concrete  curbs  and  gutters 
(§  737),  will  usually  cost  30  to  35  cents  per  square  yard  of  pavement 


452  ASPHALT   PAVEMENTS  [CHAP.  XVI 

3.  The  drainage  will  usually  cost  10  to  15  cents  per  square  yard 
of  pavement.  These  three  items  may  add  50  to  60  cents  per  square 
yard  to  the  cost  of  the  pavement  proper. 

879.  Estimated  Cost.    The  estimate  of  the  cost  of  laying  an 
asphalt  pavement  shown  in  Table  45  was  prepared  for  this  volume 
by  a  man  of  acknowledged  ability  and  unquestioned  integrity,  who 
has  had  20  to  25  years'  experience  as  an  inspecting  and  consulting 
engineer  of  asphalt  paving  in  various  cities.*    The  estimate  is  for  a 
city  in  which  100,000  square  yards  are  laid  in  one  year. 

The  table  is  chiefly  interesting  as  showing  the  items  that  go  to 
make  up  the  expenses  which  are  separate  and  distinct  from  that  for 
materials  and  labor.  Of  course,  these  expenses  would  be  less  or 
more  per  square  yard,  if  the  area  of  pavements  laid  was  greater  or 
less  than  the  amount  stated.  The  estimate  is  for  first-class  work 
under  average  conditions  prevailing  in  1916. 

880.  Actual  Cost.    Table  46,  page  454,  shows  the  contract  price 
for  constructing  sheet  asphalt  pavements  in  thirty-three  cities. 

881.  COST  OF  MAINTENANCE.     The  cost  of  maintenance  will 
vary  with  the  original  quality  of  the  pavement,  its  age,  the  amount 
and  nature  of  the  traffic,  the  width  of  the  street,  the  presence  or 
absence  of  street-car  tracks,  the  frequency  with  which  the  pavement 
is  cleaned  and  sprinkled,  the  climate,  etc. 

In  nearly  all  American  cities  there  is  a  serious  lack  of  data  con- 
cerning the  cost  of  maintaining  pavements;  and  this  lack  is  more 
serious  for  sheet  asphalt  pavements  than  other  forms,  since  this  type 
involves  more  variables.  A  few  cities  attempt  to  keep  a  record  of 
the  cost  of  maintaining  pavements;  but  such  records  are  often  so 
incomplete  and  so  incorrectly  compiled  as  to  be  valueless.  Some  of 
the  reasons  for  the  dearth  and  incompleteness  of  the  data  on  the 
cost  of  maintaining  sheet  asphalt  pavements  are  as  follows: 

1.  The  pavement  is  usually  built  under  a  long-time  guarantee, 
and  the  city  pays  comparatively  little  attention  to  the  quality  of 
the  materials  used  and  the  methods  of  construction  employed; 
consequently  there  is  no  satisfactory  record  of  the  quality  of  the 
pavement.  In  some  cases  the  date  when  the  pavement  was  laid  or 
re-surfaced  is  unknown ;  and  in  many  cities  no  adequate  records  are 
kept  of  the  location  of  repairs  or  patches.  In  recent  years  cities 
are  improving  in  this  respect;  but  the  usual  absence  of  such  records 

*A.  W.  Dow,  for  a  number  of  years  Inspector  of  Asphalt  and  Asphalt  Paving  for  the  Dis- 
trict of  Columbia,  and  for  several  years  past  a  consulting  asphalt  chemist  and  asphalt  paving 
engineer  in  New  York  City. 


ART.    1]                                 SHEET   ASPHALT   PAVEMENTS  453 

TABLE  45 

ESTIMATED  COST  OP  SHEET  ASPHALT  PAVEMENT 
Plant  and  Capital  Charges: 

Interest  on  cost  of  fixed  plant, — 5%  on  $13,500 $675.00 

Interest  on  cost  of  rollers,  tools,  etc., —5%  of  $3,000 150 . 00 

Taxes,— 1%  of  $10,000 100.00 

Insurance,^%  of  $10,000 400.00 

Depreciation,— 8%  of  $16,500 1,320.00 

Rental  or  interest  on  real  estate, — 5%  of  $4,000 200.00 

Interest  for  6  months  on  working  capital, — 5%  of  $6,000.  .  .  150.00 

Current  repairs 500 . 00 

Watchman  for  1  year 400 . 00 

Total  for  100,000  square  yards $3,895.00 

Total  for  1  square  yard .039 

Local  Management  and  Clerical  Expenses: 

Rent  of  office  1  year $400.00 

Telephone,  light,  water,  etc 100 . 00 

Salary  of  superintendent, — 1  year 2,000 . 00 

Cashier  in  charge  of  office, — 1  year 1,200 . 00 

Clerks,  timekeepers,  etc., — 6  months 700.00 

Proportionate  part  of  winter  pay  roll 750 . 00 

Total  for  100,000  square  yards $5,150. 00 

Total  for  1  square  yard .051 

General  Officers  and  Offices: 

Laboratory  and  general  expenses .  030 

Expense  Securing  Contracts: 

Agent's  commission,  legal  and  traveling  expenses,  etc .  050 

Material  and  Labor  per  Square  Yard: 

Subgrade, — 0.25  cubic  yard,  grading,  rolling,  etc 0. 125 

Foundation,— 6  inches  of  concrete  (1  P.  C.:  3  S.;  6  B.  S.)..  .700 

Binder — 1  inch  complete .  170 

Wearing  surface — 2  inches: 

22  Ib.  of  asphalt  cement  at  $20  per  ton .220 

0.083  cubic  yard  sand  at  $1.20 .100 

16  Ib.  pulverized  limestone  at  $3.50  per  ton .  030 

Fuel  used  at  plant 020 

Oil,  waste  and  sundries .  002 

Labor  at  plant .  060 

Hauling  material  to  street .  030 

Laying  and  rolling .  050 

Total  for  materials  and  labor $1 . 507 

Cost  of  Guaranty: 

5  years  at  2|  cents  per  year 1 . 00 

Total  cost  of  pavement,  per  square  yard $1 . 777 


454 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


TABLE  46 

COST  OF  CONSTRUCTION  OF  SHEET  ASPHALT  PAVEMENTS  * 
Laid  in  1916 


Ref. 

No. 

1 

2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 

LOCALITY. 

Amount 
Laid' 
Sq.  Yd. 

CONCRETE 
BASE. 

BINDER 
COURSE. 

WEARING 
COAT. 

Cost  of  Base,  Binder 
and  Wearing  Coat, 
per  Sq.  Yd. 

State. 

City. 

Thickness. 

Proportions. 

Thickness. 

+3    « 

II 

r 

Thickness. 

Per  Cent 
Bitumen. 

California  

Connecticut.  .  . 

Dist.  of  Col  .  ".  '.  . 
Illinois  

Indiana  

Long  Beach.  .  . 
San  Diego.  .  .  . 
Santa  Monica. 
Hartford  
New  Haven..  . 
Washington.  .  . 
Chicago  

11  972 
12043 
34910 
32  195 
92742 
154  076 
1  248  000 
36059 
45  112 
24046 
18  702 
107  506 
131  489 
11  000 
113  200 
47  747 
9257 
12408 
21300 
23  130 
25000 
7407 
8000 
2351 
13  790 
43  357 
25000 
74  134 
66727 
10585 
146  173 
6450 
20851 

4' 
5' 
5' 
6' 
6' 
6 
6 
5 
6 
6 
6 
5 
5 
5 
4 
6 
4-6 
6 
6 
5 
6 
6 
4 
6 
6 
6 
6 
6 
5 
5 
5 
5 
5 

1     3     6 
135 
1     3     6 
2     3 
3     6 
3     7 
3     6 
3     6 
3     6 
3     5 
7G 
5 
3     6 
2i  :4 
2     4 
3     6 
3     6 
3     6 
3     6 
3     6 
3     6 
3     6 
3     6 
3     6 
3     6 
3J  :6 
3     6 
3     6 
2*  :5 
3     6 
3     6 
2*  :6 
1:3     5 

1" 
1" 
1" 
1* 
li 

li 

11 

li 
1 

T 

i* 

1 
fr 
9 

li 

li 

-jr 

l 

5^-8' 

4-6 
5.5 
6 

"e" 
"s" 

5-8 
5-8 

;' 

/ 

! 

)  • 

\ 

1 
\ 

\ 

\    . 

;.';;: 

$1.56 
1.50 
1.22 
1.58 
1.84 
1.66 
1.89 
1.81 
1.70 
1.56 
1.81 
1.66 
1.68 
2.10 
1.26 
1.74 
1.37 
2.04 
2.14 
1.90 
1.60 
1.68 
1.73 
2.25 
1.99 
1.99 
1.98 
2.15 
1.90 
1.80 
1.40 
1.86 
1.99 

10-12 

"ii 

10.5 

11 

Moline  . 

Ft.  Wayne  
Goshen  
South  Bend.  .  . 
Vincennes.  .  .  . 
Mason  City.  .  . 
Sioux  Citv.  .  .  . 

Iowa  

Kansas  
Kentucky  

Newton  
Louisville  
Baltimore.  .  .  . 
Detroit  

.  .^.  . 

16^13 
9-is 

Michigan  
Minnesota  
Nebraska  
New  Jersey  .... 
New  York 

Duluth  

Omaha  
Elizabeth  
Syracuse  
Durham  
Cincinnati.  .  .  . 
Lakewood  .  .  . 
Toledo  
Xenia 

North  Carolina. 
Ohio  

Pennsylvania.  . 
Texas  
Utah  
Washington.  .  .  . 
West  Virginia  .  . 
Wisconsin  

Pittsburg  
Beaumont.  .  . 
Ogden  
Seattle  
Charleston.  .  .  . 
Racine  

in  the  past  makes  it  impossible  to  make  comparisons  extending  over 
any  considerable  length  of  time. 

2.  There  are  almost  no  data  as  to  the  amount,  nature  and  dis- 
tribution of  the  travel  on  city  pavements  (§  34);   and  without  such 
data  it  is  impossible  to  determine  the  service  obtained  from  a  pave- 
ment or  to  make  any  accurate  comparisons  as  to  the  cost  of  its 
maintenance. 

3.  Many  of  the  records  that  have  been  kept  fail  to  discriminate 
between  pavements  on  streets  with  and  without  street-car  tracks; 
and  even  if  they  do  state  the  presence  of  a  street-car  track,  they 
often  fail  to  state  the  method  of  computing  the  area  maintained. 

4.  Some  cities  include  in  the  cost  of  maintenance  the  expense  of 

*Municipal  Engineering,  Vol.  52  (1917),  p.  63-65. 


ART.    1]  SHEET   ASPHALT   PAVEMENTS  455 

repairing  cuts  made  by  plumbers,  gas-fitters,  electricians,  etc.,  which 
has  nothing  to  do  with  the  cost  of  maintenance  proper,  i.  e.,  with  the 
durability  or  wear  of  the  pavement. 

5.  In  some  cities  the  repairs  are  made  by  contract,  and  in  some 
by  the  city's  force.     In  some  of  the  former  cases,  the  amount  of 
repairs  is  so  small  and  other  conditions  are  such  as  to  eliminate  com- 
petitive bidding;    and  hence  the  cost  is  abnormally  high  and  con- 
sequently valueless.     Sometimes  when  the  repairs  are  made  by  the 
city's  force,  the  record  does  not  include  all  the  elements  of  the  cost. 
For  example,  the  following  items  of  cost  are  often  omitted:    (a) 
Interest  on  the  cost  of  the  plant  and  equipment  employed  in  making 
the  repairs;    (6)  interest  on  working  capital;   (c)  a  charge  for  depre- 
ciation of  plant  and  equipment;    (d)  a  charge  for  supervision  and 
office  expense;  and  (e)  material  and  labor  supplied  by  other  municipal 
departments. 

6.  Almost  no  records  of  the  cost  of  maintenance  state  anything 
as  to  the  condition  of  the  pavement  at  the  beginning  or  the  end  of 
the   period   considered.     It   is   probably   impossible   accurately   to 
make  such  an  inventory;  but  the  bearing  of  such  an  inventory  upon 
the  results  should  at  least  be  considered.     The  failure  to  consider 
this  phase  of  the  subject  is  the  same  as  though  a  merchant  should 
attempt  to  compute  his  annual  profits  without  an  inventory  at  the 
beginning  and  the  end  of  his  fiscal  year.     See  §1232. 

882.  Obviously  it  is  practically  impossible  in  any  city  to  segre- 
gate the  cost  of  maintenance  according  to  the  quality  and  the  age 
of  the  pavement,  the  amount  of  travel,  and  the  presence  or  absence 
of  car  tracks;    but  a  considerable  improvement  upon  the  present 
practice  of  most  cities  is  entirely  feasible  and  very  desirable. 

Owing  to  the  dearth  of  accurate  and  definite  data,  it  will  not  be 
possible  to  give  much  reliable  or  valuable  information  on  the  cost  of 
maintenance  of  sheet  asphalt  pavements.  Further,  since  few 
contractors  are  equipped  especially  for  making  repairs,  and  since  the 
public  has  no  knowledge  of  the  cost  of  the  work  to  the  contractor, 
the  only  data  submitted  will  be  those  obtained  with  municipal  repair 
plants. 

In  this  connection  it  is  not  wise  to  consider  pavements  laid  before 
about  1895,  since  before  that  time  the  importance  of  a  proper 
gradation  of  the  mineral  aggregate  was  not  understood,  and  con- 
sequently such  pavements  are  likely  to  be  much  inferior  to  the  bast 
pavements  laid  later. 

883.  Municipal  Repair  Plant.     A  sheet  asphalt  pavement  when 


456  ASPHALT   PAVEMENTS  [CHAP.    XVI 

constructed  in  accordance  with  good  practice  is  a  reasonably  satis- 
factory and  economical  form  of  pavement;  but  it  usually  requires 
repairs  at  an  earlier  period  in  its  life  than  most  other  pavements, 
and  the  total  cost  of  maintenance  during  its  useful  life  is  also  some- 
what greater.  It  is  a  high-grade  pavement,  and  therefore  should 
have  more  careful  and  skilful  attention  than  most  other  pavements. 
For  these  reasons  the  repair  of  a  sheet-asphalt  pavement  when 
needed  is  a  vitally  important  matter.  In  the  early  history  of  asphalt 
pavements  the  repairs  were  made  by  the  contractor  whose  chief 
business  was  to  build  new  pavements;  but  in  recent  years  many 
cities  have  established  municipal  asphalt  repair  plants.  The  main 
conditions  leading  to  this  change  in  practice  were  as  follows : 

1.  Usually  there  was  little  or  no  competition  among  contractors 
for  the  contract  to  repair  asphalt  pavements.     The  equipment  and 
skill  required  in  laying  sheet  asphalt  pavements  is  so  great  as  usually 
to  limit  the  number  of  contractors  doing  this  kind  of  work;    and 
while  competition  with  other  forms  of  pavements  is  likely  to  keep 
the  first  cost  of  asphalt  pavements  within  reasonable  limits,  there 
are  so  few  asphalt-paving  contractors  that  usually  there  is  no  com- 
petition for  the  maintenance  of  such  pavements.     With  a  municipally 
owned  repair  plant  the  city  virtually  becomes  a  competitor  of  the 
ordinary  contractor. 

2.  The  plant  and  the  other  equipment  for  pavement  construction 
is  not  suitable  for  pavement  repairs.     The  construction  plant  and 
equipment  is  designed  for  turning  out  large  quantities  of  material, 
while  a  repair  plant  should  be  designed  for  turning  out  small  quan- 
tities. 

3.  With  a  municipal  repair  plant  old  material  may  be  used  in 
repairing  pavements  that  have  about  reached  the  end  of  their  eco- 
nomical life,  and  thus  utilize  the  old  material  without  injuring  the 
pavement. 

4.  Since  the  repair  plant  is  small,  repairs  may  be  made  con- 
tinually and  when  needed,  instead  of  being  allowed  to  accumulate 
as  usual  under  the  contract  method  until  enough  repairs  are  called 
for  to  warrant  either  starting  up  a  large  plant  or  diverting  the  plant 
from  new  construction  to  repair  work. 

884.  The  saving  through  the  municipal  repair  plant  has  usually 
been  quite  considerable.  Table  47  shows  the  results  for  Washing- 
ton, D.  C.,  and  is  fairly  representative  of  the  results  obtained  in 
other  cities.  For  a  more  detailed  comparison  for  the  experience 
of  Brooklyn,  N.  Y.,  showing  a  more  marked  saving,  see  Engineering 


ART.    1] 


SHEET  ASPHALT   PAVEMENTS 


457 


and  Contracting,  Vol.  38  (1912),  page  68;  and  for  somewhat  similar 
data  for  Niagara  Falls,  N.  Y.,  see  Engineering  Record,  Vol.  69  (1914), 
page  256. 

TABLE  47 

COST  OF  REPAIRS  OF  SHEET  ASPHALT  PAVEMENTS  IN  WASHINGTON,  D.  C.* 
Including  coal-tar  surface,  and  excluding  all  pavements  under  guaranty. 


REPAIRED  BY  CONTRACT. 

REPAIRED  By  MUNICIPAL 
PLANT. 

Year. 

Cents  per 
Sq.  Yd. 

Year. 

Cents  per 
Sq.  Yd. 

1908 

3.8 

1913 

2.0 

1909 

2.3 

1914 

1.9 

1910 

2.6 

1915 

1.9 

1911 

2.2 

1916 

1.8 

1912 

2-4J 

885.  Cost  of  Repairs.  In  Brooklyn  with  Municipal  Plant.  Table 
48  shows  the  detailed  cost  of  making  repairs  to  sheet  asphalt 
pavements  in  Brooklyn,  N.  Y.,  with  a  municipal  repair  plant. 

TABLE  48 

COST  OF  MAINTENANCE  OF  SHEET  ASPHALT  PAVEMENTS 
In  Brooklyn,  N.  Y.,  in  1911,  with  Municipal  Repair  Plant  f 


COST  IN  PLACE  PER  CUBIC  FOOT  OF 
UNCOMPRESSED  MIXTURE. 


Items  of  Expense. 

Repairs. 

Repaying. 

Wearing 
Coat. 

Binder 
Course. 

Wearing 
Coat. 

Binder 
Course. 

Supervision  and  fixed  charges  
Supplies,  repairs,  etc  

$0.025 

0.067 
0.178 
0.046 
0.182 
0.060 

$0.024 
0.057 
0.097 
0.043 
0.174 
0.057 

$0.025 
0.059 
0.178 
0.046 
0.213 
0.080 

$0.024 
0.057 
0.097 
0.043 
0.203 
0.076 

Materials 

Plant  labor 

Street  labor 

Trucking 

Total 

$0.558 

$0.452 

$0.601 

$0.500 

886.  In  Buffalo  by  Contract.  Buffalo,  N.  Y.,  has  long  been  noted 
for  the  extent  of  its  sheet  asphalt  pavements,  and  also  for  the 
accuracy  and  completeness  of  its  records  concerning  the  cost  of 

*  Private  letter  from  Capt.  J.  J.  Loving,  Corps  of  Engineers,  U.  S.  A.,  Assistant  to  the  Engi- 
neer Commissioner  of  the  District  of  Columbia. 

t  Engineering  and  Contracting,  Vol.  38  (1912),  p.  68. 


458 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


repairs  of  pavements.  The  annual  reports  of  the  Bureau  of  Engi- 
neering contain  voluminous  statistics  on  the  cost  of  construction 
and  repair  of  all  kinds  of  pavements.  For  example,  the  report  for 
the  fiscal  year  ending  June  30,  1916,  contains  179  pages  of  tabular 
matter  showing  the  width,  length,  area,  original  cost,  and  the  cost 
of  repairs  for  each  year  after  the  expiration  of  the  guaranty,  of 
all  the  sheet  asphalt  pavements  in  the  city.  The  report  also 
gives  a  summary  of  similar  data  for  several  years  back;  and  in 
addition  contains  several  pages  of  data  concerning  the  amount  and 
age  of  all  pavements,  by  whom  built,  the  kind  of  materials 
used,  etc. 

The  annual  cost  of  repairs  in  Buffalo  from  1902  to  1916  for 
substantially  3,000,000  sq.  yd.,  varied  from  1.30  to  8.27  cents  per 
sq.  yd.,  the  average  for  the  fourteen  years  being  5.06  cents  per 
sq.  yd. 

The  cost  of  maintaining  2,369,191  sq.  yd.  was  4.45  cents  per  sq.  yd. 
per  year;  and  the  cost  of  repairs  was  6.46  cents  per  sq.  yd.  per  year. 
The  average  life  of  sheet  asphalt  pavements  replaced  between  1878 
and  1906,  was  20.51  years,  being  21.77  years  for  streets  without  car 
track  and  18.36  for  streets  having  car  tracks. 

Table  49  gives  some  of  the  details  concerning  the  cost  of  repairs 
of  sheet  asphalt  pavement  for  the  year  ending  June  30,  1916. 

TABLE  49 

DATA  ON  REPAIRS  OF  SHEET  ASPHALT  PAVEMENTS 
Buffalo,  N.  Y.,  1915-16,  by  contract  * 


REF 
No. 

ITEMS. 

CONTRACT 
PRICE. 

AVERAGE 
QUANTITY 
PER  SQ.  YD. 

COST 

PER 

SQ.  YD. 

1 

2 

3 

July  1  to  December  31,  1915: 
Wearing  coat,  per  cubic  foot  
Binder  course  (open),  per  cubic  foot.  .  .  . 
Asphalt  cement,  per  gallon 

SO.  37 
0.17 
0  18 

1.3673 
0.7276 
0  1205 

$0.5059 
0.1237 
0  0217 

4 

Labor,  per  square  yard.      .    .    . 

0  51 

0  51 

5 

Total  

$1  1613 

6 

January  1  to  June  30,  1916: 
Wearing  coat,  per  cubic  foot 

0  33 

1  3843 

0  4^68 

7 
8 

Binder  course  (open),  per  cubic  foot  .... 
Asphalt  cement,  per  gallon 

0.17 

0  18 

0.8570 
0  1204 

0.1456 
0  0217 

9 

Labor,  per  square  yard  

0  47 

0  47 

10 

Total  

$1  0941 

*  Annual  Report  Bureau  of  Engineering,  1915-16,  p.  68. 


ART.    1] 


SHEET   ASPHALT   PAVEMENTS 


459 


Fig.  162    shows    the    cost    of    repairs    of    sheet    asphalt  pave- 
ments at  different  ages.     The  pavements  laid  before   1898  were 


ZO 


25 


5  10  15 

Age  of  Paremenf- 

FIG.  162. — COST  OF  REPAIRS  OF  SHEET  ASPHALT  PAVEMENTS  IN  BUFFALO. 


on  a  5-year  guaranty,  and  those  after  on  a  10-year  guaranty.     The 

A% 

curve  in  Fig.  162  is  that  for  -— -,  in  which  A  is  the  age  of  the  pave- 

1«5 

ment  in  years. 

887.  MAXIMUM  GRADES  FOR  ASPHALT  PAVEMENTS.  Until 
within  a  few  years,  it  has  been  assumed  that  the  maximum  per- 
missible grade  for  a  sheet  asphalt  pavement  was  2  or  2^  per  cent; 
but  experience  has  shown  that  this  limit  is  too  low.  It  is  now  gen- 
erally conceded  that  sheet  asphalt  may  be  laid  on  grades  of  5  or 
6  per  cent,  particularly  in  residence  streets — where  a  clean,  smooth, 
noiseless  pavement  is  specially  desirable,  and  where  there  is  usually 
no  great  amount  of  travel.  With  a  5  or  6  per  cent  grade,  there 
may  be  a  few  days  each  year  when  the  pavement  is  icy  and  too 
slippery  for  either  comfortable  or  safe  use.  In  New  York  City,  on 
a  street  having  a  6  per  cent  grade  paved  with  asphalt  on  the  sides 


460  ASPHALT   PAVEMENTS  [CHAP.   XVI 

and  granite  in  the  center,  the  traffic  as  a  rule  seeks  the  asphalt 
rather  than  take  the  granite;  and  in  the  same  city  traffic  has  de- 
serted one  street  having  a  5  per  cent  grade  paved  with  granite  for 
another  having  a  6  per  cent  grade  paved  with  asphalt.  A  number 
of  cities  have  sheet  asphalt  pavements  upon  a  7  per  cent  grade,  as, 
for  example,  Peoria,  111.,  Grand  Rapids,  Mich.,  Syracuse,  N.  Y., 
Troy,  N.  Y.;  and  Omaha,  Neb.,  and  St.  Joseph,  Mo.,  have  asphalt 
pavements  on  an  8  per  cent  grade.  Scranton,  Pa.,  has  a  short  piece 
of  asphalt  on  a  13  per  cent  grade;  San  Francisco,  Cal.,  a  piece  on  a 
16  per  cent  grade;  and  Pittsburg,  Pa.,  one  on  a  17  per  cent  grade. 
A  committee  of  the  American  Society  of  Civil  Engineers  recom- 
mends 5  per  cent  as  the  permissible  maximum  grade  for  sheet  asphalt 
—see  Table  15,  page  57. 

888.  CROWN  FOR  SHEET  ASPHALT  PAVEMENTS.    The  special 

committee  of  the  American  Society  of  Civil  Engineers  recommends 
that  the  crown  shall  be  between  £  and  t  of  an  inch  per  foot  of  the 
half  width,  see  Table  16,  page  65. 

889.  MERITS  AND  DEFECTS  OF  SHEET  ASPHALT  PAVEMENTS. 

The  advantages  possessed  by  monolithic  asphalt  pavements  con- 
structed as  described  above  are:  (1)  they  produce  neither  dust  nor 
mud;  (2)  they  are  comparatively  noiseless,  except  for  the  clicking 
of  the  horses'  shoes;  (3)  they  do  not  absorb  or  retain  noxious  liquids, 
but  facilitate  their  prompt  discharge  into  the  gutters  and  storm- 
water  sewers;  (4)  they  reduce  the  force  of  traction  to  a  moderate 
amount  (see  Table  8,  page  21);  and  (5)  they  afford  a  reasonably 
good  foothold  for  horses. 

The  defects  of  sheet  asphalt  pavements  are:  1,  the  first  cost  is 
comparatively  great;  2,  the  cost  of  maintenance  is  large;  and  3, 
such  pavements  are  generally  considered  too  smooth  for  steep  grades. 

For  a  discussion  of  the  relative  merits  of  the  different  pavements, 
see  Chapter  XX. 

890.  SPECIFICATIONS  FOR  SHEET  ASPHALT  PAVEMENTS.    The 
American   Society   for   Municipal   Improvements   on   October   14, 
1915,   adopted   Specifications   for   Sheet   Asphalt   Paving,   printed 
copies  of  which  may  be  had  of  the  Secretary  of  the  Society  for  a 
nominal  sum.     The  specifications  cover  only  the  selection,  prepara- 
tion, and  laying  of  the  materials  for   the   binder  course   and  the 
wearing  coat.     These  specifications  are  of  the  so-called  blanket  type, 
that  is,   they  contain   general   requirements   for  the   asphalt   and 
asphalt  cement  which  are   intended    to  include   all  the  different 
kinds  of  asphalt. 


ART.  2]  ASPHALT-CONCRETE   PAVEMENTS  461 

For  a  statement  of  the  objections  to  this  form  of  specification, 
see  §  532;  and  for  alternate  restricted  specifications  for  asphalt  for 
other  kinds  of  pavements,  see  §  534-41. 


ART.  2.    ASPHALT-CONCRETE  PAVEMENTS 

891.  An  asphalt-concrete  pavement  consists  of  a  foundation  of 
either  bituminous  or  hydraulic-cement  concrete,  and  a  wearing  coat 
of  asphalt  concrete.     There  are  two  differences  between  asphalt 
concrete  and  the  bituminous  concrete  discussed  in  Chapter  X, — 
Bituminous  Macadam  and  Bituminous  Concrete  Roads.     1.  Asphalt 
concrete  is  made  with  asphaltic  cement,  while  the  bituminous  con- 
crete may  be  made  with  either  asphalt  or  tar.     2.  Bituminous  con- 
crete is   made  with    only   the   care  usually  employed  in  making 
hydraulic  cement  concrete,  while  asphaltic  concrete  is  usually  pro- 
portioned, mixed  and  laid  with  approximately  as  much  care  as  sheet 
asphalt  pavements. 

The  difference  between  a  sheet  asphalt  pavement  and  an  asphalt- 
concrete  pavement  is  in  the  maximum  size  of  the  aggregate.  In 
the  former  all  of  the  aggregate  will  pass  a  No.  8  sieve,  while  the  latter 
may  contain  1^-inch  stone.  The  ordinary  sheet  asphalt  pavement 
could  with  some  propriety  be  called  a  sand  asphalt  pavement  or  an 
asphalt  mortar  pavement,  to  distinguish  it  from  an  asphalt  con- 
crete pavement. 

892.  DEFINITIONS.     There  are   several  types    of  asphalt-con- 
crete pavements,  the  best  known  of  which  are :  Bitulithic,  Warrenite, 
Amiesite,  and  asphalt  concrete. 

893.  Bitulithic  Pavement.     This  is  a  patented  form  of  pave- 
ment in  which  the  wearing  coat  is  composed  of  bitumen  and  mineral 
aggregates  ranging  from  3  inches  down  to  an  impalpable  powder. 
The  bituminous  mixture  is  usually  mixed  in  a  non-portable  plant. 
The  aggregate  is  chiefly  crushed  stone;    and  the  aggregate  of  the 
seal  coat  is  stone  chips. 

Six  patents  (No.  727,505  to  727,512)  were  issued  to  Frederick  F. 
Warren,  Newton,  Mass.,  between  May  16,  1901,  and  April  14,  1902, 
for  preparing  asphalt  for  paving  purposes  and  for  closely  related 
forms  of  asphalt-pavement  construction.  Of  the  eighty-two  speci- 
fications in  these  patents,  sixty-five  relate  to  the  proportions  and 
method  of  laying  the  pavement.  Apparently  the  intention  is  to 
cover  such  gradations  of  the  ingredients  as  will  secure  maximum 
density  and  maximum  stability.  The  density  of  a  sheet  asphalt 


462  ASPHALT   PAVEMENTS  [CHAP.   XVI 

pavement  made  with  limestone  filler  is  2.20  to  2.22  (§  832);  but  the 
density  of  a  bitulithic  pavement  made  of  the  same  materials  may  be 
2.28,  and  if  the  bitulithic  is  made  of  trap  the  density  may  be  2.50 
or  even  more.  This  type  of  pavement  is  designed  for  city  streets 
and  it  has  been  largely  used  for  this  purpose  (see  page  320). 

894.  There  has  been  much  controversy  and  considerable  litigation 
as  to  the  scope  and  meaning  of  these  patents,  and  it  is  impossible 
to  state  in  a  single  series  all  the  gradations  included.  Two  gradings 
actually  employed  in  laying  bitulithic  pavements  are  shown  in 
the  following  table. 


Bitun 

Minei 
u 

it 
tt 
n 
it 
tt 
tt 
tt 
tt 
it 

GRADINGS  OF  WEARING  COAT  OF 
ien               

BITULITHIC  PAVEMEJ 

No.   1. 

7.6% 

STS* 
No.  2. 
7.02% 
4.58 
3.99 
2.76 
7.88 
1.27 
2.39 
2.13 
3.77 
4.85 
12.75 
46.61 

•al  aggregate 
tt 

it 
tt 
tt 
tt 
it 
it 
n 
tt 
tt 

Total  

passing 

n 

tt 
tt 
it 
it 
it 
tt 
tt 
it 
•i 

200-mesh     .  . 

4  9 

100-mesh. 

4  6 

80-mesh 

3  2 

50-mesh. 

7  3 

40-mesh. 

3  1 

30-mesh  

24 

20-mesh. 

2  2 

10-mesh  

5.1 

j-inch  , 

91 

i-inch. 

19  3 

1^-inch 

31  2 

100  0% 

100.00% 

*  Agg's  Construction  of  Roads  and  Pavements,  p.  308. 

895.  Warrenite  Pavement.     This  type  of  pavement  is  covered 
by  the  Warren  patents  (§  893);   and  is  especially  designed  for  rural 
roads.     The  mixing  is  usually  done  in  a  semi-portable  plant;    and 
the  ingredients  are  not  proportioned  with  as  much  care  as  for  bitu- 
lithic pavements  (§  893).     Sand  is  largely  used  in  the  body  of  the 
wearing  course  and  wholly  for  the  seal  coat. 

896.  Amiesite   Pavement.     This   is   a    proprietory   mixture   of 
asphalt  cement,  sand,  and  broken  stone  up  to  lj  inches  in  diameter. 
It  is  mixed  at  a  plant,  and  is  shipped  in  cars  to  the  city  where  it  is 
to  be  laid.     During  transit  the  mixture  cements  into  a  solid  mass, 
and  must  be  heated  before  it  can  be  shoveled  from  the  car.     To 
heat  it,  holes  are  dug  into  the  mass  and  steam  is  blown  into  them 
and  permeates  the  whole  mass  and  softens  it.     This  type  of  pavement 
is  laid  without  a  concrete  base.     The  mixture  is  laid  cold,  usually  in 
two  courses,  the  lower  being  2J  or  3  inches  thick,  and  the  wearing 
coat  1  or  \\  inch. 


ART.    2] 


ASPHALT-CONCRETE    PAVEMENTS 


463 


897.  Topeka  Mixture.  An  asphalt  pavement  somewhat  like 
the  patented  bitulithic  pavement  was  laid  in  Topeka  and  Emporia, 
Kansas;  and  as  a  result  of  a  suit  for  infringement  of  patent  No. 
727,505,  the  U.  S.  Circuit  Court  in  1910  decided  that  the  following 
gradation  did  not  infringe  said  patent: 

Bitumen 7  to  11  per  cent 

Mineral  aggregate  passing  200-mesh 5  to  11  " 

40-mesh 18  "  30  " 

"  "  "          10-mesh 25  "  55  " 

4-mesh 8  "  22  " 

"  "  "  2-mesh not  over  10 

The  sieves  are  to  be  used  in  the  order  named. 


Notice  that  the  aggregate  is  mostly  sand,  and  J-inch  and  J-inch 
stone.  Notice  also  that  a  wide  variation  in  the  grading  is  possible 
under  the  above  specifications;  and  consequently  many  somewhat 
different  gradings  have  been  designated  as  Topeka  mixture.  Since 
1911  many  thousands  of  square  yards  of  roads  and  pavements  have 
been  laid  under  the  above  so-called  Topeka  specifications. 

898.  The  American  Society  of  Municipal  Improvements  recom- 
mends the  following  grading,*  which  the  Warren  Brothers  Co.  has 
agreed  does  not  infringe  its  patents. f 

Bitumen 7  to    9  per  cent 


Mineral  matter  passing  200-mesh 7 

"  "  "        80-mesh 10 

"  "  "        40-mesh 10 

20-mesh 10 

"  "  "       8-mesh 10 

"  "  "          4-mesh 15 

2-mesh..  .  5 


10 
20 
25 
25 
20 
20 
10 


899.  Asphalt  paving  blocks  (see  Art.  4  of  this  chapter)  have  long 
been  made  of  a  mixture  substantially  the  same  as  the  so-called 
Topeka  mixture. 

900.  Stone-filled  Sheet-Asphalt  Pavement.    The  so-called  To- 
peka mixture  is  frequently  described  as  the  wearing-coat  mixture 
for  a  sheet  asphalt  pavement  to  which  has  been  added  j-inch  and 
^-inch  broken  stone;   and  is  sometimes  referred  to  as  a  stone-filled 
asphalt  mixture,  and  also  as  a  fine  asphaltic  concrete. 

Mr.  Clifford  Richardson,  a  recognized  authority  on  asphalt 
paving,  in  Engineering  Record,  Vol.  65  (1912),  page  718,  and 
again  in  Vol.  70  (1914),  page  634,  shows  that  for  the  best 


*  Proceedings,  1915,  p.  415. 
t  Ibid.,  p.  423. 


464 


ASPHALT    PAVEMENTS 


[CHAP,  xvi 


results  possible  within  the  limits  of  the  above  decree,  the  finer 
part  of  the  mixture  should  conform  to  the  standard  grada- 
tion for  sheet  asphalt  (§  827)  and  that  as  much  as  J-inch  and  J-inch 
broken  stone  should  be  added  as  the  ruling  of  the  court  will 
permit.  He  states  that  the  two  gradings  in  Table  50  have  given 
satisfaction.  The  one  on  Riverside  Drive  was  laid  in  1913;  and 
the  one  in  Rochester  has  been  in  use  since  about  1902.  Mixtures 
of  this  type  have  been  laid  extensively  in  the  last  few  years. 

TABLE  50 

COMPOSITION  OF  STONE-FILLED  SHEET  ASPHALT  PAVEMENT 
(Best  Topeka  Mixture) 


Ingredients. 

RIVERSIDE   DRIVE, 
N.   Y.   CITY. 

ROCHESTER,   N.   Y. 

Total 
Mixture. 

Finer 
Portion. 

Total 
Mixture. 

Finer 
Portion. 

Bitumen  

8.9% 
11.9 
14.5 
18.6 
18.9 
19.1 
8.1 

11.1% 

16.5 
20.1 
25.9 
26.4 

8.9% 
12.3 
10.8 
24.2 
16.2 
21.5 
5.4 

H.1% 

17.1 
15.0 
33.7 
22.7 

Mineral  matter  passing 

u                           ( 

it                           i 
(i                           i 
tf                           t 

«                           t 

Total    

200-mesh  
80-mesh. 

40-mesh  
10-mesh. 

4-mesh  
2-mesh  

100.0% 

100.0% 

100.0% 

99.6% 

901.  Asphalt-concrete    Pavement.     An    asphalt-concrete  pave- 
ment, in  a  narrower  sense  than  the  way  in  which  that  term  is  used  in 
the  heading  of  this  article,  is  a  pavement  in  which  the  gradation  of 
the  aggregates  is  not  as  carefully  made  as  in  the  four  forms  just 
mentioned.     It  is  essentially  a  bituminous  concrete  (Art.  2,  Chapter 
X),  in  which  the  bituminous  cement  is  asphalt. 

The  standard  specifications  of  the  American  Society  of  Municipal 
Improvements  for  asphalt  concrete,  provide  a  wearing  surface  of 
asphalt  cement  and  broken  stone,  and  require  the  following  gradation 
of  the  broken  stone:  "  All  of  the  broken  stone  shall  pass  a  IJ-inch 
screen;  not  more  than  10  per  cent,  nor  less  than  1  per  cent,  shall  be 
retained  upon  a  1-inch  screen;  and  not  more  than  10  per  cent,  nor 
less  than  3  per  cent,  shall  pass  a  J-inch  screen. " 

902.  MIXING  AND  LAYING  ASPHALT  CONCRETE.    The  method 
of  mixing  and  laying  is  substantially  the  same  whatever  the  gra- 
dation of  the  aggregate.     With  the  most  careful  grading  the  aggre- 
gate is  usually  heated  and  then  separated  into  three  sizes,  not  includ- 


ART.    2]  ASPHALT-CONCRETE  PAVEMENTS  465 

ing  the  dust  or  filler.  Each  size  is  stored  in  a  separate  bin.  The 
predetermined  weight  of  each  size  is  drawn  from  the  bin  into  a  box 
resting  upon  a  scale  platform.  The  proper  amount  of  asphalt  cement 
is  determined  by  weight.  The  mineral  matter  and  the  cement  are 
heated  separately  to  the  proper  temperatures,  and  are  then  put  into 
a  mixer  somewhat  like  that  shown  in  Fig.  146,  page  419.  The  lim- 
iting temperatures  are  substantially  the  same  as  for  the  wearing 
coat  of  sheet  asphalt  pavements  (§  838). 

An  asphalt  concrete  pavement  is  usually  laid  without  any  binder 
course,  as  the  wearing  course  has  sufficient  stability  to  prevent  its 
flow  or  displacement  under  travel. 

The  mixture  is  hauled  to  the  street,  dumped,  spread  and  raked, 
much  the  same  as  for  the  wearing  coat  of  a  sheet  asphalt  pavement 
(§  839^0). 

The  rolling  of  an  asphalt  concrete  pavement  is  a  very  important 
matter.  The  weight  of  the  roller  and  the  temperature  at  which 
the  concrete  is  rolled  depends  upon  the  type  of  the  mixture.  The 
roller  should  be  as  heavy  as  possible  without  displacing  the  paving 
material.  The  large  aggregate  gives  the  mixture  comparatively 
great  stability,  and  hence  it  does  not  squeeze  out  under  the  roller 
as  does  the  sand-  or  sheet-asphalt  mixture;  and  therefore  it  is  best 
to  begin  rolling  as  soon  as  the  mixture  is  spread.  The  rolling  is 
usually  done  with  a  tandem  roller  weighing  8,  10,  or  15  tons;  or 
with  a  roller  of  the  three-wheel  type  weighing  10  tons.  The  rolling 
should  be  continued  until  the  roller  no  longer  leaves  a  mark  upon 
the  surface.  The  early  and  heavy  rolling  aids  in  securing  a  firm 
union  between  the  asphalt  concrete  and  the  base  or  foundation. 

903.  After  the  rolling  is  completed,  a  coating  of  hot  asphalt 
cement  is  applied  to  the  surface  of  the  pavement  at  the  rate  of  about 
J  to  \  gallon  per  square  yard,  which  is  immediately  covered  with  a 
layer  of  hot  J-  to  f-inch  stone  chips  at  the  rate  of  about  25  Ib.  per 
square  yard.     The  pavement  is  then  again  rolled  until  the  chips  are 
firmly  bedded  in  the  cement,  and  until  the  surface  is  dense  and 
waterproof.     This  seal  coat  is  an  essential  feature  of  an  asphalt- 
concrete  pavement,  since  except  the  best  bitulithic  the  body  of  the 
asphalt  concrete  is  not  waterproof. 

904.  COST    OF    ASPHALT-CONCRETE    PAVEMENTS.    Topeka 
Mixture.    Table  51,  page  466,  is  an  accurate  analysis  of  the  cost  to  the 
contractor  of  laying  18,000  square  yards  of  Topeka  asphalt-concrete 
pavement   in  Albany,  Oregon,  in  1916.     The   pavement  consisted 
of  a  3§-inch  asphalt-concrete  base,  and  a  IJ-inch  top  of  Topeka 


466 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


mixture.  The  base  and  top  were  spread  and  rolled  separately.  For 
similar  data,  giving  almost  the  same  results,  for  another  contractor 
in  the  same  city  the  preceding  year,  see  Engineering  News,  Vol.  73 
(1915),  page  1038. 

905.  Various  Cities.     Table  52,  page  467,  shows  the  composition 
and  cost  of  the  several  types  of  patented  asphalt-concrete  pavements 
in  33  cities.     Table  52  was  compiled   from  the  statistics  from  142 
cities,  and  contains  all  the  places  for  which  the  data  were  complete. 

906.  MERITS  AND  DEFECTS  OF  ASPHALT-CONCRETE  PAVE- 
MENTS.   The  merits  and  defects  claimed  for  asphalt-concrete  pave- 
ments are:   1.  An  asphalt  concrete  pavement,  in  the  broader  mean- 
ing of  that  term,  is  cheaper  than  the  sheet  asphalt  pavement,  since 
the  use  of  the  coarser  material  reduces  the  voids  and  also  the  surface 


TABLE  51 

DETAILED  COST  OF  TOPEKA  ASPHALT  PAVEMENT  * 
Average  for  one  week's  run 


Items. 

Si-inches 
Base. 

li-inches 
Top. 

5-inches. 
Total. 

MATERIALS: 
Gravel  @  $0  .  86  per  cubic  yard  
Sand,  fine  @  $1.50  per  cubic  yard  
coarse  @  $1.80  per  cubic  yard  
Asphalt  @  $12  per  ton 

$0.062 
O.C26 
0.024 
0  078 

$0.013 

0.032 
0.058 
0  089 

$0.075 

0.058 
0.082 
0  167 

Fuel  oil  @  $1.25  per  bbl  
Wood  @  $4  per  cord. 

0.016 
005 

0.008 
0  002 

0.024 
0  007 

Coal     

005 

0  003 

0  008 

Total  for  materials  

.216 

205 

421 

LABOR: 
Plant  

0  052 

0  031 

0  083 

Street. 

0  044 

0  032 

0  076 

Teams 

0  013 

0  008 

0  0021 

Total  for  labor 

0  109 

0  071 

0  180 

GENERAL  EXPENSE: 
Mixing  plant,  interest,  depreciation,  repairs 

0  10 

Roller  and  small  tools  

0  05 

Overhead. 

0  08 

Profits                .    .    . 

0  08 

Total  general  expense  

31 

Grand  total  

96 

COST  PER  SQUARE  YARD. 


*  Engineering  News,  Vol.  76  (1916),  p.  103. 


ART.   2] 


ASPHALT-CONCRETE    PAVEMENTS 


467 


to  be  covered  with  cement,  and  hence  less  bitumen  is  required.  The 
difference  in  bitumen  is  from  1  to  3  per  cent.  2.  An  asphalt-con- 
crete pavement  is  cheaper  than  a  sheet  asphalt  one,  since  the  former 
is  laid  in  a  single  course.  3.  An  asphalt-concrete  pavement  is  less 
slippery  than  a  sheet  asphalt  pavement,  owing  to  the  use  of  the  stone 
fragments. 

But,  on  the  other  hand,  an  asphalt-concrete  pavement  will  not 
endure  under  heavy  travel,  particularly  horse-drawn  steel-tired 
traffic,  as  well  as  a  sheet  asphalt  pavement,  since  sooner  or  later 
the  larger  stones  will  be  fractured  and  leave  two  uncemented  sur- 
faces which  will  permit  motion,  wear  and  disintegration. 

TABLE  52 

COST  OF  PATENTED  ASPHALT-CONCRETE  PAVEMENTS  * 
Laid  in  1916 


Ref. 
No. 

LOCALITY. 

Area 
Laid, 
Sq.  Yd. 

CONCRETE  BASE 

Thick- 
ness of 
Wear- 
ing 
Coat. 

Cost  of 
base 
and  top 
Sq.  Yd. 

State 

City. 

Thick- 
ness. 

Propor- 
tions. 

BITULITHIC  PAVEMENT 


2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 

26 
27 
28 
29 
an 

California 

Tucson  

67219 
104  672 
95682 
15870 
47431 
13660 
27600 
20004 
4626 
1  864 
35893 
18512 
7472 
89319 
8  147 
8680 
17132 
25000 
35712 
10914 
4  112 
42734 
11  880 
4941 

IVEMENT 

23900 
9801 
18830 
16644 
80000 

5 
4-6 
5 
5 
4 
5 
5 
6 
5 
6 
4 
6 
6 
5 
6 
4 
6 
5 
5 
6 
6 
6 
5 
5 

6 
6 
4 
5 
4 

1:3:6 
1:3:6 
1:2  :4 
1:3:6 
1:3:5 
1:3:5 
1:3:5 
1:2:4 
1:3:5 
1:2:4 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:2:4 
1  :6 
1:3:6 
1:3:6 
1  :2§  :5 
1:3:6 
1:3:5 

1:3:6 
stone 
1:3:5 
1  :  2J  :  5 
1:3:6 

2i 

2 
2 
2 
2 
2 
2 
2 

I" 

2 
2 
2 
2 
2 
2 
2 
2 
2 
2 
2 
2 

2 
2 
2 
2 
?, 

Los  Angeles  
Richmond  
Santa  Monica  
Creston  
Knoxville 

Iowa 

•i 

Mt.  Pleasant  
Eveleth  
St.  Cloud  
Virginia  
Billings 

Minnesota 

New  Jersey  

Harrison  
Irvington  
New  Brunswick  .  .  . 
Binghamton  
Herkimer  
New  Rochelle  
Bismarck.  :  
Fargo 

<>  • 

New  York  

North  r^akota.  .'.'... 

Ohio  

Cincinnati  

Lakewood  
Sioux  Falls 

South  Dakota  
Texas                  

San  Antonio  
Sheridan 

Wyoming  

WARRENITE  PJ 
Berkeley 

Connecticut  
Montana  
New  York  
North  Carolina..  . 

Winsted  
Great  Falls  .  . 

Elmira  
Raleieh.  . 

$2.35 
2.16 
1.58 
2.16 
1.40 
1.89 
1.98 
1.89 
2.50 
2.15 
2.44 
1.91 
2.29 
2.40 
2.65 
2.30 
2.18 
1.50 
2.38 
2.14 
2.57 
2.20 
2.10 
.30 


2.12 


1.40 
1.35 
1.90 
2.25 
1.52 


AMIESITE  PAVEMENT 


31 

Connecticut        

Danbury  

8000 

macadam 

2 

1  05 

32 

New  York  

New  Rochelle  

19  297 

2 

1  29 

33 

Salamanca  

12215 

stone 

2i 

1  65 

*  Municipal  Engineering,  Vol.  52  (1917),  p.  248-49. 


468 


ASPHALT  PAVEMENTS 


[CHAP,  xvi 


Table  53  shows  the  composition  and  cost  of  the  non-patented 
asphalt-concrete  pavements  in  32  cities. 

TABLE  53 

COST  OF  NON-PATENTED  ASPHALT  PAVEMENTS  * 
Laid  in  1916 


LOCALITY. 

CONCRETE  BASE. 

«*- 

Cost  of 

Ref. 
No. 

State. 

City. 

Amount 
Laid, 
Sq.  Yd. 

hickness. 

Propor- 
tions. 

hickness  o 
Binder 
Course.. 

hickness  o 
Wearing 
Coat. 

Binder, 
and 
Wear- 
ing 
Coat 

M 

H 

H 

TOPEKA  MIXTURE 


1 

9 

California  

Berkeley  
Fort  Dodge.  .  .  . 

36660 
40000 

5' 
6 

3 

Kansas  

Great  Bend.  .  .  . 

41800 

4 

4 
5 

Michigan 

Topeka  
Ludington  

64  120 
700 

5 
6 

6 

Missouri  

Springfield  

7073 

4 

7 
8 
9 

Nebraska  
North  Carolina.  .  .  . 
Texas.  . 

Norfolk  
Raleigh  
Taylor  .  . 

63000 
70000 
130  000 

6 
4 
4 

10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 
34 
35 

Alabama 

ASPHAL 
Gadsden  
LaGrange  
Oak  Park  
South  Bend.  .  .  . 
Cedar  Rapids  .  . 
Webster  City  .  . 
Emporia  
Manhattan  .... 
Louis  ille  
Grand  Rapids.  . 
St.  Paul  
Greenwood  .... 
Beatrice  
Trenton  
Batavia  
Niagara  Falls.  . 
Toledo 

r  CONCRE 

23  i?i  ' 

185  241 
37444 
35000 
16622 
9875 
39930 
20092 
14  110 
91  690 
8930 
36100 
23334 
1800 
4560 
12  174 
18313 
62000 
26512 
32  126 
126917 
12200 
39593 
96  690 
22475 

TE 
5' 

6 
6 
5 
4 
5 
4 
5 
6 
5 
5 
4 
5 
5 
5 
6 
6 
5 
4 
5 
5 
6 
4 
4 
5 
5 

Illinois  

Indiana  

Iowa  

Kansas  

Michigan  
Minnesota  
Mississippi  
Nebraska  
New  Jersey  
New  York  

Ohio 

Oregon  
South  Carolina.  .  .  . 
South  Dakota  
Tennessee  
Texas 

Portland  
Greenville  
Mitchell  
Chattanooga.  .  . 
Houston  
Salt  Lake  
N.  Yakima.  .  .. 
Charleston  
Fond  du  Lac.  .  . 

Utah  
Washington  
West  Virginia  
Wisconsin  

1 

:  3  :  6 

2" 

$1  18 

1 

3  :5 

2 

1.57 

1 

1 

21  :5 
21  :5 

2  ' 
2 

1.26 
1.35 

1 

6 

2 

1.05 

1 

3  :  7 

2 

1.22 

1 

5G 

2 

1.51 

1 

3  :6 

2 

.63 

1 

2  :4 

2 

1.40 

2  :  5 

2" 

2" 

1.36 

3  :6 

2 

1.36 

3  :  6 

.  .  „  . 

2 

1.55 

7G 

.  .  „  . 

1.45 

3  :5 

1.49 

3  :  6 

*! 

1.65 

2*  :5 



2 

1.27 

5 

2 

1.22 

3  I  6 

2 

1.68 

31  :7 
21  :5 

2 
2 

1.65 
1.60 

3  :5 

2 

1.48 

3  :6 

2 

1.57 

3  :  6 

2 

1.75 

21  :5 

"2  ' 

seal 

1.75 

3  :6 

2 

2.40 

31  :6 

.  „  .  . 

2 

1.85 

3  :6 

11 

1.30 

3  :  6 

.  2 

1.30 

51 

2 

1.80 

3  :6 

2 

1.46 

3  :6 

2 

1.80 

3  :6 

2 

1.75 

4  :  6 

2 

1.17 

21  :5 

2 

1.78 

3  :  5 

2 

1.53 

*  Municipal  Engineering,  Vol.  52  (1917),  p.  193-95. 

907.  SPECIFICATIONS.  The  American  Society  of  Municipal 
Improvements  on  October  14,  1915,  adopted  complete  "  Specifica- 
tions for  Bituminous  Paving,"  in  which  the  binding  material  was  the 
asphalt  specified  therein;  but  in  1916  the  Society  amended  the  gen- 
eral specifications  by  eliminating  the  special  requirements  for  asphalt, 
and  stating  that  the  asphalt,  the  flux,  and  the  asphalt  cement  should 
conform  to  the  requirements  for  these  materials  given  in  the  Spe- 
cifications for  Asphalt  Paving  ( §  542) .  Printed  copies  of  the  amended 


ART.   3]  ROCK   ASPHALT  PAVEMENTS  469 

specifications  for  "  Bituminous  Asphalt  Concrete  Paving  "  (Asphalt 
Concrete  Pavements)  may  be  had  of  the  Secretary  of  that  Society 
for  a  nominal  sum. 

With  the  above  Specifications  are  printed  also  specifications  for 
the  wearing  coat  of  Bitulithic  Pavements. 

908.  The  above  Society  also  publish  Specifications  for  Bituminous 
Concrete  Pavements  in  which  the  mineral  aggregate  is  the  com- 
plete product  of  a  stone  crusher  and  in  which  the  binding  material 
may  be  any  one  of  the  asphalts  described  in  §  539-40  or  either  of 
the  two  tars  described  in  §  574-75. 

ART.  3.    ROCK  ASPHALT  PAVEMENTS 

910.  A  rock  asphalt  pavement  is  made  by  crushing  asphaltic 
limestone  or  sandstone,  and  laying  it  while  hot  upon  a  concrete 
foundation.     In  Europe  this  is  the  common  form;    and  when  the 
term  asphalt  pavement  is  used  there,  this  kind  is  intended. 

Rock  asphalt  pavements  have  been  laid  only  to  a  comparatively 
small  extent  in  America.  Rock  asphalt  pavements  have  been  used 
in  a  small  way  in  California  for  many  years,  and  San  Francisco, 
Los  Angeles,  and  other  cities  have  several  miles  of  such  pavements. 
Apparently  both  asphaltic  limestones  and  sandstones  are  used  in 
California;  but  the  most  of  the  so-called  rock  asphalts  used  for 
paving  purposes  are  asphaltic  limestones. 

A  bituminous  limestone  to  be  suitable  for  paving  purposes 
should  be  as  coarse-grained  as  possible,  should  contain  between  9 
and  10  per  cent  of  bitumen  soluble  in  carbon  bisulphide,  and  should 
contain  very  little  matter  volatile  below  400°  F.  Often  one  or  more 
natural  rocks  are  mixed  to  secure  the  proper  proportion  of  bitumen; 
and  sometimes  a  natural  asphalt  is  added  to  the  natural  rock  to 
increase  the  proportion  of  bitumen. 

911.  CONSTRUCTION.    The    asphaltic    rock    is    quarried,    and 
then  crushed  to  about  egg  size  by  toothed  rollers.     These  pieces 
are  first  reduced  to  powder  and  then  sifted  to  uniform  fineness. 
The  powder  is  dropped  through  a  hopper  into  a  revolving  cylinder 
like  a  coffee  roaster,  which  is  about  6J  feet  in  diameter,  and  is  sur- 
rounded by  a  chamber  the  air  in  which  is  heated  by  a  movable 
furnace  placed  just  below  it.     The  cylinder  itself  revolves  and, 
since  it  is  provided  with  blades  arranged  in  screw  form,  the  pow- 
dered rock  is  well  mixed  with  hot  air  and  is  thus  thoroughly  heated 
to  a  temperature  between  300°  and  350°  F.     Specifications  fre- 


470  ASPHALT   PAVEMENTS  [CHAP.   XVI 

quently  permit  the  rock  asphalt  to  be  heated  to  but  200°  to  250°  F. 
When  the  powder  &  hot  enough,  the  furnace  is  removed  from  under 
the  heater  and  a  cart  replaces  it,  into  which  the  asphalt  powder  is 
discharged  and  hauled  to  the  street.  The  powder  will  retain  its 
heat  for  several  hours  and  so  admits  of  being  carted  long  distances 
without  losing  its  heat,  thus  doing  away  with  the  necessity  of  having 
roasters  at  the  point  where  the  surface  is  to  be  laid,  as  was  at  one 
time  the  practice.  For  the  best  results,  the  mixture  should  be 
delivered  upon  the  street  at  a  temperature  of  not  less  than  250°  F., 
although  specifications  sometimes  permit  a  temperature  of  but 
190°  F. 

The  heated  powder  is  spread  upon  the  concrete  base  to  a  uniform 
thickness  about  40  per  cent  greater  than  that  required  for  the  fin- 
ished pavement.  This  must  be  done  with  great  care  in  order  that 
the  material,  which  while  hot  has  a  great  tendency  to  consolidate, 
may  not  be  denser  in  one  spot  that  another.  The  material  is  com- 
pacted by  rolling  in  much  the  same  way  as  is  described  for  the  arti- 
ficial asphaltic  paving  compound,  except  that  as  a  rule  the  natural 
rock  asphalt  is  not  consolidated  to  so  great  an  extent  as  is  customary 
in  laying-  the  artificial  mixture.  The  evidence  of  this  is  that  a  rock 
asphalt  pavement  will  continue  to  shrink  in  thickness  under  traffic 
for  a  year  or  two;  while  the  artificial  mixture  shrinks  but  little,  if 
any,  after  completion. 

912,  The   general   appearance   of   the   completed   pavement  is 
much  the  same  as  that  of  the  pavement  made  of  the  artificial  mix- 
ture, except  that  the  European  rock  pavements  are  lighter  in  color. 
The  claim  is  that  European  natural  rock  asphalt  pavements  are 
more  slippery  and  less  susceptible  to  changes  in  temperature  than 
are  American  artificial  asphalt  pavements. 

Not  infrequently  the  term  rock  asphalt  pavement  is  inappro- 
priately applied  to  a  pavement  made  of  an  artificial  mixture  of  sand 
and  of  asphalt  extracted  from  a  natural  rock. 

ART.  4.     ASPHALT-BLOCK  PAVEMENTS 

913.  There  are  two  general  forms  of  asphalt  pavements,  the 
sheet  or  monolithic  and  the  block.     Three  forms  of  the  first  have  been 
fully  described  in  the  three  preceding  articles.     The   asphalt-block 
pavement  is  constructed  by  first  molding  rectangular  blocks  com- 
posed of  asphaltic  cement  and  crushed  stone,  and  then  placing  these 
blocks  side  by  side  upon  a  gravel  or  concrete  foundation.     In  1909 


ART.    4] 


ASPHALT-BLOCK   PAVEMENTS 


471 


there  were  in  this  country  about  5,500,000  square  yards  of  asphalt 
block  pavement,  see  page  320. 

Fig.  163  shows  a  perspective  view  and  cross  section  of  a  carriage 
way  and  foot  way  paved  with  asphalt  blocks. 


Fia.  163. — ASPHALT-BLOCK  PAVEMENT. 

914.  THE  BLOCKS.  At  first  crushed  limestone  was  used,  but 
now  the  blocks  are  made  with  crushed  trap,  granite,  or  gneiss. 
The  asphaltic  cement  is  mixed  substantially  as  for  sheet  pavements. 
The  proportions  employed  in  making  the  blocks  vary  slightly  with 
the  climate,  and  considerably  with  the  fineness  of  the  crushed  stone; 
but  are  about  as  follows: 

Asphaltic  cement 6 ,5  to    8  per  cent 

Limestone  dust 10  "  15    "    " 

Crushed  stone 67  "  78     "     " 

Since  the  blocks  contain  larger  fragments  than  sheet  pavements, 
they  contain  a  smaller  per  cent  of  voids,  and  hence  can  be  made 
with  a  slightly  smaller  per  cent  of  bitumen. 

The  ingredients  are  mixed  and  heated  about  as  for  sheet  pave- 
ments, and  are  then  molded  while  hot  under  heavy  pressure.  For- 
merly the  blocks  were  made  5  X  12  X  4  inches  deep;  and  later  they 
were  4  X  12  X  3  inches  deep,  and  also  5  X  12  X  3  inches  deep; 
but  the  blocks  are  usually  5  inches  wide,  12  inches  long,  and  2,  2J 
or  3  inches  deep  according  to  the  traffic  conditions.  Tiles  are  made 
now  8X8X2  inches  deep;  and  also  with  a  hexagonal  top  surface 
having  the  same  area  as  the  square  tile.  The  blocks  are  used  for 
carriage  ways,  and  the  tiles  for  foot  ways. 

The  blocks  are  usually  moulded  under  a  pressure  of  about  2 


472 


ASPHALT   PAVEMENTS 


[CHAP,  xvi 


tons  per  square  inch;  and  must  be  manufactured  at  such  a  temper- 
ature that  the  materials  will  press  together  in  a  mass  having  a  spe- 
cific gravity  of  not  less  than  2.5  if  made  of  trap,  and  of  not  less 
than  2.35  if  made  of  limestone. 

915.  The  blocks  are  laid  on  a  sand  cushion  or  on  a  half-inch 
mortar  bed  on  a  portland  concrete  foundation — recently  the  latter 
is  more  common.     After  the  blocks  are  laid,  the  surface  of  the  pave- 
ment is  covered  with  clean,  fine  sand  or  hard  fine  stone  screenings, 
which  are  swept  into  the  joints.     The  joints  are  not  usually  filled 
with  a  bituminous  cement,  as  the  blocks  are  malleable,  and  there- 
fore travel  soon  seals  the  joints  and  makes  the  pavement  prac- 
tically water-tight. 

916.  COST.    Table  54  shows  the  cost  of  asphalt  block  pave- 
ments in  several  representative  cities. 

TABLE  54 

COST  OP  ASPHALT  BLOCK  PAVEMENTS  * 
Laid  in  1912 


LOCALITY. 

Amount 
Laid 
in  1912 
Sq.  Yd. 

CONCRETE  BASE. 

Depth  of 
of 
Blocks. 

Averate 
Cost  in- 
cluding 
Base  and 
Grading 
per  Sq.  Yd. 

State. 

City. 

Thick- 
ness. 

Propor- 
tions. 

New  York  

Jamestown  
Niagara  Falls  
Port  Chester  
Rye  
Rye 

13408 
56619 
7  117 
9800 
18  500 
4800 
3800 
5083 
19000 
51800 
6000 
5992 
13000 
6  185 

5" 
5 

? 

4 

2 

6 
5J 

1     2i  :5 
136 
1     3     6 

'  '  2>V  '  ' 

I1 

3 
2 
2* 
2| 

S2.941 
3.201 
2.132 
2.56 
2.20 
2.65 
1.94 
2.251 
2.08 
1.73 
2.10 
2.95 
3.42 
2.65 

New  Jersey  
Ohio  
Michigan     .  . 

W.  New  York  
Ashland 

1     3     5 
1     2J  :  5 
1     3     5 
1     3     4 

Highland  Park.... 
Mt.  Clemens  
Savannah  

Georgia  

Way  Cross  
Kingston  
Toronto  

"2J" 

Canada  

4 
4 

136 
1     3     7 

Regina  

1  Includes  curbs. 

2  Does  not  include  curbs. 

*  Engineering  and  Contracting,  Vol.  39  (1913),  p.  373. 

917.  MERITS  AND   DEFECTS.    The   advantages   claimed   for   a 
pavement  of  asphalt  blocks  over  a  continuous  asphalt  sheet  are: 

1.  It  is  less  slippery,  owing  partly  to  the  joints  and  partly  to  the 
roughening  of  the  surface  due  to  the  use  of  a  hard  crushed  stone. 

2.  It  can  be  laid  like  any  block  pavement,  and  at  the  same  time  has 
almost  the  continuity  of  a  sheet  asphalt  pavement.     3.  It  can  be 
laid  in  cities  where  there  is  no  asphalt  plant.     4.  It  can  be  repaired 
by  removal  of  individual  units  by  common  labor  without  expensive 


ART.   4]  ASPHALT-BLOCK  PAVEMENTS  473 

plant  and  expert  labor.     5.  Having  numerous  joints,  it  is  free  from 
irregular  and  unsightly  contraction  cracks. 

The  disadvantages  of  an  asphalt  block  pavement  in  comparison 
with  a  continuous  asphalt  sheet  are:  1.  Its  first  cost  is  more,  since 
the  wearing  coat  of  the  block  pavement  is  2  inches  or  more  thick, 
while  that  of  the  continuous  sheet  is  at  most  only  2  inches.  2. 
The  edges  of  the  blocks  chip  off,  and  the  pavement  wears  rough. 
3.  It  is  slightly  more  noisy.  4.  Owing  to  its  numerous  joints, 
it  is  less  sanitary.  5.  It  is  more  expensive  to  clean.  6.  Unless 
bedded  with  unusual  care,  the  blocks  have  a  tendency  to  crack. 
7.  It  is  less  durable,  particularly  under  heavy  or  even  moderate 
traffic.  8.  The  blocks  are  not  rigid,  and  do  not  hold  their  shape 
as  well  as  other  paving  blocks;  and  hence  the  pavement  is  not  as 
easy  to  repair  as  other  block  pavements. 


CHAPTER  XVII 
BRICK  PAVEMENTS 

920.  A  brick  pavement  consists  of  brick  set  on  edge  on  a  suit- 
able  foundation — either    concrete,   gravel,    or   native   soil.      Brick 
pavements  have  been  used  in  Holland  for  perhaps  a  century,  and  to 
a  much  less  extent  and  for  a  shorter  period  in  northern  England. 
Brick  pavements  were  first  used  in  the  United  States  in  1870  at 
Charleston,  W.  Va.,  a  place  having  a  population  of  12,000.     An 
experiment  was  tried  with  a  short  section — less  than  a  block; — 
and  in  1873  a  block  on  the  principal  business  street  was  laid  with  a 
good  quality  of  building  brick,  and  remained  in  service  until  1909 — 
36  years.     A  block  of  brick  pavement,  laid  in  1875  on  a  leading 
business  street  of  Bloomington,  Illinois,  a  place  of  20,484  popula- 
tion in  1890,  though  constructed  of  an  inferior  building  brick  made 
of  a  superior  clay,  continued  in  service  for  20  years. 

At  present  brick  is  the  chief  paving  material  employed  in  most  of 
the  smaller  cities  of  the  Mississippi  Valley,  and  it  is  used  exten- 
sively in  many  of  the  larger  cities  in  that  territory.  In  all  parts  of 
this  country,  the  use  of  brick  for  residence  streets  and  light  traffic 
business  streets  is  rapidly  increasing.  For  the  relative  amount  of 
brick  pavements  in  use,  see  page  320.  Notice  that  in  yardage  of 
what  may  be  called  durable  pavements,  brick  ranks  second.  There 
are  in  this  country  nearly  two  hundred  plants  devoted  to  the  man- 
ufacture of  paving  brick,  some  having  annual  outputs  of  60,000,000 
to  100,000,000  bricks. 

921.  The  term  brick  pavements  will  be  used  in  this  chapter  as 
including  the  brick  wearing  coat  of  both  rural  roads  and  city  streets. 
The  discussion  will  primarily  be  made  with  reference  to  city  pave- 
ments, since  in  extent  they  exceed  rural  roads;  but  later  brick-paved 
rural  roads  will  be  considered. 

474 


ART.    1]  THE    BRICK  475 


ART.  1.     THE  BRICK 

922.  A  paving  brick  is  simply  a  brick  which,  owing  to  careful 
selection  of  the  clay  and  to  skilful  manufacture,  is  so  hard  and 
tough  that  it  will  resist  the  crushing  and  the  abrading  action  of 
the  travel. 

923.  THE  CLAY.     Three  distinct  classes  of  clay  may  be  em- 
ployed in  the  manufacture  of  paving  brick:    surface  clays,  impure 
fire  clays,  and  shales.     Surface  clays  are  almost  exclusively  used  for 
the  manufacture  of  building  bricks;  and  are  not  ordinarily  suitable 
for  making  paving  bricks,  since  it  is  practically  impossible  to  burn 
them  hard  enough  without  their  losing  their  shape.     On  account 
of  its  infusibility,  pure  fire  clay  is  unsuitable  for  making  paving 
brick,  the  brick  being  expensive  to  burn  and  lacking  density,  hard- 
ness, and  strength;   but  quite  impure  fire  clay  makes  a  fair  quality 
of  paving  brick,  although  the  process  of  manufacture  is  compara- 
tively expensive.     Bricks  made  from  impure  fire  clay  are  usually 
light  in  color,  varying  from  cream  to  buff,  and  ordinarily  are  quite 
porous,  absorbing  from  2.5  to  5.0  per  cent  of  water.     Most  paving 
bricks   are   made   from   shale, — an   impure,   hard,   laminated   clay 
which  has  been  subjected  to  great  pressure  by  the  superincumbent 
earth  strata.      Shale   is  widely  distributed,  and  usually  makes  a 
better  and  cheaper  paving  brick  than  either  surface  or  fire  clay, 
although  some  fire  clays  make  excellent  paving  bricks. 

The  different  classes  of  clay  so  shade  by  imperceptible  degrees 
one  into  the  other  that  it  is  impossible  sharply  to  discriminate  be- 
tween them.  Surface  clays  are  soft  and  unconsolidated,  and  are 
found  at  or  near  the  natural  surface.  Shales  are  dense  and  rock-like; 
but  are  easily  reduced  to  powder,  and  are  readily  worked  into  a 
plastic  mass  when  mixed  with  water.  Shale  is  often  incorrectly 
called  soapstone,  from  which  it  differs  in  nearly  every  respect. 
Shale  is  also  frequently,  but  erroneously,  called  slate,  from  which 
it  differs  only  slightly  in  origin  and  composition;  but  slate,  unlike 
shale,  can  not  be  rendered  plastic  by  mixing  it  with  water.  The 
only  method  of  distinguishing  between  shale  and  impure  fire  clay, 
except  by  a  kiln  test,  is  the  fact  that  shale  gives  a  conchoidal  frac- 
ture while  fire  clay  does  not. 

924.  Chemical  Composition.     It  is  not  wise  to  enter  into  any 
extended  consideration  of  the  chemical  composition  of  brick  clays, 
since  the  subject  is  very  complicated,  and  since  the  engineer  is 


476  BRICK    PAVEMENTS  [CHAP.    XVII 

interested  only  in  the  physical  properties  of  the  finished  product 
and  should  not  attempt  to  prescribe  the  materials  or  to  limit  the 
methods  employed  by  the  manufacturer. 

925.  Physical  Properties.     A  chemical  analysis  of  a  clay  may 
furnish  sufficient  evidence  upon  which  to  condemn  it  for  brick- 
making    purposes,    but   never   enough    for   its   indorsement.     The 
following  physical  properties  are  important  factors  in  determining 
the  value  of  a  brick  clay:   1,  its  plasticity;   2,  the  amount  of  water 
required  to  make  a  plastic  mass;   3,  the  amount  of  shrinkage,  both 
in  burning  and  in  drying;  4,  the  rapidity  of  drying  and  also  of  burn- 
ing;   5,   the  temperature  of  incipient  and  complete  vitrification; 
6,  the  density  before  and  after  burning;   and  7,  the  strength  of  the 
burned  brick. 

926.  MANUFACTURE  OF  THE  BRICK.    Soft,  homogeneous  clay 
may  be  run  through  rollers,  to  crush  the  lumps,  and  from  the  crusher 
it  may  go  directly  to  the  moulding  machine;  but  it  is  usually  desirable 
to  run  it  first  through  a  pug  mill,  where  it  is  mixed  and  worked 
with  water  into  a  plastic  mass.     Hard  clays  and  shales  are  usually 
reduced  to  a  powder  in  a  dry  pan,  which  consists  of  two  heavy 
rollers  or  wheels  running  in  a  revolving  pan  having  a  perforated 
bottom.     It  is  important  to  have  the  clay  finely  pulverized,  because 
it  will  then  fuse  at  a  lower  temperature,  and  also  because  fineness 
is  necessary  to  the  production  of  ari  even  and  close-grained  texture 
which  conduces  to  make  the  brick  tough  and  impervious.     The 
powdered  clay  is  screened  and  then  tempered  with  water  in  the  pug 
mill  or  a  wet  pan.     Fire  clays  are  sometimes  both  crushed  and 
tempered  in  a  wet  pan,  which  is  similar  to  a  dry  pan  except  that 
the  bottom  is  water  tight.     The  wet  pan  gives  better  results  than 
the  pug  mill,  as  the  clay  can  be  retained  in  the  pan  until  it  is  thor- 
oughly tempered,  but  as  it  requires  a  large  plant,  and  takes  more 
labor  and  power,  it  is  not  usually  employed  in  making  paving  brick. 
The  more  thoroughly  the  clay  is  worked  or  tempered,  the  more 
uniform  and  better  will  be  the  brick. 

927.  Moulding.     Paving  brick  are  now  made  by  the  stiff-mud 
process.     The  moulding  is  done  by  an  auger  machine  which  forces 
the  tempered  clay  or  stiff  mud  through  a  die,  thus  giving  a  contin- 
uous  bar    of   compressed   clay.     Fig.    164  shows   an   auger  brick- 
moulding  machine.     At  the    right  is  the    auger  machine,  in  the 
middle  the  bar  of  clay,  and  at  the  left  the   cutting  table   or  the 
machine  for  cutting  the  bar  into  bricks. 

Instead  of  an  auger  producing  a  continuous  stream  of  clay, 


AET.    1] 


THE   BRICK 


477 


reciprocating  plungers  are  sometimes  employed,  which  give  an  inter- 
mittent bar.  The  auger  machine  is  the  cheapest,  and  is  almost  uni- 
versally used. 

928.  The  weak  point  of  the  stiff-mud  process  is  the  laminations 
that  must  inevitably  result  from  pushing  a  stream  of  clay  through 
a  fixed  die.  The  friction  of  the  sides  of  the  die  will  cause  differential 
speeds  in  the  flow  of  the  clay,  and  these  variations  must  necessarily 
result  in  laminations  in  the  clay  bar.  If  the  air  has  been  expelled 


FIG.  164. — AUGER  BRICK-MOULDING  MACHINE. 

from  the  clay  by  the  pug  mill,  these  lines  can  be  largely  closed  up 
again  by  a  properly  shaped  die,  and  a  first-class  brick  will  result  in 
which  the  laminations  will  be  inconspicuous  and  of  no  importance; 
but  if  the  air  has  not  been  expelled,  or  if  the  tapering  former  and 
the  die  are  not  properly  designed,  there  will  be  a  number  of  concentric 
lines  that  divide  the  cross  section  of  the  brick  into  a  series  of  shells 
or  concentric  cylinders  which  greatly  weaken  the  brick.  These 
laminations  vary  with  the  character  and  the  condition  of  the  clay; 
and  as  a  rule,  the  more  plastic  the  clay  the  more  prominent  the 
laminations. 

929.  Cutting  the  Brick.  Formerly  the  size  of  the  die  was  such 
as  to  give  a  bar  of  clay  about  4J  by  2J  inches,  which  the  automatic 
machine  cut  into  lengths  of  about  9J  inches  by  forcing  a  wire  through 
it,  thus  producing  what  is  called  an  end-cut  brick.  But  now  the 
die  is  usually  a  little  greater  than  8J  by  4  inches,  the  excess  depend- 
ing upon  the  shrinkage  of  the  clay  in  drying  and  burning,  and  the 
bar  is  cut  into  sections  3J  inches  thick,  thus  producing  a  side-cut 
brick.  Substantially  all  paving  brick  are  now  side-cut. 


478 


BRICK   PAVEMENTS 


[CHAP,  xvii 


930.  Kinds  of  Brick.     The  first  brick  made  especially  for  paving 
purposes  was  a  square-edged  side-cut  brick  with  plane  sides;    but 
now  several  kinds  or  forms  are  in  use. 

931.  Re-pressed   Brick.     A   re-pressed   brick  is   one   that  after 
being  moulded  is  subjected  to  a  heavy  pressure.     The  re-pressing 
makes  the  brick  more  symmetrical  in  form  and  of  better  appearance. 

In  the  early  history  of  paving  brick  industry,  it  was  claimed  that 
one  of  the  advantages  of  re-pressing  was  that  it  enabled  the  man- 
ufacturer to  form  grooves  in  the  faces  of  the  brick  which  facilitated 
the  introduction  of  the  joint  filler  and  also  increased  the  power  of 
the  filler  to  hold  the  brick  in  place  in  the  pavement.  Fig.  165 
shows  three  forms  of  grooved  paving  brick. 


FIG.  165. — OLD-STYLE  GROOVED  PAVING  BRICKS. 

The  above  form  of  brick  proved  undesirable,  since  the  joints 
between  the  bricks  when  set  or  laid  in  the  pavement  were  so  narrow 
that  it  was  difficult  to  get  the  filler  into  the  joints.  The  demand 
next  was  for  a  brick  having  projections  on  the  side  which  would 
automatically  space  the  joints  so  they  could  be  easily  and  com- 
pletely filled  by  the  filler.  In  answer  to  this  demand  manufacturers 
formed  lugs,  or  buttons,  or  raised  letters  on  the  side  of  a  re- 
pressed brick,  which  served  to  keep  the  bricks  a  uniform  distance 
apart  and  thus  made  joints  into  which  the  filler  could  easily  be 
poured  or  swept.  Fig.  166  shows  re-pressed  paving  bricks  which  are 
much  used. 

932.  Even  though  re-pressed  under  a  great  total  pressure,  the  re- 
pressing does  not  increase  the  density  of  the  brick.  In  fact  the  re- 


ART.    1]  THE   BRICK  479 

press  invariably  increases  the  volume  of  the  brick.  The  reasons 
for  this  are:  1.  The  box  in  which  the  brick  is  re-pressed  must  be 
slightly  larger  than  the  die-moulded  clay  block,  so  that  the  block 
can  be  easily  dropped  in;  and  hence  re-pressing  compresses  the 
block  in  one  direction,  but  expands  it  in  the  other  two.  2.  For 


FIG.  166. — MODERN  RE-PRESSED  PAVING  BRICKS. 

practical  reasons  the  re-pressed '  block  must  have  rounded  edges; 
and  the  forming  of  the  round  edges  disrupts  the  original  structure 
and  doubtless  opens  up  some  of  the  laminations.  However,  in 
1916,  a  few  manufacturers  began  to  make  re-pressed  brick  with 
square  edges.  The  practice  seems  to  have  been  abandoned.  3. 
The  formation  of  the  lugs  or  buttons,  or  the  making  of  the  raised  or 
sunken  letters  breaks  the  original  bond  of  the  clay  and  opens  up  the 
laminations. 

Experiments  show  that  bricks  not  re-pressed  are  stronger  and 
freer  from  structural  defects  than  re-pressed  brick.  The  cost  of 
re-pressing  is  about  2  cents  per  square  yard  of  pavement.  There 
is  little  or  no  advantage  to  compensate  for  the  decreased  strength 
and  increased  cost  due  to  re-pressing. 

933.  A  marked  disadvantage  of  a  re-pressed  brick  is  that  it  has 
rounded  edges,  which  makes  it  impossible  to  maintain  the  joint 
level  full  of  filler.     The  filler  chips  out  of  such  a  joint  much  more 
easily  than  a  joint  between  square-edged  bricks.     Re-pressed  paving 
bricks  were  almost  exclusively  used  for  a  quarter  of  a  century. 

934.  Wire-cut  Lug  Brick.     In  1910  a  method  of  cutting  the  bar 
of  clay  into  bricks  was  introduced  which  gives  a  square-edged  brick 
having  on  one  of  its  faces  four  lugs  integral  with  the  body  of  the 
brick,  and  also  having  a  groove  adjacent  to  each  lug.     The  lugs 
project  TQ  of  an  inch,  which  insures  a  joint  at  least  Y&  of  an  inch 
wide;   but  in  1918  the  lugs  are  to  project  only  £  of  an  inch,  which 
will  give  a  minimum  joint  of  J  inch.     The  brick  also  has  a  double 
bevel  or  bulge  of  ^  of  an  inch  at  each  end,  which  insures  an  end 


480 


BRICK   PAVEMENTS 


[CHAP,  xvii 


joint  at  least  &  of  an  inch  wide  at  the  top  and  bottom  of  the  end 
joint.  Fig.  167  shows  this  form  of  brick;  and  Fig.  168  shows  such 
brick  laid  in  the  pavement. 


FIG.  167. — WIRE-CUT  LUG  PAVING  BRICK. 


FIG.  168. — WIRE-CUT  LUG  BRICK  IN  PAVEMENT. 

Fig.  169,  page  481,  shows  the  machine  for  cutting  the  bar  of  clay 
into  wire-cut  lug  bricks.  The  clay  is  cut  by  forcing  a  wire  horizon- 
tally through  the  bar,  the  wire  being  guided  by  narrow  slots  in 
plates  above  and  below  the  clay.  The  slots  on  one  side  of  the  brick 
are  wavy  and  form  the  lugs;  while  the  slots  on  the  other  side  are 
straight  and  form  a  plane  surface. 

The  advantages  of  the  wire-cut  lug  brick  are:  1.  There  are  no 
new  laminations.  2.  The  lugs  space  the  bricks  when  laid  in  the 
pavement  so  as  to  make  joints  of  uniform  width  and  the  beveled  ends 


AET.    1] 


THE  BRICK 


481 


and  the  grooves  on  the  vertical  faces  make  it  easy  to  fill  the  joints 
completely.  3.  The  square  edge  of  the  brick  makes  a  joint  that 
holds  the  filler  better  than  the  round-edge  of  the  re-pressed  brick, 
since  the  filler  does  not  feather  out  at  its  wearing  surface.  4.  The 
wire-cut  face  is  rougher  than  the  smooth  face  of  the  re-pressed  bricks, 
and  therefore  the  joint  filler  adheres  better.* 


FIG.  169. — MACHINE  FOR  CUTTING  WIEE-CUT  LOG  BRICK. 

On  the  other  hand,  the  wire-cut  lug  bricks  require  considerably 
more  filler  than  the  re-pressed  brick. 

The  wire-cut  lug  brick  is  patented;  but  many  manufacturers 
are  licensed  to  make  it,  and  it  is  sold  in  unrestricted  competition. 
Something  like  three  fourths  of  all  the  paving  bricks  used  east  of 
the  Mississippi  river  are  of  this  type.  Many  manufacturers  make 
both  the  wire-cut  lug  and  the  re-pressed  paving  brick. 

935.  Vertical-fiber  Brick.  The  vertical-fiber  brick  is  one  cut 
from  a  die-moulded  bar  of  clay  by  wires  traveling  in  straight  par- 
allel slots,  which  is  laid  in  the  pavement  with  the  wire-cut  face  up, 
and  hence  the  wear  comes  upon  the  end  of  the  laminations  or  fibers. 
Lugs  are  formed  on  one  side  of  the  brick  by  notches  or  grooves  in 
one  side  of  the  die.  Apparently  this  form  of  brick  is  made  only  by 
members  of  the  Western  Paving  Brick  Manufacturers  Association, 
and  promoted  by  it. 

The  advantages  officially  claimed  for  this  type  of  brick  are:    1. 


*  For  experimental  data  proving  this,  see  Engineering  Record,  Vol.  69  (1914),  p.  607. 


482  BRICK   PAVEMENTS  [CHAP.    XVII 

It  is  not  patented,  and  therefore  can  be  made  by  any  manufacturer 
without  paying  royalty.  2.  The  depth  of  the  brick  as  laid  in  the 
pavement  can  easily  be  changed  by  simply  changing  the  spacing  of 
the  cutting  wires.  3.  The  brick  can  be  set  in  the  kiln  so  that  all 
kiln  marks  come  upon  the  vertical  surfaces,  and  hence  leaves  the 
bedding  and  wearing  faces  free  from  such  marks  and  makes  a  smoother 
pavement.  4.  In  the  vertical-fiber  brick  the  wear  comes  upon  the 
end  of  the  laminations,  instead  of  on  the  sides  as  in  other  kinds. 
5.  The  wire-cut  surface  gives  a  good  foothold  for  horses.  6.  If  a 
bituminous  filler  is  used,  the  wire-cut  surface  aids  in  retaining  a 
carpet  of  bituminous  material  on  the  surface. 

Of  the  above  claims  1,  2,  3,  and  4  must  be  admitted  as  being  true; 
but  there  is  a  great  difference  of  opinion  as  to  the  importance  of  the 
advantage  claimed.  Claim  5  is  of  doubtful  value,  since  all  brick 
pavements  afford  a  satisfactory  foothold  for  horses.  Claim  6  is  a 
disadvantage  rather  than  an  advantage,  since  the  smoother  the  sur- 
face of  a  brick  pavement  the  better,  and  since  at  best  a  thin  film  of 
bituminous  cement  can  not  endure  long  on  a  brick  pavement  (see 
§  579).  It  is  a  reversal  of  good  practice  to  place  the  rougher  face 
horizontal  and  the  smoother  vertical. 

The  weight  given  to  the  above  claims  seems  to  be  largely  a  matter 
of  locality.  The  wire-cut  lug  brick  is  favored  by  at  least  most  of  the 
members  of  the  National  Paving  Brick  Manufacturers  Association, 
while  the  vertical-fiber  brick  seems  to  be  preferred  by  the  members 
of  the  Western  Paving  Brick  Manufacturers  Association.  The  ter- 
ritory of  the  former  is  east  of  the  Mississippi  river,  and  that  of  the 
latter  west  of  that  river.  However,  as  only  about  4  per  cent  of  the 
paving  bricks  made  in  the  United  States  are  manufactured  between 
the  Mississippi  river  and  the  Rocky  Mountains,  this  difference  of 
opinion  is  not  important.  Many  re-pressed  and  wire-cut  lug  bricks 
are  used  west  of  the  Mississippi  river. 

936.  Hill-side  Brick.  To  afford  a  better  foothold  for  horses  on 
steep  grades,  a  special  hillside  brick  is  made.  There  are  two  forms: 

1.  A  brick  laid  with  its  long  dimension  across  the  street  and  hav- 
ing one  edge  each  of  its  top  and  bottom  edges  chamfered,  which  gives 
a  series  of  continuous  parallel  grooves  running  transversely  across 
the  road  or  street.     These  bricks  are  usually  re-pressed,  the  cham- 
fered corners  being  produced  by  filling  up  opposite  edges  of  the 
mould. 

2.  A  brick  laid  with  its  long  dimension  lengthwise  of  the  road 
or  street,  and  having  one  or  more  transverse  grooves  on  each  of 


ART.    1] 


THE   BRICK 


483 


its  two  edges,  thus  producing  a  series  of  non-continuous  parallel 
grooves  across  the  road.  These  brick  are  die-moulded  side-cut, 
the  grooves  being  produced  by 
metal  lugs  on  the  sides  of  the  die. 
Fig.  170  shows  a  wire-cut  lug  hill- 
side paving  brick;  and  Fig.  171 
shows  such  brick  in  the  pave- 
ment before  rolling  and  before 
the  application  of  the  joint  filler. 
Fig.  171  is  a  street  in  Toronto, 
Canada,  on  a  6  per  cent  grade. 
Sometimes  a  strip  of  hill-side 
bricks  is  laid  on  each  side  of  the 
street  with  a  strip  of  ordinary 
paving  bricks  in  the  center. 

937.  After  being  moulded,  or 
after  being  re-pressed,  the  bricks 

,  FIG    170. — WIRE-CUT  LUG  HILL-SIDE    PAVING 

are    placed    on    trucks    or    cars,  BRICK.. 

and  conveyed  to  the  dry  house. 

A  paving  brick  immediately  after  being  moulded  contains  20  to  30 

per  cent  of  water;    and  hence  thorough  drying  greatly  facilitates 

the  burning  of  the  brick. 


FIG.  171. — HILL-SIDE  BRICK  IN  A  PAVEMENT. 


938.  Burning.     Paving  bricks  are  usually  burned  in  down-draft 
brick-ovens  having  fire  pockets  or  furnaces  built  in  their  outer  walls. 


484  BRICK   PAVEMENTS  [CHAP.   XVII 

The  bottoms  of  the  kilns  are  perforated  to  allow  the  gases  to  pass 
through  the  flues,  which  are  beneath  the  floor,  and  which  lead  to  the 
chimney.  The  fire  passes  up  from  the  furnaces  into  the  kiln,  then 
down  through  the  brick  to  be  burned  to  the  flues,  and  thence  to  the 
chimney.  The  burning  is  the  most  critical  step  in  the  manufacture 
of  paving  brick.  At  first  the  heat  is  applied  slowly  in  order  to  drive 
off  the  remaining  water  without  cracking  the  brick.  A  low  heat  is 
continued  until  the  smoke  passing  off  shows  no  further  signs  of  steam 
or  "  water-smoke,"  after  which  the  fires  are  gradually  increased 
until  the  temperature  throughout  the  kiln  is  sufficient  to  vitrify  the 
brick.  Most  shales  vitrify  at  from  1,500°  to  2,000°  F.;  but  impure 
fire-clays  require  from  1,800°  to  2,300°  F.  From  seven  to  ten  days 
are  required  to  raise  the  entire  kiln  to  the  vitrifying  temperature. 

There  has  been  much  discussion  as  to  the  meaning  of  the  term 
vitrification  as  applied  to  brick  making.  Literally  speaking,  to 
vitrify  means  to  render  glassy;  but  as  applied  to  clay  working, 
vitrification  has  come  to  mean  incipient  fusion  of  the  particles  of 
the  clay  into  a  new  chemical  compound.  The  degree  of  vitrification 
increases  with  the  temperature,  and  the  logical  end  of  the  process 
is  complete  fusion.  A  clay  is  partially  vitrified  if  its  constituents 
have  begun  to  unite  by  heat  into  a  compound  silicate,  even  though 
it  may  not  have  a  glassy  fracture.  The  physical  peculiarities  which 
mark  vitrification  in  a  burned  clay  are  the  conchoidal  fracture,  the 
absence  of  pores,  and  the  blending  of  the  ingredients  into  one  mass. 
Cracks,  fissures,  and  cavities  may  be  found,  but  porosity  must  not 
exist  in  a  well  vitrified  brick;  and  the  original  particles  must  have 
begun  to  cohere  by  the  bond  of  heat  instead  of  the  bond  of  plas- 
ticity. Within  limits  which  are  different  for  different  clays,  the 
degree  of  vitrification  in  a  burned  clay  is  measured  by  its  ability  to 
absorb  water.  A  lightly  burned  brick  will  greedily  absorb  water, 
and  the  greater  the  degree  of  vitrification  the  less  the  water  absorbed, 
a  fully  vitrified  brick  absorbing  absolutely  no  water. 

After  the  bricks  have  been  vitrified  entirely  through,  the  kiln  is 
tightly  closed  and  allowed  to  cool  very  slowly.  Rapid  cooling 
renders  the  brick  brittle;  but  by  slow  cooling  they  are  annealed  and 
rendered  tough.  Slow  cooling  is  the  secret  of  toughness,  and  the 
slower  the  cooling  the  tougher  the  brick.  The  annealing  is  fre- 
quently unduly  hurried,  much  to  the  detriment  of  the  toughness 
of  the  brick.  The  kiln  is  often  cooled  in  three  to  five  days,  when 
seven  to  ten  would  materially  improve  the  product  and  usually 
would  be  worth  the  extra  cost. 


ART.    1]  THE    BRICK  485 

With  the  utmost  care  a  considerable  per  cent  of  the  contents  of 
the  kiln  are  unsuitable  for  paving  purposes,  because  of  some  being 
under-burned  and  some  over-burned.  With  shale  80  to  90  per  cent 
of  first-class  paving  brick  is  a  high  average,  while  with  impure  fire 
clay  85  to  90  per  cent  may  be  produced. 

939.  Size  of  the  Brick.     Formerly  there  was  considerable  differ- 
ence of  opinion  as  to  the  best  size  for  paving  brick,  some  advocating 
2±X4X8",  others  3iX4X8J",  and  a  few  4X5X12".     The  first 
size  is  always  referred  to  as  a  brick,  but  the  last  two  are  sometimes 
called  paving  blocks.     The  last  was  never  made  in  any  quantity, 
and  has  been  entirely  abandoned.     There  is  no  conventional  line  by 
which  to  distinguish  bricks  from  blocks.     It  was  often  claimed  that 
one  or  the  other  size  made  the  better  pavement,  but  there  is  no 
material  difference  in  the  quality  of  the  pavement  between  the  dif- 
ferent sizes. 

The  advantages  of  the  building-brick  size  are:  (1)  being  smaller 
they  are  more  easily  vitrified,  and  therefore  a  little  cheaper  to  man- 
ufacture; and  (2)  brick  unsuitable  for  use  in  the  pavement  can  be 
more  readily  disposed  of  for  building  purposes,  a  fact  which  tends  to 
cheapen  the  cost  of  the  brick  used  in  the  pavement.  The  advan- 
tages of  the  block-size  to  the  manufacturer  are  that  there  are  fewer 
pieces  to  handle;  and  in  the  pavement  the  blocks  chip  or  spall  on 
the  edges  less  than  the  bricks,  particularly  if  the  filler  is  not  rigid 
(see  §  1014).  The  manufacturer  of  the  block  sometimes  places 
building  brick  in  that  part  of  the  kiln  in  which  it  is  difficult  to  burn 
blocks  thoroughly  (the  bottom  of  a  down-draft  kiln),  a  process  which 
decreases  the  per  cent  of  blocks  unsuitable  for  paving  purposes,  and 
at  least  partially  eliminates  the  second  advantage  of  the  building- 
brick  size  as  above.  In  the  early  history  of  brick  paving,  bricks 
were  most  in  favor;  but  now  the  blocks  are  used  almost  exclusively, 
and  usually  they  are  called  bricks. 

940.  Uniformity  of  size  is  very  desirable  to  prevent  confusion  in 
buying  and  bidding,  and  particularly  for  convenience  in  making 
repairs.     Unfortunately  the  sizes  of  building  bricks  and  also  of  paving 
bricks  or  blocks  vary  considerably  in  different  parts  of  the  country. 

941.  The  Specifications  of  the  National  Paving  Brick  Manu- 
facturers Association,  which  have  been  widely  adopted  by  engineers, 
prescribe  re-pressed  and  wire-cut  lug  paving  blocks  shall  be  8J 
inches  long,  4  inches  deep,  and  3J  inches  wide,  with  the  provision 
that   shallower   brick  may   be   used    (see    §  942).     The   specifica- 
tions of  the  Western  Paving  Brick  Manufacturers  Association  seem 


486  BRICK    PAVEMENTS  [CHAP.    XVII 

to  have  no  standard  size  for  vertical-fiber  brick;  but  seem  to  make 
such  brick  from  3f  to  4f  inches  wide,  from  8  to  9  inches  long,  and 
2|,  3  or  4  inches  deep,  a  depth  of  2|  inches  seeming  to  be  the 
most  common. 

Until  1916  the  Specifications  for  Brick  Pavements  adopted  by 
the  American  Society  of  Municipal  Improvements  permitted  the 
use  of  either  bricks  or  blocks;  but  in  1916  the  specifications  were 
amended  so  as  to  permit  the  use  of  only  blocks  8J  inches  long,  4 
inches  deep,  and  3f  inches  wide. 

The  width  is  always  exclusive  of  lugs  or  buttons. 

942.  Thus  far  in  the  history  of  brick  pavements,  the  depth  of 
the  brick  has  quite  uniformly  been  4  inches;    but  some  engineers 
claim  that  no  brick  pavement  ever  failed  through  the  wear  on  the 
brick,  and  therefore  the  depth  of  the  brick  should  be  reduced. 

In  1915  a  new  type  of  brick  pavement  was  introduced  (§  982), 
which  would  safely  permit  a  reduction  in  the  total  thickness  of  the 
pavement.  This  and  relative  matters  are  discussed  in  §  1028. 

943.  TESTING  THE  BRICK.     It  is  important  to  have  a  definite 
method  of  testing  the  qualities  of  any  artificial  material,  since  then 
all  parties  may  know  exactly  the  grade  called  for,  and  since  the 
results  obtained  by  different  observers  with  different  materials  may 
be  compared.     This  is  particularly  true  of  brick,  since  the  clays 
differ  greatly  in  quality,  and  also  since  a  slight  variation  in  each  step 
of  the  manufacture  materially  affects  the  result.     The  object  of 
testing  paving  brick  is  two-fold:   (1)  to  determine  whether  the  mate- 
rial is  suitable  for  use  in  a  pavement;  and  (2)  to  enable  comparisons 
to  be  mad©  between  different  classes  of  brick. 

Several  te'sts  formerly  employed  have  now  been  practically 
abandoned;  but  for  the  sake  of  completeness  these  will  be  briefly 
considered. 

944.  General  Appearance.    A  critical  examination  of  a  paving 
brick  by  the  experienced  eye  aided  by  a  hand  hammer  is  a  fair  method 
of  determining  the  relative  merits  of  different  bricks  of  a  particular 
kind;   but  unfortunately  experience  with  one  make  is  not  of  much 
value  with  brick  made  by  a  different  process  or  of  a  different  kind  of 
clay,  and  further  the  results  by  this  method  of  testing  admit  of  no 
numerical  evaluation  or  even  of  being  described  accurately.     It  is  a 
method  of  selecting  or  inspecting  rather  than  of  testing. 

The  brick  should  be  reasonably  straight;  and  have  flat  sides  and 
square  corners;  be  uniform  in  size,  texture  and  shape;  and  be  hard, 
tough,  and  evenly  burned.  If  the  edges  of  the  bricks  are  square, 


ART.    1]  THE   BRICK  487 

they  should  be  smooth  and  free  from  serrations  or  "  ragging,"  due 
to  friction  in  the  die.  If  the  edges  are  rounded,  the  radius  should 
not  exceed  three  sixteenths  of  an  inch.  Kiln  marks  or  impressions 
from  the  over-lying  brick  in  the  kiln  must  not  be  more  than  three 
sixteenths  of  an  inch  deep.  One  face  should  have  not  less  than  two 
nor  more  than  four  projections,  which  should  be  not  more  than  one 
fourth  nor  less  than  one  eighth  inch  high,  nor  exceed  one  half  square 
inch  in  area. 

When  broken  the  interior  of  the  brick  should  show  a  uniform 
fracture,  be  free  from  lime,  and  contain  no  uncrushed  or  lumpy 
material,  especially  if  such  material  is  not  united  by  vitrification 
with  the  remainder  of  the  material.  There  should  be  no  marked 
laminations. 

945.  Size.     The  brick  should  closely  conform  to  the  standard 
size  (§  941)  or  to  the  specified  size.     The  brick  from  any  one  man- 
ufacturer should  be  of  uniform  size;   and  the  brick  for  any  one  job 
should  be  of  practically  the  same  size.     The  usual  specifications  are 
as  follows:    "A  brick  shall  not  vary  from  standard  or  specified 
dimensions  more  than  J  an  inch  in  length,  nor  more  than  f  inch  in 
width  or  depth."     Sometimes  it  is  also  specified  that  "  the  bricks 
in  any  one  shipment  must  not  vary  in  width  or  depth  more  than 
|  of  an  inch." 

946.  Color.     The  color  is  no  criterion  of  the  value  of  a  paving 
brick,  when  comparing  bricks  of  various  makes;    but,  in  inspecting 
bricks  from  a  single  factory,  the  color  will  usually  furnish  a  fairly 
safe  guide  as  to  the  relative  hardness,  when  the  inspector  is  thor- 
oughly acquainted  with  the  particular  manufacture.     The  knowl- 
edge gained  regarding  the  relation  of  color  and  quality  in  inspecting 
one  make  of  brick,  however,  can  seldom  be  used  with  that  of  another 
make  from  a  different  locality,  as  clays  vary  greatly  in  kind  and 
degree  of  color.     The  popular  belief  is  that  hardness  is  proportional 
to  the  darkness  of  the  color  of  the  brick,  and  that  light  color  is  prima 
facie  evidence  of  softness.     As  a  rule  ,the  impure  fire  clays  make 
excellent  paving  material,  although  the  bricks  are  light   colored, 
usually  buff,  while  shale  bricks  are  reel  or  brown.     For  a  particular 
clay,  the  color  of  the  bricks  indicates  the  degree  of  heat  they  have 
received,  provided  they  were  burned  with  the  same  fuel  and  under 
the  same  conditions;   and  ordinarily  the  higher  the  heat  the  darker 
the  color,  and  presumably  the  better  the  brick.     The  uniformity 
of  the  color  of  the  interior  of  the  brick  is  more  important  than  the 
color  of  the  exterior. 


488  BRICK   PAVEMENTS  [CHAP.   XVII 

The  color  of  the  outside  of  the  brick  is  sometimes  valueless 
owing  to  the  sand  employed  to  prevent  sticking  in  the  kiln,  or  to 
the  effect  of  sulphur  in  the  coal  used  in  burning,  or  to  salt  glazing. 
Salt  glazing  is  a  trick  occasionally  employed  to  give  a  dark  gloss  to 
the  outside  which  is  very  attractive  to  the  superficial  observer, 
but  which  is  practically  worthless,  since  it  is  only  skin  deep  and 
soon  wears  off.  Salt  glazing  makes  it  more  difficult  to  detect  soft 
brick,  and  should  never  be  allowed  on  paving  brick. 

947.  Specific  Gravity.  In  a  general  way,  the  more  dense  a  brick 
the  harder  and  stronger  it  is;  and  consequently  early  in  the  history 
of  brick  testing  it  was  believed  that  a  knowledge  of  the  specific. 
gravity  would  be  of  value  in  judging  of  the  quality  of  a  paving 
brick.  It  is  now  known  that  the  specific  gravity  reveals  nothing 
not  determined  by  other  tests;  and  further  that  the  density  depends 
upon  the  character  of  the  clay,  the  kind  of  fuel,  etc.,  and  in  no  way 
measures  the  quality  of  the  product.  The  specific  gravity  may  be 
computed  by  the  formula  : 

Wa 
Specific  gravity  =  > 


in  which  Wa  represents  the  weight  of  the  dry  brick  in  air,  Ws  the 
weight  of  the  saturated  brick  in  air,  Wi  the  weight  of  the  brick 
immersed  in  water.  The  specific  gravity  of  shale  brick  ranges 
from  2.05  to  2.55,  and  usually  from  2.20  to  2.40;  and  that  of  brick 
made  from  impure  fire  clay  ranges  from  1.95  to  2.30,  and  generally 
from  2.10  to  2.25. 

948.  Crushing  Strength.  The  results  for  the  crushing  strength 
vary  more  with  the  details  of  the  method  employed  than  any  other 
test  of  paving  brick.  There  is  no  standard  method  of  making  this 
test.  For  experimental  data  showing  the  marked  effect  of  the  dif- 
ferent methods  of  testing,  see  the  author's  Treatise  on  Masonry 
Construction,  tenth  edition,  §  10-17,  and  §  78-81. 

Tests  on  cubes  cut  from  paving  brick  show  that  the  best  paving 
brick  have  a  crushing  strength  of  10,000  to  20,000  Ib.  per  square 
inch.  This  is  the  crushing  strength  when  the  load  is  applied  uni- 
formly over  the  surface  of  the  test  specimen;  but  if  the  pressure  is 
applied  to  only  a  small  part  of  the  upper  surface  of  a  brick,  the 
strength  will  be  much  greater.*  Any  brick  that  is  likely  to  be 

*  See  Baker's  Masonry  Construction,  tenth  edition,  §  657. 


ART.    1]  THE   BRICK  489 

accepted  for  paving  purposes  by  any  of  the  tests  hereafter  described, 
is  in  no  danger  of  being  crushed  by  the  pressure  of  the  wheel  of  a 
vehicle.  For  example,  the  surface  of  contact  between  a  wheel 
having  a  1^-inch  tire  loaded  with  half  a  ton  is  about  one  half  square 
inch,  which  gives  a  pressure  on  the  brick  of  only  about  2,000  Ib. 
per  square  inch. 

If  the  crushing  strength  could  be  easily  and  accurately  found, 
it  would  be  of  value  in  determining  the  relative  strength,  and  hence 
would  be  useful  in  comparing  the  quality  of  different  brick;  but 
owing  to  the  difficulty  of  making  the  experiments  and  to  the  uncer- 
tainty of  the  results,  the  test  has  been  abandoned. 

949.  Absorption  Test.     In  the  early  days  of  the  paving  brick 
industry,  many  of  the  brick  used  were  so  porous  and  brittle  that  it 
was  feared  they  would  be  disintegrated  by  the  action  of  frost;   and 
consequently  the  absorption  test  was  employed  to  eliminate  porous 
brick.     Subsequent  tests  by  repeatedly  freezing  and  thawing  paving 
bricks  showed  that  any  brick  which  was  likely  to  be  accepted  for 
paving  purposes  would  not  be  appreciably  injured  by  the  action 
of  frost.     There  are  probably  two  elements  that  prevent  frost  from 
seriously  injuring  even  a  soft  paving  brick;   viz.:    (1)  the  cushion- 
ing effect  of  the  air  remaining  in  the  pores  of  the  brick,  and  (2)  the 
strength  of  the  brick  may  be  greater  than  the  disrupting  effect  of 
the  frost.     Alternate  freezing  and  thawing  might  injure  a  non- 
vitrified  brick,  which  is  not  only  very  porous  but  is  also  deficient  in 
strength;    but  such  a  brick  would  be  rejected  for  paving  purposes 
as  the  result  of  a  casual  inspection.    The  absorption  test  is  no 
longer  regarded  of  importance  as  measuring  the  ability  of  the  brick 
to  resist  freezing  and  thawing. 

Different  bricks  vary  widely  in  their  rate  of  absorption.  For 
example,  one  brick  absorbed  in  one  day  80  per  cent  of  its  total 
amount,  while  another  absorbed  only  8.7  per  cent;  and  two  other 
specimens  absorbed  71.8  and  19.5  per  cent  respectively  in  the  same 
time.  The  absorption  of  whole  brick  is  slightly  less  than  that  of 
half  brick,  and  the  absorption  of  half  brick  is  considerably  less 
than  that  of  small  chips.  For  the  above  reasons  and  for  other 
minor  ones,  results  for  the  absorptive  power  are  likely  to  be  untrust- 
worthy. 

950.  Transverse  Strength.     This  is  determined  by  resting  the 
brick  upon  two  knife-edges  and  applying  a  steady  pressure  on  the 
upper  side  of  the  brick  through  a  third  knife-edge  placed  midway 
between  the  other  two.     The  results  are  expressed  in  terms  of  the 


490  BKICK   PAVEMENTS  [CHAP.    XVII 

modulus  of  rupture,  which  is  computed  by  the  following  formula: 

3JFJ 
~26tP 

in  which  R  represents  the  modulus  of  rupture  in  pounds  per  square 
inch,  W  the  breaking  load  in  pounds,  I  the  distance  between  sup- 
ports in  inches,  b  the  breadth  of  the  brick  in  inches,  and  d  the  depth 
of  the  brick  in  inches.  The  brick  may  be  tested  edgewise  or  flat- 
wise, although  the  former  is  usually  the  better  method,  since  then 
W  is  larger.  The  knife-edges  should  be  rounded  transversely  to 
a  radius  of  about  one  sixteenth  of  an  inch  and  longitudinally  to  a 
radius  of  about  12  inches,  to  secure  better  contact  and  to  prevent 
the  brick  from  being  crushed  at  the  edges.  Some  authorities  rec- 
.ommend  grinding  opposite  edges  of  the  brick  to  parallel  planes, 
but  this  is  a  useless  expense.  If  the  brick  is  warped,  the  contact 
between  the  brick  and  the  knife-edges  can  easily  be  made  entirely 
satisfactory  by  placing  pieces  of  metal  under  the  blocks  carrying 
the  lower  knife-edges,  or  by  shifting  the  brick  longitudinally,  or  by 
turning  it. 

The  modulus  of  rupture  of  bricks  that  have  given  excellent 
service  in  a  pavement  varies  from  1,500  to  3,500  Ib.  per  square  inch, 
usually  from  2,000  to  3,000.  Owing  to  apparently  unavoidable 
variations  in  the  structure  of  the  brick,  it  is  not  possible  to  attain 
closely  concordant  results  in  making  this  test;  and  with  the  utmost 
care  in  selecting  the  brick  and  in  making  the  tests,  the  variation 
from  the  mean  ranges  from  8  to  30  per  cent,  and  on  the  average  is 
about  20  per  cent. 

The  cross-breaking  test  furnishes  a  means  of  comparing  the 
toughness  of  various  kinds  of .  paving  brick.  The  uniformity  of 
the  results  for  any  particular  kind  of  brick  indicates  its  structural 
soundness,  freedom  from  air  checks,  etc.,  and  shows  whether  the 
material  has  been  properly  treated  in  the  various  stages  of  manu- 
facture. The  transverse  strength  indicates  the  resistance  of  the 
brick  to  cross  breaking  when  laid  in  the  pavement  on  an  unyield- 
ing and  uneven  surface;  but  this  element  is  not  entitled  to  much 
consideration,  since  brick  are  seldom  thus  broken  in  the  pave- 
ment, at  least  not  until  nearly  worn  out. 

The  test  is  comparatively  easy  to  make,  and  is  a  valuable  check 
upon  the  rattler  test  (§951). 

951.  Rattler  Test.  This  test  is  made  by  rolling  or  tumbling  the 
bricks  in  a  foundry  rattler,  i.  e.,  a  revolving  cast-iron  barrel;  and  it 


ART.    1]  THE    BRICK  491 

greatly  exceeds  in  importance  all  the  other  tests  combined.  It 
imitates  more  closely  than  any  other,  the  impact  due  to  the  horse's 
hoofs  and  shoes,  and  to  the  bumping  of  the  vehicle  wheels,  and  also 
the  abrasion  due  to  the  slipping  of  the  horse's  feet  and  the  sliding 
of  the  wheels.  This  test  could  with  propriety  be  called  an  impact 
and  abrasion  test.  The  result  of  the  test  is  jointly  dependent  upon 
the  toughness  of  the  brick — its  ability  to  resist  shock, — and  its 
hardness — its  ability  to  resist  abrasion. 

To  make  this  test  of  any  scientific  value,  it  is  necessary  to  have 
some  standard  method  of  conducting  the  experiments.  Several 
methods  of  standardizing  this  test  have  been  proposed.  The  first 
test  was  made  by  the  author.*  Brick  that  had  seen  service  in  a  pave- 
ment and  pieces  of  well-known  natural  stones  used  for  paving  pur- 
poses, together  with  small  pieces  of  scrap  cast  iron,  were  rolled  in  a 
rattler.  Shortly  after  being  proposed,  this  method  was  quite  widely 
adopted;  but  it  did  not  give  satisfactory  results,  chiefly  because  the 
original  experiments  were  made  with  a  rattler  having  wooden  staves, 
while  subsequent  tests  were  made  with  rattlers  having  cast-iron 
staves.  The  method  was  objectionable  on  account  of  the  trouble 
and  expense  of  preparing  the  test  pieces  of  natural  stone.  Later 
each  of  four  radical  modifications  of  the  test  gained  prominence  in 
succession  for  a  time.  Finally  it  was  found  that  seemingly  unim- 
portant details  materially  affected  the  results,  as,  for  example,  the 
chemical  composition  of  the  cast  iron  in  the  staves  and  abrading 
material,  the  stiffness  of  the  staves,  the  frequency  of  renewal  of  the 
staves  and  abrading  material,  the  speed  of  rotation,  the  method  of 
driving  the  rattler,  etc. 

952.  In  1910  after  a  very  elaborate  series  of  tests,  the  National 
Paving  Brick  Manufacturers  Association  proposed  specifications 
which  set  forth  in  great  detail  the  method  of  constructing  and  using 
the  rattler;  and  in  1915  substantially  these  specifications  were 
adopted  by  the  American  Society  for  Testing  Materials,  and  they 
have  been  generally  accepted  as  the  standard,  f 

Fig.  172,  page  492,  shows  the  standard  rattler  and  abrasive  mate- 
rial. The  latter  consists  of  two  sizes  of  cast  iron  spheres,  the  larger 
weighing  7.5  Ib.  each  and  the  smaller  0.95  Ib.  The  total  abrasive 

*  Durability  of  Paving  Brick,  by  Ira  O.  Baker,  pp.  46,  5"  X  8".  T.  A.  Randall  &  Co., 
Indianapolis,  Ind.,  1891.  Out  of  Print. 

t  Proc.  Amer.  Soc.  for  Testing  Materials,  Vol.  XV,  Year  Book  1915,  Report  of  Committee 
C-3,  pp.  396-407.  Copies  of  complete  specifications  for  the  inspection  and  testing  of  paving 
brick  may  be  had  by  addressing  Secretary  Amer.  Soc.  for  Testing  Materials,  Philadelphia.  Pa., 
or  Secretary  National  Paving  Brick  Mfrs.  Assoc.,  Cleveland,  Ohio. 


492 


BRICK    PAVEMENTS 


[CHAP,  xvii 


charge  consists  of  10  large  spheres  and  245  to  260  small  ones,  the 
collective  weight  being  as  nearly  as  possible  300  Ib. 


FIG.  172. — STANDARD  BRICK  RATTLER. 

In  consulting  the  literature  concerning  tests  of  paving  brick, 
it  is  necessary  to  carefully  distinguish  between  the  present  and  the 
former  standard.  The  latter  gives  trie  smaller  loss. 

953.  Making  the  Test.     To  make  the  rattler  test,  the  bricks  are 
thoroughly  dried,  weighed,  placed  in  the  rattler,  and  turned  1,800 
revolutions  at   a  speed   of  30  revolutions  per  minute,   and   then 
weighed.     The  percentage  of  loss  indicates  the  quality  of  the  brick. 

Fig.  173  shows  the  brick  charge  before  and  after  testing.  Ten 
brick  of  the  so-called  block-size  constitute  a  charge. 

954.  The  object  of  the  rattler  test  is  twofold,  viz.:   (1)  to  deter- 
mine whether  the  bricks  are  tough  enough  for  use  in  a  pavement, 
and  (2)  to  determine  whether  the  material  is  uniform  in  quality. 
The  first  is  determined  by  the  average  loss  of  a  charge,  and  the  sec- 
ond by  the  uniformity  of  loss  of  the  several  bricks.     Uniformity 
of  wear  is  an  important  quality,  for  a  single  soft  brick  may  wear 
so  as  to  make  a  hole  in  the  pavement,  and  then  each  passing  wheel 
will  rapidly  destroy  adjacent  bricks  even  though  they  themselves 
are  of  excellent  quality. 

To  determine  the  uniformity  of  wear,  the  rattler  test  should  be 


ART.    1] 


THE   BRICK 


493 


so  conducted  as  to  find  the  loss  of  each  brick.     This  requires  the 
marking  of  the  bricks  so  they  can  be  identified  after  being  tested. 


FIG.  173. — BRICK  CHARGE  BEFORE  AND  AFTER  TESTING. 

955.  Marking  the  Brick.  There  are  several  schemes  in  use  for 
marking  the  several  bricks  of  a  charge. 

The  following  method  is  used  by  William  H.  Howell,  Engineer 
of  Streets  and  Highways,  Newark,  N.  J.*  "The  holes  may  be 
made  with  a  small  cold  chisel,  after  a  little  experience,  in  twenty 
to  twenty-five  minutes."  Fifteen  holes  are  required. 

1.  One  drill  hole  on  one  side. 

2.  One  drill  hole  on  one  edge. 

3.  One  drill  hole  on  each  side. 

4.  One  drill  hole  on  each  edge. 

5.  One  drill  hole  on  one  end. 

6.  One  drill  hole  on  each  end. 

7.  Two  drill  holes  on  one  side. 

8.  Two  drill  holes  on  one  edge. 

9.  One  drill  hole  each  on  one  edge  and  one  end. 
10.  Blank. 

The  method  shown  graphically  in  Fig.  174,  page  494,  was  pro- 
posed by  C.  A.  Baughman,  Instructor  in  Civil  Engineering,  Iowa 
State  College. f  The  holes  are  made  with  a  small  diamond  drill, 

*  Proc.  Amer.  Soc.  of  Municipal  Improvements,  1911,  p.  95. 
t  Engineering  and  Contracting,  Vol.  44  (1915),  p.  470. 


4S4 


BRICK   PAVEMENTS 


ICHAP.  xvn 


and  are  about  one  eighth  of  an  inch  deep.     Eighteen  holes  are 
required. 


o 

o 

o 

o 

7 


0 

0 

FIG.   174. — BAUGHMAN'S  METHOD  OF  MARKING  BRICK. 

Fig.  175  shows  the  method  proposed  by  Mr.  B.  L.  Bowling, 
Assistant  in  Road  Laboratory,  University  of  Illinois,  for  marking 
wire-cut  lug  brick.  Brick  No.  10  is  not  marked.  Notice  that  only 
nine  holes  are  required.  The  holes  are  one  fourth  of  an  inch  in 
diameter,  and  one  fourth  of  an  inch  deep;  and  can  be  made  with  a 
compressed-air  percussion  drill  in  about  forty  minutes. 


J/c/e 


Edge 


Fio.  175. — BOWLING'S  METHOD  OP  MARKING  WIRE-CUT  LUG  BRICK. 

Of  course,  determining  the  loss  of  each  brick  in  the  charge  re- 
quires extra  time  in  marking  and  weighing;  but  it  is  believed  that 
the  additional  cost  is  abundantly  justified.  It  is  sometimes  claimed 
that  the  brick  can  not  be  marked  so  as  to  identify  them  after  the 
test  without  weakening  them  and  increasing  the  loss  in  the  rattler; 


ART.    1]  THE   BRICK  495 

but  it  has  been  proved  that  this  is  not  true  to  an  appreciable  extent 
in  any  of  the  three  methods  of  marking  mentioned  above. 

956.  Limit  of  Loss.     The  standard  specifications  do  not  pre- 
scribe any  limit  for  the  permissible  loss;    but  distinctly  state,  that 
such  limit  shall  be  determined  by  the  contracting  parties.     The 
standard  specifications  give  "  the  following  scale  of  losses  to  show 
what  may  be  expected  of  tests  executed  under  the  foregoing  speci- 
fications : 

For  bricks  suitable  for  heavy  traffic 22  to  24  per  cent. 

For  bricks  suitable  for  medium  traffic 24  to  26  per  cent. 

For  bricks  suitable  for  light  traffic 26  to  28  per  cent. 

"Which  of  these  grades  should  be  specified  in  any  given  district  and  for  any 
given  purpose,  is  a  matter  wholly  within  the  province  of  the  buyer;  and  should  be 
governed  by  the  kind  and  amount  of  traffic  to  be  carried,  and  the  quality  of  paving 
bricks  available." 

957.  The  limit  that  should  be  specified  for  the  average  loss  in 
any  particular  case  will  depend  upon  the  following:    (1)  the  traffic 
to  be  carried,  (2)  the  ordinary  quality  of  the  brick  available,  (3) 
the  expense  to  be  incurred  in  culling  or  selecting  the  brick,  (4)  the 
size  of  the  brick  or  block,  (5)  the  form  of  the  edge  of  the  brick,  (6) 
the  minimum  dimension  of  the  brick,  (7)  the  uniformity  of  the  loss. 

1.  The  amount  and  character  of  the  travel  may  be  such  as  to 
make  it  unwise  to  specify  the  highest  grade  of  brick. 

2.  The  locality  may  be  such  that  the  ordinary  paving  bricks  are  of 
a  high  quality,  and  hence  no  appreciable  increase  of  expense  will  be 
incurred  by  requiring  a  high  grade  of  brick,  i.  e.,  a  low  rattler  loss. 
For  example,  in  a  certain  year  the  average  loss  of  all  the  bricks  sub- 
mitted to  a  testing  laboratory  in  an  eastern  state  was  18  per  cent, 
and  several  lots  of  each  of  three  brands  gave  an  average  loss  of 
only  14.3  per  cent;   but  on  the  other  hand,  all  the  bricks  submitted 
in  a  year  to  a  laboratory  in  a  western  state  gave  an  average  loss  of 
22.56  per  cent,  with  only  three  charges  having  a  loss  less  than  17 
per   cent.     In  some    localities,    specifying    a  small   loss  may  limit 
competition  and  thus  increase  the  price  of  the  bricks. 

3.  If  the  bricks  available  are  not  uniformly  good,  and  if  the 
service  required  of  the  proposed  pavement  is  severe,  it  may  be  wise 
to  specify  a  quality  which  will  require  careful  selection  and  possibly 
include  only  a  comparatively  small  percentage  of  the  kiln  run.     Of 
course,  the  last  method  is  expensive,  because  of  the  cost  of  culling, 
and  also  because  the  better  bricks  should  bear  part  of  the  possible 
loss  on  the  rejected  bricks. 


496  BRICK   PAVEMENTS  [CHAP.    XVII 

4.  The  limit  to  be  set  for  the  loss  depends  upon  the  size  of  the 
bricks  or  blocks,  i.  e.,  whether  the  pavement  is  to  be  built  of  bricks 
or  blocks.     However,  as  but  few  bricks  are  now  used  in  pavements, 
this  phase  of  the  subject  is  not  important.     Apparently  the  relative 
loss  of  bricks  and  blocks  has  not  been  determined  with  the  1910 
standard  rattler;   but  in  view  of    data  obtained  with  the   former 
standard,  some  authorities  permit  a  differential  of  2  per  cent  in  favor 
of  brick  in  comparison  with  block. 

5.  The  limit  varies  also  with  the  form  of  the  edge  or  corner  of 
the  brick.     If  a  brick  has  square  corners,  it  will  lose  more  in  the 
rattler  than  one  having  rounded  corners.     The  standard  re-pressed 
brick  has  corners  of  a  j%-inch  radius,  and  the  absent  corner  represents 
about  If  per  cent  of  the  volume.      Therefore  a   square-cornered 
brick  could  lose  If  per  cent  in  the  rattler  before  being  on  a  par  with 
a  standard  round-edge    re-pressed   brick.     For    this   reason,  some 
claim  that  the  former  should  be  allowed  1  or  2  per  cent  greater 
rattler  loss  than  the  latter.     The   Illinois   Highway   Commission 
allows  the  standard  wire-cut  lug  brick  a  differential  of  1  per  cent 
over  the  standard  re-pressed  brick. 

6.  The  limit  should  depend  upon  the  minimum  dimension  of  the 
brick.     Since  the  general  use  of  a  concrete  foundation  for  brick 
pavements,  and  particularly   since  the  introduction  of  the  semi- 
monolithic  and  the  monolithic  construction,  there  has  been  a  marked 
tendency  to  use  a  shallower  brick.     Formerly  paving  brick  were 
quite  uniformly  4  inches  deep;    but  now,  bricks  of  various  depths 
are  being  used,  3J,  3,  2}  inches  (§  1028).     The  prescribed  limit  of 
rattler  loss  should  be  less  the  thinner  the  brick.     The  loss  in  the 
rattler  is  mainly  due  to  the  edges  being  worn  or  broken  off  (Fig. 
173,  page  493).     The  central  portion  of  a  brick  loses  almost  nothing 
except  at  its  edges  or  corners.     A  brick  3JX4X8J  inches  has  64 
inches  of  edges  or  corners,  while  a  brick  3J  X3  X8J  inches  has  only 
60  inches  of  edges,  or  6J  per  cent  less.     Apparently  then  for  this 
reason  a  difference  should  be  made  in  the  limiting  loss  between  a 
4-inch  and  a  3-inch  brick.     Further,  it  is  claimed  that  the  form  of 
the  rattler  test  is  unjust  to  a  brick  thinner  than  the  3|X4  XSj-inch 
standard,  since  if  a  thin  brick  becomes  bridged  in  the  rattler  it  is 
much  more  likely  to  be  broken  than  a  thicker  one.     The  Ohio  High- 
way Department   takes   account    of  these   two    factors,   at  least 
approximately,  as  follows:    It  specifies  a  loss  for  standard  4-inch 
brick  of  not  more  than  22  per  cent,  and  then  inserts  the  following 
clause  in  its  standard  specifications:   "  If  other  than  standard  sized 


ART.    1J  THE   BRICK  497 

blocks  are  required  by  the  plans  or  specifications,  the  average  rattler 
loss  allowed  shall  be  22  per  cent  multiplied  by  the  ratio  of  the  volume 
of  the  standard  block  to  the  volume  of  the  block  specified,  less  2 
per  cent."  For  another  method  of  testing  3-inch  brick,  see  the 
second  paragraph  of  §  960. 

7.  The  limit  depends  also  upon  the  uniformity  of  loss  of  the  sev- 
eral bricks  of  a  charge.  A  low  rattler  loss  with  a  wide  range  will 
probably  not  give  as  durable  a  pavement  as  a  larger  average  loss 
with  a  narrower  range.  A  single  soft  or  brittle  brick  will  soon  wear 
below  those  adjacent  to  it,  and  then  each  passing  wheel,  particularly 
a  steel-tired  one,  in  dropping  into  the  depression  chips  and  crushes 
the  adjoining  bricks  (however  good  they  are)  and  tends  to  destroy 
the  pavement.  The  Illinois  State  Highway  Department  recog- 
nizes this  principle,  and  specifies  that  the  average  loss  of  wire-cut 
lug  bricks  may  be  23  per  cent  provided  the  individual  bricks  have 
losses  between  17  and  27,  or  25  per  cent  provided  the  individual  losses 
are  between  20  and  28,  or  27  per  cent  provided  the  individual  losses 
are  between  23  and  29.  The  degree  of  uniformity  of  the  rattler  loss 
depends  upon  the  quality  of  the  bricks,  but  chiefly  upon  the  care 
and  skill  employed  in  culling  the  brick  at  the  kiln.  For  data  on 
the  degree  of  uniformity  obtained  in  practice,  see  §  959. 

958.  Loss  Found  in  Laboratory.  It  is  presumable  that  the  pav- 
ing brick  sent  to  a  laboratory  to  be  tested  are  usually  samples  pro- 
posed for  use  in  a  pavement;  and  hence  the  average  losses  are  in- 
structive as  showing  the  quality  of  the  material  available  in  that 
locality,  and  the  range  of  loss  in  any  charge  is  evidence  of  the  skill 
employed  in  culling  the  brick  at  the  kiln.  Sometimes  the  sample 
may  be  sent  to  obtain  for  the  manufacturer  information  concerning 
some  point  in  manufacture;  but  usually  the  manufacturer  will 
make  such  tests  at  home,  and  hence  the  samples  tested  at  a  public 
laboratory  may  be  considered  fairly  representative.  However, 
the  results  obtained  in  any  state  or  city  laboratory  will  depend 
somewhat  upon  the  maximum  rattler  loss  permitted  by  the  official 
specifications  of  that  state  or  city.  For  example,  if  one  state  or  city 
specifies  a  lower  rattler  loss  than  another  state  or  city,  manufac- 
turers when  shipping  brick  for  work  under  the  former  specifications 
are  likely  to  select  a  better  quality  or  cull  the  bricks  more  carefully 
than  when  shipping  to  the  other  state  or  city. 

In  1916  the  Illinois  Highway  Department  tested  59  charges  of 
re-pressed  blocks,  which  gave  an  average  loss  of  20.9  per  cent;  and 
71  charges  of  wire-cut  lug  brick,  which  gave  an  average  loss  of  19.9 


498  teRicK  PAVEMENTS  [CHAP,  xvn 

per  cent.*  The  three  smallest  losses  for  re-pressed  brick  were  16.8, 
16.8,  and  17.0  per  cent;  and  the  three  smallest  for  wire-cut  lug  brick 
were  17.4,  17.4,  and  17.5  per  cent.  The  three  least  ranges  in  loss  of 
the  individual  bricks  in  a  charge  were:  for  re-pressed  brick  3.5, 
3.8,  and  3.8  per  cent;  and  for  wire-cut  lug  brick  4.0,  5.7,  and  6.0 
per  cent.  The  three  greatest  ranges  in  loss  of  individual  bricks 
were:  for  re-pressed  bricks  25.1,  27.8,  and  31.1  per  cent,  and  for  wire- 
cut  lug  bricks  23.9,  24.1,  and  26.2  per  cent.  The  last  results  simply 
show  that  probably  some  of  the  samples  were  not  carefully  culled, 
although  it  is  well  known  that  it  is  sometimes  practically  impos- 
sible by  appearance  alone  to  eliminate  all  the  poor  brick. 

In  1916,  Vermilion  County,  Illinois,  tested  124  charges  of  blocks 
which  were  practically  all  wire-cut  lug  brick  from  a  local  plant.  The 
average  loss  of  all  was  18.06  per  cent.  One  charge  had  a  loss  be- 
tween 14  and  15  per  cent,  7  between  15  and  16,  27  between  16  and 
17,  31  between  17  and  18,  and  25  between  18  and  19.  The  average 
loss  of  the  brick  used  in  8  miles  of  rural  road  was  17J  per  cent.  The 
range  in  the  five  best  lots  was  3.5,  3.7,  3.7,  4.1,  and  4.3  per  cent,  and 
in  the  three  worst  was  21.1,  29.2,  and  31.5  per  cent.f 

The  Iowa  Engineering  Experiment  Station  in  1916  tested  85 
different  charges  writh  an  average  loss  of  22.56  per  cent.  The  three 
smallest  were:  16.31,  16.78,  and  16.80.  Twelve  charges  of  re- 
pressed brick  gave  an  average  loss  of  19.44;  and  33  charges  "  that 
were  not  re-pressed"  gave  an  average  loss  of  24.06  per  cent,  and 
omitting  four  of  the  largest  the  average  is  22.33  per  cent.t  The 
kind  of  brick  in*  the  remainder  of  the  charges  is  not  known;  and  the 
results  for  individual  bricks  are  not  known. 

In  1917  the  Ohio  Highway  Commission  tested  302  lots  of  23 
different  brands  of  blocks  4  inches  deep,  the  average  loss  of  all  brands 
being  21.14  per  cent.  The  three  smallest  average  losses  were  17.47, 
18.98,  and  19.34;  and  the  three  largest  were  23.95,  25.08,  and  27.84. 
The  range  of  average  losses  for  the  three  brands  having  the  smallest 
losses  were  respectively:  3.20  for  four  lots;  5.96  for  26  lots,  and 
10.04  for  14  lots.  Apparently,  no  results  were  obtained  for  individual 
bricks.  § 

In  one  year  the  Maryland  Roads  Commission  tested  19  charges  of 
six  different  brands  of  blocks,  the  average  being  18.7  per  cent.  Four 

*  Data  from  F.  L.  Roman,  Engineer  of  Tests. 

t  Data  from  P.  C.  McArdle,  Superintending  Engineer,  Danville,  111. 

J  Data  from  R.  W.  Crum,  in  charge  of  testing. 

§  Data  from  A.  S.  Rea,  Engineer  ofl  Tests. 


ART.    1J  THE   BRICK  499 

charges  of  one  brand  average  14.5  and  three  charges  of  another  14.3 
per  cent.  In  another  year  the  Commission  tested  12  charges  of  one 
brand  which  averaged  20.83  per  cent,  one  lot  was  rejected,  and  the 
average  for  the  11  lots  accepted  was  20.5  per  cent.* 

The  New  York  State  Highway  Commission  in  the  past  few  years 
has  tested  many  paving  blocks,  in  one  year  making  exactly  400 
duplicate  tests.  For  1914-17  the  average  loss  for  wire-cut  lug 
blocks  was  21.3  per  cent,  and  for  re-pressed  20.9  per  cent.  The 
average  loss  in  a  duplicate  test  usually  varied  from  about  17  to  24 
per  cent,  with  a  few  as  low  as  15  and  a  few  as  high  as  27  per  cent.f 
In  consideration  of  the  way  in  which  the  samples  were  obtained  it  is 
not  permissible  to  attempt  to  draw  any  conclusions  from  these  data 
as  to  the  relative  quality  of  re-pressed  and  wire-cut  lug  brick.  The 
results  seem  to  show  a  slight  improvement  in  quality  from  year  to 
year;  and  the  results  from  several  other  laboratories  agree  with  this 
conclusion. 

959.  Loss  Allowed  in  Practice.    The  classification  of  losses  sug- 
gested by  the  American  Society  for  Testing  Materials  is  stated  in 
§  956.     These  limits  have  been  adopted  by  many  cities.     The  rat- 
tler loss  allowed  by  the  Illinois  Division  of  Highways  for  wire-cut 
lug  brick  is   stated   in  the   previous   section;   and   the  permissible 
loss  for  round-edged  re-pressed  brick  is  1  per  cent  less  in  each  case. 

Vermilion  County,  Illinois,  in  1916,  built  8  miles  of  monolithic 
brick  rural  roads,  and  specified  that  the  average  loss  of  a  charge 
should  not  exceed  23  per  cent,  and  that  no  single  brick  should  exceed 
27  per  cent.  The  general  average  loss  of  the  brick  used  was  17J 
per  cent.{  For  a  summary  of  all  the  rattler  tests  made,  see  the  third 
paragraph  of  §  958. 

The  Ohio  Highway  Department  specifies  that  the  average  loss 
of  standard  brick  shall  not  exceed  22  per  cent;  and  that  the  range 
shall  not  exceed  8  per  cent.  For  brick  thinner  than  the  standard,  a 
differential  is  allowed  as  stated  in  paragraph  6  of  §  957. 

The  New  York  State  Highway  Department  specifies  a  maximum 
average  loss  of  24  per  cent. 

960.  Changes  in  Test  Proposed.     In  the  early  history  of  brick 
pavements  the  joints  were  usually  filled  with  sand,  and  hence  the 
wear  of  the  brick  in  the  pavement  was  due  largely  to  impact;   and 
therefore  a  form  of  rattler  test  was  adopted  in  which  the  wear  was 

*  Data  from  H.  G.  Shirley,  Chief  Engineer. 

t  Data  from  H.  E.  Breed,  First  Deputy  Commissioner. 

t  Engineering  Record,  Vol.  74  (1916),  p.  678. 


500  BEICK  PAVEMENTS  [CHAP.   XVII 

largely  due  to  impact.  But  now  the  joints  of  brick  pavements  are 
usually  filled  with  grout  or  at  least  a  comparative  hard  bituminous 
cement;  and  hence  the  wear  on  the  brick  in  the  pavement  is  mainly 
abrasion.  Therefore  some  engineers  claim  that  the  present  standard 
rattler  test  does  not  reasonably  well  represent  the  conditions  in  the 
pavement.  Consequently  several  changes  have  been  proposed  in 
the  method  of  testing  paving  brick. 

The  City  of  Baltimore  has  modified  the  standard  rattler  test 
by  leaving  out  the  large  balls  and  replacing  them  by  an  equal  weight 
of  the  small  spheres.  This  change  was  made  because  it  was  believed 
that  the  7J-lb.  spheres  were  unduly  severe  on  brick  thinner  than 
the  standard  paving  block.  The  City  of  Baltimore  allows  3-inch 
brick  when  tested  in  this  way  a  differential  of  1J  per  cent  over  a  4- 
inch  brick.  This  differential  was  arrived  at  by  measuring  the  loss 
of  the  middle  inch  in  a  4-inch  brick,  somewhat  as  described  in  para- 
graph 6  of  §  957.  This  introduces  a  new  method  of  testing,  which 
is  unfortunate  since  it  makes  confusion  and  limits  comparisons.  It 
is  unfortunate  that  a  differential  for  the  3-inch  brick  was  not  adopted 
for  temporary  use  until  the  propriety  of  the  present  standard  rattler- 
test  for  thinner  brick  could  be  fully  investigated.  Baltimore  and 
the  State  Highway  Departments  of  both  New  York  and  Pennsyl- 
vania are  making  comparative  tests  of  4-inch  and  3-inch  brick  by  this 
method;  but  at  present  there  are  insufficient  data  to  warrant  any 
definite  conclusions. 

Some  competent  authorities  claim  most  of  the  preceding  diffi- 
culties would  be  met  and  more  equitable  results  would  be  obtained, 
if  the  brick  were  tested  by  the  standard  rattler  test  using  the  same 
number  of  bricks  regardless  of  their  size,  and  were  then  compared 
by  their  absolute  loss  in  weight  rather  than  by  the  per  cent  of  their 
losses. 

Attempts  have  been  made  to  test  paving  brick  by  a  sand  blast;* 
but  not  much  progress  has  been  made. 

St.  Louis  has  discarded  the  rattler  test,  and  trusts  to  comparing 
the  brick  on  the  street  with  standard  samples  previously  selected,  f 
It  has  not  been  proved  that  this  method  is  not  subject  to  more 
objections  than  the  rattler  test. 

361.  SERVICE  TESTS.  The  relationship  between  the  loss  in 
the  rattler  and  the  service  in  the  pavement  has  not  been  indisput- 

*  Trans.  Am.  Soc.  Test.  Mat.,  Vol.  16  (1914),  Part  II,  p.  557-64;   or  an  abstract  of  the  same, 
Engineering  Record,  Vol.  70  (1914),  p.  215. 
t  Engineering  Record,  Vol.  72  (1915),  p.  200. 


ART.    1]  THE   BRICK  501 

ably  established.  From  time  to  time  several  experiments  have 
been  undertaken  to  determine  the  relative  qualities  of  different 
grades  of  paving  bricks  by  actual  service  in  the  pavement.  The 
experiment  consists  in  making  a  standard  rattler  test  of  different 
grades  of  paving  blocks,  and  then  laying  short  sections  of  pavement 
with  each  of  the  several  kinds.  For  one  reason  or  another,  all  of 
these  experiments,  except  the  one  mentioned  in  §  962,  have  failed 
to  give  a  conclusive  result. 

For  example,  the  first  of  these  experimental  sections  was  laid 
in  May,  1898,  in  Detroit,  Michigan.  Transverse  strips  of  fourteen 
kinds  of  brick  were  laid  in  a  distance  of  222  feet.  The  blocks  were 
tested  by  a  former  N.  B.  M.  A.  standard  rattler  test.  A  comparison 
between  the  results  of  the  rattler  tests  and  a  general  observation  of 
the  effect  of  three  years'  wear  in  the  pavement  failed  to  show  any 
close  agreement  between  the  rattler  test  and  service  in  the  pave- 
ment. But  this  test  was  not  conclusive,  because  it  was  later  found 
the  rattler  test  used  failed  to  discriminate  between  the  good  and  the 
bad  brick,  and  it  was  for  this  reason  abandoned. 

962.  In  1912-13  the  Office  of  Public  Roads  and  Rural  Engineering 
of  the  U.  S.  Department  of  Agriculture  directed  the  construction 
of  an  experimental  section  of  road  upon  an  extension  of  Connecticut 
Avenue  known  as  Kensington  Road,  near  Chevy  Chase,  Maryland. 
The  improvement  was  6,195  feet  long,  and  was  divided  into  six 
sections  each  having  a  different  road  surface.  Two  sections  had  a 
surface  of  bituminous  concrete,  two  oil-cement  concrete,  one  hy- 
draulic-cement concrete,  and  one  (the  one  nearest  Chevy  Chase) 
vitrified  brick.  The  details  of  the  design  and  construction  of  the 
several  sections  are  described  in  the  following  publications :  Circulars 
No.  98  and  99  of  the  Office  of  Public  Roads;  Bulletins  No.  105 
(1914),  257  (1915),  and  407  (1916)  of  the  U.  S.  Department  of  Agri- 
culture. Each  year  an  examination  is  made  of  the  condition  of  the 
several  sections,  and  a  report  of  the  same  is  published;  and  doubtless 
this  practice  will  be  continued  for  a  number  of  years.  A  census 
of  the  travel  on  the  road  is  taken  for  twenty-four  hours  on  every 
thirteenth  day. 

The  following  are  the  particulars  for  the  brick-paved  section. 
The  brick  pavement  is  18  feet  wide,  and  980  feet  long.  The  con- 
crete base  is  6  inches  thick,  and  the  proportions  are  1:3:7,  the 
coarse  aggegate  being  gravel  (pebbles).  The  bedding  course  is  2 
inches  of  sand.  Fourteen  varieties  of  paving  blocks  were  laid. 
Table  55  shows  the  character  of  the  blocks.  The  pavement 


502 


BRICK   PAVEMENTS 


CHAP.   XVII 


was  rolled  with  a  5-ton  tandem  roller;  and  then  the  joints  were 
filled  with  a  1  :  1  portland-cement  grout.  The  depth  of  the  bricks 
constituting  two  courses  of  each  variety  were  measured,  and  the 
location  of  these  courses  were  recorded,  so  that  in  the  future  these 
brick  may  be  taken  up  and  measured,  and  the  amount  of  wear  thus 
be  determined.  The  average  traffic  one-way  on  half  of  the  road 
during  1915  consisted  chiefly  of  56  horse-drawn  wagons  and  342 
motor-driven  cars. 

TABLE  55 
CHARACTERISTICS  OF  PAVING  BLOCKS  ON  CHEVY  CHASE  ROAD 


Ref. 
No. 

Description  of  Blocks. 

Absorp- 
tion — 
Average 
of  Five 
Tests. 

Loss  in 
Rattler,  — 
A  verage 
of  Three 
Tests. 

1 

2 
3 
4 
5 
6 
7 
8 
9 
10 

11 
12 
13 
14 

Shale  wire-cut  lug,  hard  burned  

1.39% 
1.31 
0.88 
1.65 
1.10 
1.81 
2.29 
3.74 
2.86 

1.56 
2.38 
4.04 
3.73 
3.68 

21.12% 
16.36 
25.57* 
17.67 
22.04* 
18.80 
27.92f 
22.68 
22.59f 

19.11 

37.68f 
38.89f 
24.31f 
31.19 

"           "        "     medium  hard  burned  
Shale,  re-pressed,  well  vitrified  
hard  burned,  coarsely  ground  
"                      very  hard  burned. 

coarsely  ground  
"                      medium  hard  burned,  even  wear.  .  .  . 
finely  ground, 
coarsely    " 
Fire-clay,  re-pressed,  medium  hard  burned,  coarsely 
ground.                            

Fire-clay,  repressed,  soft  burned,  coarsely  ground  .... 
Shale,  re-pressed,  soft  burned,  coarsely  ground  
Fire-clay,  re-pressed,  soft  burned,  finely  ground 

"         wire-cut  lug,  hard  burned,  laminated  

• 

*  Loss  due  mainly  to  chipping. 


t  Uniform. 


Three  annual  inspections  have  been  made  of  the  above  sections 
of  brick  paving  and  each  time  the  conclusion  is  that  there  is  no 
appreciable  difference  in  wear  between  the  several  varieties  of  brick. 
Apparently  time  enough  has  not  elapsed  to  justify  any  trustworthy 
conclusion,  since  the  wear  has  been  so  slight  as  to  make  it  impossible 
to  discover  any  difference  between  the  different  varieties  of  brick. 
It  has  been  asserted  that  this  experiment  already  proves  that  a 
brick  having  a  large  loss  in  the  rattler  wears  as  well  as  one  having 
a  much  smaller  loss;  but  this  conclusion  is  not  justifiable,  since  the 
wear  on  any  brick  is  as  yet  exceedingly  small,  and  the  difference 
between  different  varieties  is  too  small  to  warrant  any  conclusion 
as  to  relative  wear.  Doubtless  in  due  time  valuable  information 
will  be  obtained  as  to  the  relation  between  the  loss  in  the  rattler  and 
that  in  actual  service. 


ART.    2} 


CONSTRUCTION 


503 


ART.  2.     CONSTRUCTION 

963.  Fig.  176,  shows  the  several  parts  of  a  brick  pavement  of  the 
standard  type. 


Fio.  176. — SECTION  OF  BRICK  PAVEMENT  WITH  SAND  CUSHION  AND  CONCBETE  FOUNDATION 

964.  SUBGRADE.    An  essential  feature  in  the   construction  of 
such  a  pavement  is  the  proper  preparation  of  the  subgrade.     It 
should  be  thoroughly  underdrained,  should  be  rolled  until  it  is  solidly 
compacted,  and  the  surface  should  be  smooth  and  of  the  correct 
crown  and  grade. 

Underdrainage  has  been  fully  discussed  in  §  113-24;  and  street 
drainage  has  been  considered  at  length  in  Chapter  XIII.  The 
smoothing  and  rolling  of  the  subgrade  is  considered  in  Art.  1  of 
Chapter  XV. 

965.  FOUNDATION.     In  the   evolution  of  the  brick    pavement 
several  types  of  foundation  were  used  for  a  time. 

966.  Abandoned    Types.     The    first    brick    pavement    in    this 
country,  that  at  Charleston,  W.  Va.,  was  laid  on  a  foundation  of 
1-inch  tarred  boards  resting  on  a  layer  of  3  or  4  inches  of  sand,  with 
a  l|-inch  sand  cushion  between  the  bricks  and  the  boards.     This 
form  was  not  used  to  any  considerable  extent,  and  has  been  entirely 
abandoned. 

During  the  first  ten  or  fifteen  years  after  the  introduction  of 
brick  pavements  in  the  Middle  West,  the  foundation  consisted 
almost  exclusively  of  a  course  of  brick  laid  flatwise  on  a  thin  bed 


504  BRICK   PAVEMENTS  [CHAP.    XVII 

of  gravel  or  cinders.  Such  pavements  are  generally  known  as  two- 
course  brick  pavements.  The  layer  of  cinders  or  gravel  was  leveled, 
and  inferior  paving  brick  were  laid  flatwise  thereon;  and  then  the 
joints  of  the  bricks  were  swept  full  of  sand.  The  chief  defect  in  this 
form  of  foundation  was  that  the  joints  of  the  lower  course  were  not 
fully  filled,  and  consequently  after  the  pavement  was  in  service  the 
sand  of  the  cushion  coat  (the  layer  between  the  two  courses  of  brick) 
would  work  into  these  joints  and  permit  the  bricks  in  the  wearing 
course  to  settle.  To  cheapen  the  pavement,  broken  and  chipped 
brick  were  used  in  the  lower  course,  and  the  tendency  was  to  place 
the  larger  face  uppermost,  thus  making  it  nearly  impossible  to  fill 
entirely  the  joints  during  the  time  of  construction.  This  form  of 
foundation  was  abandoned  on  account  of  its  cost  and  inferior 
quality. 

In  some  localities  where  gravel  or  broken  stone  was  cheap,  brick 
pavements  were  laid  upon  a  layer  of  gravel  or  broken  stone;  but  the 
difficulty  and  expense  of  getting  such  a  foundation  thoroughly  com- 
pacted and  properly  shaped  led  to  the  substitution  of  a  concrete 
foundation. 

In  localities  where  the  native  soil  is  clean  sand  or  fine  gravel, 
brick  pavements  were  constructed  directly  upon  the  natural  soil. 
The  subgrade  is  simply  shaped  and  rolled.  Quite  a  number  of  cities 
in  the  North,  some  of  which  have  a  considerable  traffic,  for  example 
Cleveland,  Ohio,  and  Galesburg,  Illinois,  and  many  cities  in  the 
South,  lay  such  pavements  on  native  sand.  The  sand  is  usually 
simply  graded  and  puddled,  the  puddling  being  mainly  to  keep  the 
subgrade  hard  and  smooth  until  the  bricks  can  be  laid  and  the  joints 
filled.  Many  southern  cities  lay  brick  pavements  upon  a  1-inch 
layer  of  cement  mortar  or  fine  concrete,  this  bedding  course  being 
used  to  prevent  the  sand  subgrade  from  working  up  into  the  joints 
while  the  bricks  are  being  rolled.  Such  foundations  are  wise  only 
for  light  traffic  streets,  and  the  decreased  cost  of  portland  cement 
has  led  to  the  increasing  use  in  such  localities  of  a  concrete  founda- 
tion for  even  light  traffic  streets. 

967.  Old  Macadam  Foundation.     Not  infrequently  a  brick  pave- 
ment replaces  a  water-bound  gravel  or  macadam  surface,  in  which 
case  it  may  be  economical  to  use  the  old  pavement  for  a  founda- 
tion for  the  new.     For  a  consideration  of  this  case,  see  §  791  and 
§437. 

968.  Bituminous  Concrete  Foundation.    For  a  discussion  of  this 
type  of  foundation,  see  §  792-95. 


ART.    2]  CONSTRUCTION  505 

969.  Concrete   Foundation.    At   present   a   layer   of   portland- 
cement  concrete  is  the  almost  universal  foundation  for  brick  pave- 
ments.    This  form   of  foundation  is  fully  considered   in  Art.  2  of 
Chapter  XV — Pavement  Foundations. 

970.  BEDDING    COURSE.     The  bedding  course  is  a  layer  of  sand 
or  mortar  between  the  foundation  and  wearing  coat  to  provide 
for  slight  variations  in  the  surface  of  the  foundation  and  small 
irregularities  of  size  and  form  of  the  bricks.     There  are  three  dis- 
tinct forms  of  bedding  layer,  viz.:  sand,  a  dry  mixture  of  sand  and 
cement,  and  wet  mortar. 

971.  Sand  Bedding  Course.    The  proper  thickness  of  this  layer 
will  depend  upon  the  regularity  of  the  upper  face  of  the  concrete 
foundation  and  also  upon  the  uniformity  of  the  bricks  in  size  and 
form. 

For  reasons  stated  later  (§  977-78),  the  layer  of  sand  should  be  as 
thin  as  will  afford  a  good  bed  for  the  bricks;  and  therefore  the  top 
of  the  concrete  foundation  should  be  carefully  finished  with  a  surface 
parallel  to  the  surface  of  the  pavement.  Not  infrequently  loose 
fragments  of  stone  are  left  on  the  surface  of  the  concrete,  a  result 
which  is  very  undesirable,  since  they  necessitate  a  thicker  cushion 
and  at  best  prevent  the  bricks  from  coming  to  a  uniform  bearing. 
With  good  workmanship  in  laying  the  concrete,  there  will  be  no  loose 
pieces  of  stone  on  the  surface;  and  if  they  do  happen  to  get  there, 
they  should  be  removed  before  laying  the  cushion  coat. 

The  sand  for  the  cushion  should  preferably  be  so  fine  as  to  be  of  a 
soft,  velvety  nature  and  should  contain  no  pebbles  of  any  consider- 
able size,  or  loam,  or  vegetable  matter.  The  size  of  pebbles  permis- 
sible depends  upon  the  thickness  of  the  sand  bed.  Pebbles  will 
prevent  the  brick  from  having  a  uniform  bearing;  the  loam  is  likely 
to  be  washed  to  the  bottom  of  the  layer  and  cause  the  brick  to  settle ; 
while  the  vegetable  matter  will  decay  or  wash  away,  and  leave  the 
bricks  unsupported.  The  sand  should  be  dry  when  it  is  spread. 
Even  a  small  per  cent  of  moisture  in  the  sand  adds  considerably  to 
its  volume,  particularly  if  it  is  fine;  and  hence  if  the  sand  when  laid 
is  wet  and  dry  in  spots,  the  cushion  will  not  be  of  uniform  thickness 
when  dry.  The  shrinkage  of  the  sand  cushion  away  from  the  brick 
causes  depressions  which  are  unsightly,  unpleasant  to  users  of  the 
pavement,  and  causes  the  pavement  to  wear  more  rapidly.  Further, 
the  shrinkage  of  the  sand  cushion  away  from  the  brick  sometimes 
causes  an  unpleasant  noise  when  vehicles  pass  rapidly  over  these 
spots  (§  1055). 


506 


BRICK    PAVEMENTS 


[CHAP,  xvn 


972.  Spreading  the  Sand.  The  spreading  of  the  sand  should  be 
carefully  done,  so  as  to  secure  a  uniform  thickness  and  to  have  its 
upper  surface  exactly  parallel  to  the  top  of  the  finished  pavement. 
After  the  sand  has  been  distributed  approximately  to  the  proper 
thickness  with  a  shovel,  the  surface  should  be  leveled  by  drawing 
over  it  a  template  conforming  exactly  to  the  curvature  of  the  cross 
section  of  the  proposed  surface  of  the  pavement. 

Fig.  177  shows  a  common  form  of  template,  which  was  used  in 
constructing  a  pavement  33  feet  wide.  It  is  trussed  to  prevent  it 


FIG.  177. — TEMPLATE  FOR  STRIKING  THE  SAND  CUSHION. 


from  sagging  at  the  middle;  and  is  also  trussed  to  prevent  it  from 
deflecting  toward  either  the  front  or  rear.  The  template  is  pro- 
vided with  two  rollers  at  each  end  which  run  upon  the  top  face  of 
the  concrete  gutter.  Some  templates  are  provided  with  a  roller 
upon  a  bent  lever,  by  which  the  template  can  be  lifted  and  rolled 
back.  The  length  can  be  varied  by  means  of  fish-plates  at  each  end; 
and  the  elevation  of  the  cutting  edge  can  be  adjusted  by  the  screw 
and  hand-wheel  at  the  left. 

Practice  differs  considerably  as  to  the  length  of  the  template. 
Some  contractors  make  the  template  the  full  width  of  the  pavement, 
if  that  is  less  than  about  30  feet,  and  for  a  wider  pavement  make  the 
template  half  the  width  of  the  street.  This  form  of  template  must 


ART.   2]  CONSTRUCTION  507 

be  made  of  a  2-inch  pine  plank  of  sufficient  width  to  permit  of  the 
cutting  of  its  lower  edge  to  the  proper  curvature,  which  may  be 
determined  by  the  method  explained  in  §  718  (page  374).  If  the 
template  is  long,  it  must  be  braced  to  prevent  bending  and  sagging; 
and  it  must  have  a  long  and  substantial  handle  at  each  end  by  which 
to  draw  it  forward,  and  another  handle  at  each  end  by  which  to  carry 
it  backward.  It  is  desirable  that  the  template  shall  have  consider- 
able weight  to  keep  it  from  lifting  up  as  it  is  drawn  forward;  and 
when  being  drawn  forward,  the  face  of  it  should  lean  backward  a 
little  to  keep  it  from  lifting  up.  At  each  end  there  should  be  a  roller 
or  a  metal  runner  to  carry  the  template  along  the  top  of  the  curb  or 
along  the  edge  of  the  concrete  gutter.  The  roller  is  more  common, 
but  the  runner  is  better  since  it  eliminates  small  irregularities  in  the 
top  face  of  the  forms,  and  also  since  it  distributes  the  weight  upon 
the  forms  over  a  longer  length.  If  the  template  is  to  run  on  top  of 
the  curb,  a  roller  also  should  be  provided  to  keep  it  away  from  the 
curb.  If  the  length  of  the  template  is  equal  to  half  the  width  of  the 
street,  one  end  of  it  may  run  upon  a  screed,  or  wood  strip,  equal  in 
thickness  to  that  of  the  cushion  layer,  placed  in  the  center  of  the 
street.  If  there  is  a  car  track  in  the  street,  one  end  of  the  template 
may  be  made  to  run  on  the  rail. 

A  long  template  requires  considerable  force  to  draw  it  forward, 
and  it  is  difficult  to  move  backward.  Some  contractors,  therefore, 
use  a  template  equal  to  one  quarter  of  the  width  of  the  pavement. 
For  a  pavement  30  to  40  feet  wide,  screeds  made  of  2-inch  by  4-inch 
scantlings  are  placed  at  the  crown,  in  the  gutters,  and  also  midway 
between  the  crown  and  the  gutter,  being  bedded  on  a  thin  layer 
of  sand  so  that  their  tops  conform  to  the  finished  surface  of  the  pro- 
posed sand  cushion.  The  position  of  these  screeds  is  determined  by 
measuring  down  from  a  string  stretched  from  curb  to  curb.  The 
template  may  be  made  of  a  1-inch  by  6-inch  plank,  with  a  1-inch  by 
2-inch  handle  braced  by  two  1-inch  by  2-inch  pieces.  The  edge 
should  be  hollowed  out  to  fit  the  curved  surface  of  the  pavement, 
although  often  this  is  not  done.  The  middle  ordinate  for  the  curved 
cutting-edge  of  the  template  may  be  computed  by  the  formula 

C  d2 
m  =  -jTg-,  in  which  m  is  the  middle  ordinate  in  inches,  C  the  crown 

of  the  pavement  in  inches,  d  half  the  length  of  the  template  in  feet 
and  D  half  the  width  of  the  pavement  in  feet. 

After  the  sand  for  the  cushion  layer  has  been  distributed  with 
shovels,  the  template  should  be  drawn  slowly  over  it  several  times, 


508  BRICK   PAVEMENTS  [CHAP.    XVII 

any  depressions  that  develop  being  filled  by  sprinkling  sand  into  them 
with  a  shovel.  A  considerable  quantity  of  sand  should  be  drawn 
along  in  front  of  the  template,  as  this  aids  materially  in  packing  the 
bed.  It  is  necessary  to  draw  the  template  several  times  to  pack  the 
sand  well,  particularly  if  there  are  wet  and  dry  spots,  as  the  suc- 
cessive jarring  of  the  sand  grains  causes  them  to  settle  more  closely 
together.  When  the  sand  cushion  is  properly  packed,  it  will  have  a 
uniform,  smooth,  velvety  appearance,  and  will  not  look  rough, 
porous,  and  grainy.  No  one  should  be  allowed  to  step  on  the  sand 
cushion  after  it  has  been  spread,  nor  after  it  has  been  rolled. 

Formerly,  when  the  concrete  for  the  base  was  mixed  by  hand, 
the  template  was  pulled  forward  entirely  by  men,  or  sometimes  by 
one  or  two  horses;  but  now  it  is  moved  forward,  at  least  .for  the 
first  trip,  by  hitching  it  to  a  self-propelling  concrete  mixer,  or  better 
by  passing  a  rope  over  a  winding  drum  on  the  mixer. 

973.  The  surface  of  the  cushion  layer  is  sometimes  prepared  with 
a  short  lute  or  scraper  without  any  screeds;   but  the  template  and 
screeds  secure  a  more  uniform  surface  and  also  give  a  greater  com- 
pression and  a  more  even  bed.     With  hand  luting  the  surface  of  the 
pavement  is  almost  certain  to  be  covered  with  saucer-like  depressions 
after  it  has  been  rolled.     Hand  luting  should  be  prohibited  except 
where  the  use  of  the  template  is  impossible,  as  at  street  intersections, 
around  manhole  covers,  etc. 

A  considerable  part  of  the  difference  in  tractive  resistance  between 
brick  pavements  No.  4  and  Nos.  5  and  6  of  Table  7,  page  20,  is  due 
to  the  difference  in  the  preparation  of  the  sand  cushion,  the  remainder 
of  the  difference  being  in  the  rolling  of  the  brick  (§991). 

974.  In  adjusting  the  thickness  of  the  sand  cushion  adjoining 
concrete  gutters,  manholes,  etc.,  care  should  be  taken  that  the  upper 
surface  of  the  brick  after  being  rolled  is  not  below  the  upper  face  of 
the  gutter. 

975.  After  the  sand  cushion  has  been  struck  off  with  the  tem- 
plate, it  should  be  rolled  with  a  hand  roller  about  30  inches  long, 
24  inches  in  diameter,  and  giving  a  pressure  of  about  15  Ib.  per 
linear  inch.     An  ordinary  two-section  lawn  mower  with  a  12-foot 
handle  is  satisfactory  for  this  work. 

A  2-inch  layer  of  sand  will  compress  about  J  an  inch  under  the 
above  rolling,  and  consequently  the  height  of  the  template  should 
be  adjusted  accordingly.  To  bring  the  template  to  the  right  height, 
J-inch  strips  should  be  laid  upon  the  curbs  and  screeds;  and  then 
after  striking  the  sand  cushion  and  rolling  it,  these  strips  should  be 


ART.    2]  CONSTRUCTION  509 

removed,  and  the  template  be  again  drawn  over  the  sand  to  test  the 
surface  of  the  sand  bed.  If  the  surface  is  high  in  spots,  the  second 
drawing  of  the  template  will  plane  them  down;  and  if  there  are  low 
spots,  sand  should  be  sprinkled  over  them,  and  the  template  be 
drawn  again. 

The  spreading  and  shaping  of  the  sand  cushion  is  of  prime  im- 
portance in  securing  an  even  surface  in  the  finished  pavement; 
and  can  be  successfully  done  only  by  careful  and  skilful  men. 

976.  At  street  intersections  there  are  no  curbs  or  gutters  to  act 
as  guides  for  the  template,  and  hence  the  above  method  of  striking 
the  sand  cushion  can  not  be  applied.     In  such  cases  the  sand  cushion 
is  usually  shaped  with  a  hand  lute.     Stakes  about  J-inch  square 
should  be  driven  at  close  intervals  to  aid  in  bringing  the  top  of  the 
sand  cushion  to  the  right  elevation. 

977.  Objections  to  Sand  Cushion.     Until  comparatively  recently 
the  only  bedding  for  the  brick  was  a  layer  of  sand  about  2  inches 
thick.     The  purpose  of  the  sand  was  to  level  up  the  foundation  and 
to  give  a  good  bearing  for  the  brick;    and  in  the  early  history  of 
brick  pavements  probably  a  thickness  of  2  inches  was  required  for 
this  purpose.     However,  later  experience  proved  that  such  a  great 
thickness  was  unnecessary  and  also  inadvisable.     A  better  prepara- 
tion of  the  surface  of  the  concrete  foundation,  the  greater  uniformity 
in  paving  brick,  and  the  closer  inspection  of  the  brick  made  unneces- 
sary so  thick  a  sand  cushion. 

There  are  three  serious  objections  to  a  sand  cushion. 

1.  With  a  thick  cushion,  it  is  nearly  impossible  to  secure  uniform 
density  in  the  sand  layer.     The  sand  is  likely  to  be  more  moist  in 
some  spots  than  in  others;  and  when  it  dries  out,  it  will  shrink  and 
leave  the  brick  unsupported,  which  will  ultimately  cause  a  settle- 
ment of  the  brick  and  make  a  depression  on  the  surface  of  the  pave- 
ment.    Such  depressions  are  saucer-like,  and  can  be  seen  in  many 
brick  pavements.     Such  depressions  are  most  apparent  in  pave- 
ments having  joints  filled  with  cement  grout,  since  the  boundaries 
of  the  depression  are  indicated  by  a  break  of  the  bond  of  the  joint 
filler.     Tapping  the  surface  of  such   a   pavement,   particularly  a 
nearly  new  one,  with  a  hammer  will  reveal  many  spots  which  sound 
hollow,  showing  that  the  sand  cushion  has  shrunk  away  from  the 
brick.     Such  a  spot  is  likely  to  become  a  depression. 

2.  With  a  thick  sand  cushion,  the  rolling  of  the  pavement  is 
almost  sure  to  force  the  sand  up  into  the  bottom  of  the  vertical 
joints  between  the  bricks,  and  thus  prevent  the  cementitious  joint- 


510  BRICK   PAVEMENTS  [CHAP.   XVII 

filler  from  penetrating  the  full  depth  of  the  brick.  Examples  are  on 
record  in  which  the  sand  was  thus  forced  nearly  or  quite  to  the  top 
of  the  bricks ;  and  not  infrequently  sand  is  forced  up  half  the  depth 
of  the  bricks,  particularly  if  the  sand  of  the  cushion  is  fine,  as  is 
usually  the  case. 

This  objection  to  the  sand  cushion  is  particularly  important  if  a 
rain  should  wet  the  cushion  before  the  bricks  are  rolled;  for  if  the 
sand  cushion  is  wet,  the  bricks  can  not  be  rolled  adequately  without 
forcing  the  sand  up  into  the  joints. 

3.  Both  of  the  above  objections  apply  to  any  sand  cushion,  but 
particularly  to  a  thick  one;    and  the  following  objection  applies  to 
any  sand  cushion,  even  a  thin  one.     There  is  danger  that  the  sand 
cushion  may  leak  away  through  cracks  into  sewers,  manholes,  etc., 
particularly  if  the  wearing  course  of  brick  is  not  water-tight.     Such 
flow  of  sa^id  often  occurs  on  steep  grades.     Cases  have  been  known 
in  which  a  pavement  having  sand-filled  joints  and  being  on  a  1  per 
cent  grade,  sunk  next  to  the  curb  an  inch  in  twenty  years  from  this 
cause.     Even  with  a  water-tight  pavement,  the  sand  cushion  some- 
times leaks  into  trenches  opened  through  the  pavement  and  left 
unfilled  for  a  time.     A  street-railway  track  has  a  tendency  to  cause 
the  sand  cushion  to  flow  away.     The  vibration  due  to  passing  cars 
has  a  tendency  to  break  the  bond  of  the  joint  filling  near  the  rails 
and  make  a  crack  that  will  let  water  down  to  the  sand  cushion,  and 
then  the  water  will  flow  toward  the  curb  and  carry  the  sand  with  it. 
Further,  the  track  will  be  forced  down  by  the  weight  of  the  car  and 
will  spring  back  when  the  car  has  passed,  thus  pumping  the  water 
in  and  out,  which  forces  the  water  through  the  sand  cushion  and 
tends  to  displace  it. 

4.  Lately  some  have  claimed  that  the  sand  cushion  served  also 
to  give  elasticity  and  resiliency  to  the  pavement,  and  consequently 
protected  the  brick  from  excessive  wear  and  possible  breakage.     It 
is  likely  that  the  sand  cushion  compresses  under  use,  particularly 
if  it  is  not  thoroughly  compacted  before  the  brick  are  laid,  and  also 
if  the  brick  are  not  firmly  settled  into  the  sand  cushion  by  rolling; 
but  under  ordinary  conditions  this  compression  must  be  quite  small, 
and  takes  place  comparatively  soon  after  the  pavement  is  opened  to 
travel,  and  hence  can  not  have  any  appreciable  effect  upon  the 
durability  of  the  pavement.     Further,  little  or  none  of  this  compres- 
sion is  due  to  the  elasticity  of  the  sand  cushion,  and  hence  the  sand 
cushion  can  have  little  or  no  effect  in  absorbing  shock.     The  sand 
cushion  is  a  cause  of  shock  rather  than  an  absorber  of  shock. 


ART.    2]  CONSTRUCTION  511 

978.  In  the  early  history  of  brick  pavements  a  2-inch  cushion 
was  customary;   but  later  some  cities  reduced  it  to  1J  inches,  some 
to  1  inch,  and  a  few  to  f  inch.     At  present  the  better  practice  does 
away  with  any  mobile  sand  cushion. 

979.  Cement-sand    Bedding    Course.     This    form    of    bedding 
consists  of  a  layer  of  dry  cement  and  sand  about  J  inch  thick.     The 
cement  and  sand  are  thoroughly  mixed  dry  in  the  proportion  of 
1  :  3  or  1  :  4,  and  then  spread  and  struck  as  described  for  the  sand 
cushion  (§  972-74).     The  mixture  of  cement  and  sand,  after  being 
spread  and  struck  with  a  template,  is  so  well  compacted  and  of 
such  uniform  density  as  not  to  require  rolling;   and  besides  if  it  is 
rolled,  even  with  a  light  roller,  the  bed  is  so  hard  as  to  make  it  nearly 
impossible  to  roll  the  brick  to  a  smooth  surface.     After  the  bricks 
for  the  wearing  coat  have  been  set  in  place  upon  the  mortar  bed  and 
rolled,  the  bricks  should  be  thoroughly  sprinkled,  and  care  should  be 
taken  to  see  that  the  water  really  reaches  all  parts  of  the  sand- 
cement  bedding  course  and  converts  it  into  mortar.     This  can  be 
tested  by  taking  up  an  occasional  brick.     Subsequently  the  joints 
in  the  wearing  course  are  filled  with  hydraulic-cement  grout,  as  will 
be  described  later. 

There  are  really  two  types  of  this  form  of  construction,  viz.,  one 
in  which  the  amount  of  cement  is  sufficient  to  make  a  mortar  of 
fair  strength,  and  another  in  which  the  amount  of  cement  is  suf- 
ficient only  to  prevent  the  cement-sand  bed  from  leaking  away  or 
shifting  under  traffic.  With  the  latter  form,  the  bedding  course 
was  usually  1  or  1J  inches,  and  hence  considerable  cement  was 
required,  even  though  the  mixture  was  a  lean  one.  Gradually  the 
bedding  course  was  decreased  in  thickness  and  increased  in  richness. 

980.  The  cement-sand  bedding  course  has  two  advantages  over 
the  sand  cushion:    1.  It  is  rigid;    and  hence  can  not  compress  or 
shift  under  travel,  nor  leak  away.     2.  The  mortar  adheres  to  the 
foundation  and  also  to  the  bottom  of  the  brick,  and  binds  them 
together,  thus  converting  the  foundation  and  wearing  course  into  a 
partial  monolith.     Such  a  pavement  is  usually  called  a  semi-mono- 
lithic brick  pavement. 

A  bed  of  sand  and  cement  laid  as  described  above  never  makes 
a  really  good  mortar.  In  the  first  place,  if  the  water  is  applied 
sparingly  there  is  no  certainty  that  enough  water  reaches  the  mix- 
ture to  make  a  mortar;  and  on  the  other  hand,  if  water  is  applied 
profusely  it  may  wash  the  cement  out  of  the  sand  and  destroy  the 
mixture  as  a  mortar.  Again,  even  though  the  mixture  receives  the 


512 


BRICK    PAVEMENTS 


CHAP.    XVII 


proper  amount  of  water,  the  resulting  mortar  will  be  of  poor  quality 
owing  to  the  lack  of  mixing  after  the  addition  of  the  water. 

981.  Apparently  the  first  example  of  this  type  of  pavement 
was  constructed  in  Baltimore,  Md.,  in  1906;  but  the  most  noted 
example  is  the  vehicle  entrance  to  the  Pennsylvania  Railway 
Passenger  Station  in  New  York  City  constructed  in  1910.  Another 
innovation  in  this  pavement  was  that  the  bricks  were  only  2J  inches 
deep.  In  1916,  7,800  vehicles  passed  over  this  pavement  daily;  and 
yet  Fig.  178  shows  that  the  surface  is  still  practically  perfect. 


FIG.  178. — SEMI-MONOLITHIC  BRICK  PAVEMENT — PENNSYLVANIA  PASSENGER  STATION,  NEW 

YORK  CITY. 

This  form  of  construction  was  justly  popular  until  the  true  mono- 
lithic construction  was  developed,  as  described  in  the  next  section 
below. 

982.  Mortar  Bedding  Course.  This  consists  of  a  layer  of  cement 
mortar  on  which  the  brick  are  set  while  the  mortar  is  still  green.  The 
chief  difference  between  this  form  of  construction  and  the  cement- 
sand  type  is  that  the  layer  of  mortar  is  placed  upon  the  concrete 
foundation  before  it  has  begun  to  set;  and  then  the  brick  are  placed, 
rolled,  and  grouted  before  the  cement  in  either  the  concrete  base  of 
the  mortar  bed  has  taken  its  initial  set.  With  careful  work  it  is 
reasonably  certain  that  the  whole  construction  is  really  one  solid 
mass.  This  form  is  known  as  the  monolithic  brick  pavement. 

There  are  two  slightly  different  forms  of  this  type  of  construction. 
In  one  the  concrete  foundation  is  laid  in  the  usual  way,  and  on  it  a 
layer  of  rather  dry  cement  mortar  is  spread  by  means  of  a  template 
similar  to  that  used  in  gaging  the  thickness  of  the  sand  cushion 


ART.    2] 


CONSTRUCTION 


513 


(§  972).  This  form  was  first  used  near  Paris,  111.,  in  1914.  In 
the  other  form  of  construction  the  concrete  foundation  is  laid  about 
half  an  inch  thicker  than  the  depth  required,  and  is  then  tamped 
with  a  tamping  template  (§  462)  to  reduce  the  thickness  to  that  speci- 
fied. The  tamping  flushes  a  layer  of  mortar  to  the  top;  and  then  the 
surface  of  the  concrete,  or  rather  the  mortar,  is  struck  off  with  a 
cutting  template,  and  the  brick  are  set  on  the  mortar  surface.  This 
form  of  construction  was  first  used  near  Danville,  111.,  in  1915. 

When  gravel  is  used  for  the  coarse  aggregate  of  the  concrete 
foundation,  the  concrete  and  the  mortar  bedding  course  are  struck 
off  at  a  single  operation  with  a  double  template.  Fig.  179  shows 
this  template.  The  front  template  is  usually  a  steel  I  beam  which 


Fia.  179. — DOUBLE  TEMPLATE  FOR  STRIKING  MORTAR  BEDDING  COURSE. 

strikes  the  surface  of  the  concrete  foundation;  and  the  rear  template 
is  usually  a  steel  channel,  which  is  |  or  Y&  of  an  inch  higher  than  the 
front  one,  strikes  the  upper  surface  of  the  mortar  bedding  course. 
The  space  between  the  templates  is  2  feet  wide,  and  is  kept  full  of 
mortar,  which  is  previously  mixed  in  a  small  machine  mixer. 

Fig.  180,  page  514,  shows  the  mortar  bed  after  the  bricks  have 
been  placed  upon  it  and  rolled.  The  mortar  is  usually  forced  up 
into  the  joint  i  to  f  of  an  inch.  When  a  brick  is  thus  removed,  the 
surface  of  the  mortar  should  be  damp,  but  there  should  be  no  film  of 
water  on  the  top  of  the  mortar.  The  monolithic  type  of  construc- 
tion has  been  adopted  with  great  rapidity;  but  thus  far  it  has  been 
employed  chiefly  on  rural  roads. 

The  monolithic  brick  pavement  is  not  adapted  to  a  steep  grade, 


514 


BRICK   PAVEMENTS 


[CHAP,  xvii 


owing  to  the  difficulty  of  keeping  the  green  concrete  at  the  correct 
grade  and  cross  section. 


FIG.  180. — VIEW  OP  MORTAR  BED  AFTER  BRICK  SURFACE  HAD  BEEN  ROLLED. 

983.  Comparison  of  Types.     For  a  comparison  of  the  relative 
merits  of  brick  pavements  having  the  preceding  forms  of  bedding 
course,  see  §  1023-33.     It  is  not  wise  to  attempt  to  discuss  this 
phase  of  the  subject  until  the  complete  construction'  of  the  pave- 
ment has  been  considered. 

984.  LAYING  THE  BRICK.    Delivery.    The  brick  are  usually 
placed  in  piles  at  the  side  of  the  road  or  pavement  before  the  grading 
is  done.     The  brick  should  be  kept  clean,  and  should  be  handled 
so  as  not  to  needlessly  nick  and  break  them. 

Formerly  there  was  considerable  discussion  as  to  the  relative 
merits  of  (1)  delivering  the  brick  along  the  curb  a  considerable  time 
before  they  are  to  be  laid,  or  (2)  hauling  them  to  the  street  as  they 
are  laid.  In  the  former  case,  the  brick  were  transported  from  the 
parking  to  the  men  who  set  them,  either  in  wheelbarrows  or  by  hand 
on  a  board  or  later  in  a  pair  of  tongs.  In  the  latter  case,  the  wagon 
was  hauled  to  the  middle  of  the  street  on  planks,  and  the  bricks  were 
carried  by  hand  or  with  tongs  directly  from  the  wagon  to  the  layers. 


ART.    2]  CONSTRUCTION  515 

There  were  some  disadvantages  in  the  last  method,  but  there  was 
the  possibility  of  saving  2J-  to  3  cents  per  square  yard. 

Both  methods  have  been  superseded,  as  far  as  street,  i.  e.,  wide, 
pavements  are  concerned,  by  an  improved  method  of  delivering 
brick  from  the  parking  to  the  setters.  This  device  consists  of  a 
roller  conveyor, — a  series  of  rollers  set  in  an  inclined  frame  down 
which  the  brick  roll  by  their  own  weight  in  a  stream  from  the  parking 
toward  the  middle  of  the  street,  and  from  which  the  setters  pick 
them  to  set  them  into  place.  This  is  an  example  of  one  of  the  im- 
provements that  have  helped  to  keep  the  price  of  street  pavements 
down,  notwithstanding  the  advance  in  cost  of  materials  and  labor 
(§  637). 

For  rural  roads,  i.  e.,  for  narrow  pavements,  the  brick  are  carried 
by  hand  on  a  pallet  or  in  a  pair  of  tongs  from  the  side  of  the  pave- 
ment to  the  setter. 

985.  It  is  undesirable  to  use  wheelbarrows  in  transporting  the 
bricks  from  the  parking  to  the  setters,  since  the  bricks  are  likely  to 
be  chipped  in  placing  them  in  the  wheelbarrow  and  in  dumping 
them  out,  and  further  since  dumping  them  is  likely  to  tilt  the  bricks 
already  set,  which  will  make  the  surface  of  the  finished  pavement 
uneven  and  rough. 

986.  If  the  brick  are  delivered  to  the  setters  in  tongs  or  on  a 
pallet,  the  men  should  be  provided  with  planks  to  walk  upon ;   and 
they  should  not  be  allowed  to  step  upon  the  bricks  before  they  are 
rolled,  as  it  is  liable  to  tilt  them  and  cause  the  surface  of  the  pave- 
ment to  be  rough  after  it  is  rolled.     Further,  workmen  should  not 
be  permitted  to  track  mud  upon  the  brick.     When  the  condition  of 
the  ground  is  such  that  mud  will  be  tracked  upon  the  pavement,  the 
work  of  laying  brick  should  not  be  allowed. 

987.  Direction  of  Courses.     It  is  customary  to  lay  the  brick 
with  the  length  perpendicular  to  the  curb,  except  at  street  inter- 
sections;  but  there  are  a  few  cities  in  which  the  brick  are  laid  in 
courses  making  an  angle  of  45°  with  the  length  of  the  street,  with 
the  idea  that  the  tendency  to  form  ruts  would  be  less  if  the  wheels 
crossed  the  bricks  diagonally.     There  is  no  advantage  in  the  diagonal 
over  the  square  courses;   they  are  more  difficult  to  lay,  cutting  the 
corner  of  the  brick  in  making  the  fit  next  to  the  curb  is  wasteful  of 
material,  and  the  diagonal  courses  do  not  give  as  good  foothold  to 
the  horses. 

The  hill-side  brick  shown  in  Fig.  170  and  171,  page  483,  must  be 
laid  with  its  length  in  the  direction  of  the  street. 


516 


BRICK    PAVEMENTS 


[CHAP,  xvii 


Occasionally  a  few  courses  of  brick  are  laid  longitudinally  in  the 
gutter,  similar  to  the  practice  with  stone  blocks;  but  this  is  unneces- 
sary, since  the  brick  pavement  is  much  smoother  than  the  ordinary 
stone-block  pavement,  and  besides  the  running  joint  where  the  trans- 
verse and  the  longitudinal  sections  join  is  likely  to  develop  into  a  rut. 

988.  At  street  intersections  and  junctions  the  bricks  should  be 
laid  diagonally — a  compromise  position  between  the  directions  of 
the  travel  on  the  two  streets.  Street  intersections  need  special  care 
in  construction,  since  they  are  exposed  to  the  traffic  of  two  streets. 
Fig.  181  shows  the  usual  arrangement  of  the  courses  for  a  street 


Fia.  181. — DOUBLE-DIAGONAL  BRICK  INTERSECTION. 


intersection;  and  Fig.  182  and  Fig.  183  (page  518)  show  two  other 
arrangements  that  have  occasionally  been  used.  Slight  objections 
have  been  urged  against  all  three  plans.  The  bond  in  Fig.  181  is 
weak  along  the  middle  line  of  each  street;  Fig.  182  is  objectionable 
owing  to  the  tendency  of  ruts  to  form  along  the  lines  running  through 
the  ends  of  the  bricks;  and  Fig.  183  is  defective  since  traffic  around 
the  corners  A  and  B  is  parallel  to  the  courses  of  brick. 


ART.    2] 


CONSTRUCTION 


517 


At  a  street  junction  only  half  of  the  common  area  should  be  laid 
with  diagonal  courses.  For  example,  assuming  that  in  Fig.  181 
the  street  enters  the  lower  side  of  the  transverse  street  but  does  not 
cross  it,  then  the  lower  half  of  the  intersection  would  be  laid  with 
courses  as  in  the  diagram,  while  in  the  upper  half  the  length  of  the 
bricks  would  be  perpendicular  to  the  transverse  street. 

989.  Setting  the  Brick.  In  setting  the  brick  the  man  should 
stand  on  those  already  laid,  and  not  upon  the  sand  cushion.  Under 
no  consideration  should  the  sand  bed  be  disturbed.  The  brick  should 
be  set  on  edge  as  closely  and  compactly  as  possible,  each  being 


FIG.  182. — HERRING-BONE  BRICK  INTERSECTION. 

pressed  or  rather  bumped  both  endwise  and  sidewise  against  those 
already  laid.  The  bricks  are  stronger  and  more  durable  than  any 
material  that  can  be  used  to  fill  the  joints,  and  consequently  the 
thinner  the  joints  the  better.  The  bond  should  be  approximately  a 
half  brick.  If  the  brick  were  laid  without  bond,  ruts  would  form 
along  the  continuous  end-joints;  and  therefore  the  more  the  bond 
the  better.  No  bats  should  be  used,  except  in  making  closures;  and 


518 


BRICK   PAVEMENTS 


[CHAP,  xvii 


in  cutting  a  brick  to  close  a  course,  care  should  be  taken  to  get  a 
square  end  and  to  make  as  tight  a  fit  as  practicable.  As  far  as  pos- 
sible, the  bats  should  be  obtained  from  chipped  and  broken  brick, 
or  from  misshapen  ones  rejected  in  the  inspection  after  the  brick 
are  set.  It  is  usually  specified  that  no  bat  less  than  2J  inches  long 
shall  be  used.  Under  this  specification,  if  the  space  is  less  than  2J 
inches,  it  is  necessary  to  take  up  the  next  brick  and  chip  enough  off 
to  permit  the  use  of  a  bat  more  than  2J  inches.  Fig.  184  shows  the 
hammer  employed  in  cutting,  or  rather  in  breaking,  a  brick  to  close 
a  course. 


FIG.  183. — SINGLE-DIAGONAL  BRICK  INTERSECTION. 


In  some  cities  it  is  required  that  each  four  or  five  courses  of  brick 
shall  be  driven  up  from  the  face  by  striking  with  a  sledge  against  a 
2"  X  4"  or  4"  X  4"  timber  resting  against  the  last  course;  but  this 
is  unnecessary,  if  each  brick  when  laid  is  pressed,  or  rather  bumped, 
against  the  side  of  the  course  already  in  position.  In  any  case  the 
courses  should  be  straight  across  the  street;  and  if  they  are  not  laid 
so,  they  should  be  straightened  by  driving  up  each  four  or  five 
courses  from  the  face.  Sometimes  the  bricks  in  a  row  are  crowded 


ART.    2]  CONSTRUCTION  519 

together  endwise  by  inserting  a  crowbar  at  the  curb;  but  this  is 
unnecessary,  provided  each  brick  as  it  is  laid  is  bumped  against 
the  end  of  the  preceding  one. 

It  is  usually  specified  that  nothing  smaller  than  a  2j-inch  bat 
shall  be  used  in  making  a  closure.  The  alternative  is  either  to  close 
up  the  joints  between  the  ends  of  the 
bricks  by  prying  with  a  crowbar  until  a 
larger  bat  can  be  inserted,  or  to  open  up 
a  few  joints  until  the  space  at  the  end  of 
the  course  is  moderately  small.  The  latter 
is  undesirable,  since  it  is  likely  to  displace 
the  brick  vertically  so  as  to  make  the 
surface  of  the  pavement  rough  after  it 
has  been  rolled. 

Of  course,  lug  brick  should  be  set  with        FIG.  184—  BRICK  PAVEB'S 
the  lugs  always  in  the  same  direction. 

990.  Inspecting.     After  the  bricks  are  laid,  the  pavement  should 
be  inspected,   all  soft,   broken,  and  badly  misshaped  brick  being 
marked  for  removal.     To  reveal  the  soft  brick,  it  is  customary  to 
sprinkle  the  pavement  heavily  with  a  hose.     While  the  water  is 
being  applied,  the  soft  brick  will  appear  comparatively  dry;  but  after 
the  sprinkling  is  stopped,  the  soft  brick  will  appear  to  be  the  wetter. 
A  brick  having  only  a  small  piece  chipped  from  the  corner  or  edge 
may  be  turned  over.     Objectionable  brick  may  be  marked  with 
chalk,  a  cross  or  circle  indicating  a  brick  to  be  removed  and  a  single 
straight  line  one  to  be  turned.     Rejected  brick  are  removed  with 
tongs  having  broad  flat  noses  and  long  stout  handles. 

991.  Rolling.     After  all  rejected  brick  have  been  removed  and 
the  pavement  has  been  swept,  it  is  ready  for  rolling.     The  purpose 
of  the  rolling  is  to  settle  the  bricks  uniformly  into  the  bedding  course. 
A  heavy  roller  is  undesirable,  at  least  in  the  beginning  of  the  rolling, 
since  the  first  passage  of  it  tilts  the  bricks  to  one  side  so  much  that  it 
is  nearly  impossible  to  straighten  them  up  again.     Unless  the  top 
faces  of  the  bricks  are  brought  to  a  plane,  the  pavement  will  be  rough 
and  noisy,  and  will  lack  durability. 

It  is  important  that  the  rolling  shall  closely  follow  the  spreading 
of  the  bedding  course,  so  that  a  shower  may  not  wet  it.  Whatever 
the  material  of  the  bedding  course,  if  it  is  rained  upon,  the  bricks 
with  unfilled  joints  should  be  taken  up  and  a  new  bedding  course  be 
spread.  For  much  the  same  reason  the  filling  of  the  joints  (§  994- 
1005)  should  closely  follow  the  rolling. 


520 


BEICK  PAVEMENTS 


[CHAP.   XVII 


If  the  bedding  course  is  sand  or  dry  sand  and  cement,  the  -roller 
should  weigh  3  to  5  tons;  but  if  the  bedding  course  is  green  mortar 
(§  982),  the  rolling  should  be  done  with  a  hand  roller  about  30  inches 
in  diameter,  24  inches  long  and  weighing  from  600  to  900  Ib.  The 
rolling  should  be  continued  until  the  surface  of  the  pavement  is 
smooth. 

The  pavement  should  first  be  rolled  longitudinally,  beginning 
at  the  crown  and  working  toward  the  gutter,  taking  care  that  each 
return  trip  of  the  roller  covers  exactly  the  same  area  as  the  preceding 
trip  so  that  the  second  passage  of  the  roller  may  neutralize  any 
careening  of  the  brick  due  to  the  first  passage.  Pavements  that 
have  been  rolled  only  once  or  always  in  one  direction,  are  very  much 
rougher  and  more  noisy  than  when  properly  rolled.  If  a  spot  is 
skipped  on  the  return  passage  of  the  roller,  it  can  be  detected  by  a 
casual  inspection  or  by  the  noise  of  a  passing  vehicle.  The  first 
passage  of  the  roller  should  be  made  at  a  slow  speed  to  prevent  undue 
canting  of  the  brick.  After  the  pavement  has  been  rolled  longi- 
tudinally, it  should  be  rolled  back  and  forth  transversely,  or  at  least 
in  both  directions  at  an  angle  of  45°  from  curb  to  curb.  If  the 
pavement  is  narrow,  and  particularly  if  it  has  a  high  crown,  it  may 
not  be  wise  to  roll  it  transversely  or  even  diagonally,  because  of  the 

time  required  and  also  because  of  the  probability  of 

breaking  brick.     In  this  case  the  longitudinal  rolling 

should  be  more  thorough. 

If  the  rolling  is  well  done,  the  sand  cushion  will 

be  pushed  up  between  the  brick  J  to  f  of  an  inch. 

992.  There  are  sometimes    places  which  can  not 
be  reached  by  the  roller,  for  example  around  manhole 
covers;   and  in  these  cases  the  brick  should  be  settled 
to  place  by  ramming.     The  ramming  should  be  done 
with  a  paver's  rammer,  Fig.    185,   wighing  not   less 
than  50  Ib.     The  ramming  should  be  done  on  a  2-inch 
plank  10  to  12  inches  wide  and  6  feet  long  laid  parallel 
to  the  curb.     The  ramming  should  be  continued  until 
the  pavement  has  a  good  surface  at  the  proper  ele- 
vation. 

993.  After   the   rolling  is   completed,   the  joints 
snould  be  inspected;    and  if  the  bedding  course  has 
been  forced  up  between  the  bricks  more  than  \  an 

inch,  the  bricks  should  be  taken  up  and  relaid. 

It  is  usually  specified  that  after  the  final  rolling  the  surface  of 


ART.   2] 


CONSTRUCTION 


521 


the  pavement  shall  be  tested  with  a  10-foot  straight  edge  laid 
parallel  to  the  length  of  the  street,  and  any  depression  exceeding 
|  of  an  inch  shall  be  taken  up  and  re-laid. 

Fig.  186  shows  a  monolithic  brick  pavement  after  it  has  been 
rolled  and  before  the  application  of  the  filler.  The  two-section 
hand  roller  used  in  rolling  the  bricks  stands  in  the  foreground. 


FIG.  186. — MONOLITHIC  BRICK  PAVEMENT  ROLLED  AND  READY  FOR  THE  FILLER. 

994.  JOINT  FILLER.     The  joints  should  be  filled  (1)  to  keep  the 
brick  in  the  proper  position,  (2)  to  lessen  the  chipping  of  the  edges  of 
the  brick,  and  (3)  to  prevent  water  from  penetrating  to  the  cushion 
coat  and  to  the  foundation.     Three  forms  of  filler  are  in  common 
use,  viz.:    sand,  hydraulic-cement  grout,  and  bituminous  cement, 
i.  e.,  asphalt  or  tar;  and  recently  a  mixture  of  tar  and  sand  has  been 
used. 

995.  Sand  Filler.     Sand  was  the  first  filler  employed  for  brick 
pavements,  and  in  the  Middle  West  is  even  yet  largely  used.    The 
sand  should  be  fine  and  dry,  and  be  worked  into  the  joints  by  sweep- 
ing it  over  the  pavement,  which  also  should  be  dry.     Although  the 
sand  is  nominally  swept  into  the  joints,  it  is  usually  simply  spread 
upon  the  surface  and  left  to  be  worked  in  by  travel,  which  is  unde- 
sirable since  the  joints  are  then  partially  filled  with  manure  and 
street  dirt.     The  sand  can  be  swept  into  the  joints  effectively  and 
economically  with  a  revolving  machine  sweeper.    After  the  joints 


522  BKICK   PAVEMENTS  [CHAP.   XVII 

have  been  filled,  the  surface  of  the  pavement  is  covered  with  a  layer 
of  sand  J  to  J  inch  thick,  which  is  left  on  for  a  few  weeks  after  the 
street  is  thrown  open  to  travel,  to  secure  the  thorough  working  of  the 
sand  into  every  joint. 

The  cost  of  sweeping  the  pavement  and  filling  the  joints  with  sand 
is  0.2  to  0.3  cent  per  square  yard,  and  the  cost  of  a  |-inch  layer  of 
sand  at  $1.08  per  cubic  yard  is  1.5  cents  per  square  yard.  To  cover 
waste  and  contingencies,  the  sand  joint-filler  is  usually  estimated  at 
2  cents  per  square  yard. 

The  advantages  of  a  sand  filler  are:  1.  It  is  cheap.  2.  The 
pavement  may  be  thrown  open  to  traffic  as  soon  as  the  bricks  are 
laid.  3.  The  pavement  may  be  taken  up  easily  and  without  break- 
age of  the  brick.  4.  It  is  practically  water  tight,  particularly 
after  being  in  service  a  short  time.  Whenever  a  brick  pavement 
having  a  sand  filler  is  opened,  the  sides  of  the  brick  are  always  found 
dry  and  clean  a  little  distance  below  the  wearing  surface. 

The  disadvantages  of  a  sand  filler  are:  1.  It  does  not  protect  the 
edges  of  the  brick  from  chipping.  2.  It  may  wash  out  from  the  crown 
toward  the  gutter.  3.  It  is  removed  from  the  top  of  the  joints  by  the 
street  sweeper — either  the  broom  or  the  pneumatic, — and  by  auto- 
mobiles. 

996.  Cement  Grout.  The  grout  should  be  composed  of  part 
Portland  cement  and  1  or  1J  parts  sand.  If  the  sand  is  well  graded, 
1  part  of  cement  will  fill  the  voids  of  1J  parts  of  sand,  and  give  a 
filler  of  maximum  strength;  but  if  the  sand  is  not  well  graded,  the 
grout  should  be  1  :  1.  The  cement  should  meet  the  requirements  of 
the  standard  specifications.  The  sand  should  not  contain  more  than 
1  per  cent  by  weight  of  clay  or  loam,  and  should  contain  such  gra- 
dation of  sizes  that  all  will  pass  a  20  standard  sieve  and  all  be  re- 
tained on  a  100  standard  sieve.  Some  contractors  place  the  sand  in 
bags  when  unloading  it  from  the  car,  and  distribute  the  bags  of  sand 
with  an  equal  number  of  bags  of  cement  at  proper  intervals  along 
the  pavement,  which  secures  the  proper  proportion  of  ingredients 
and  prevents  loss  of  materials  and  time  in  measuring.  The  sand 
and  cement  should  be  mixed  dry  until  the  mass  is  homogeneous  and 
of  a  uniform  color.  The  cement  and  sand  should  be  mixed  dry  so 
that  the  cement  will  not  ball  up  when  the  water  is  added.  For  the 
best  results  a  considerable  amount  of  this  mixture  should  be  pre- 
pared at  one  time,  but  not  more  than  will  be  used  up  in  two  hours. 

To  prepare  the  grout,  a  small  batch  of  the  mixed  sand  and  cement, 
preferably  not  more  than  2  cubic  feet,  should  be  placed  in  a  suitable 


ART.    2] 


CONSTRUCTION 


523 


box  or  machine,  and  water  should  be  added  to  make  a  grout  that  will 
freely  flow  to  the  bottom  of  the  joints  without  separation.  It  is 
important  that  the  water  be  added  slowly  and  that  the  grout  be 
mixed  thoroughly.  For  the  best  results,  the  dry  mortar  should  first 
be  reduced  to  a  uniform  and  plastic  mortar,  and  then  more  water 
should  be  added,  while  the  mass  is  mixed  vigorously,  until  the  desired 
consistency  is  attained. 

997.  Mixing  Box.    The  mixing  may  be  done  in  a  box  made  for 
the  purpose,  which  should  be  3J  to  4  feet  long,  27  to  30  inches  wide, 
and  14  inches  deep,  and  should  have  legs  of  different  lengths,  so  that 
the  mixture  will  readily  flow  to  one  corner  of  the  box,  which  should 
be  8  to  10  inches  above  the  pavement.* 

The  grout  should  be  removed  from  the  box  to  the  pavement 
with  a  scoop  shovel,  and  not  by  overturning  the  box  upon  the  pave- 
ment; since  by  the  last  process  the  sand,  cement,  and  water  are 
separated,  and  are  deposited  in  different  portions  of  the  pavement. 
While  the  box  is  being  emptied,  the  grout  should  be  constantly 
stirred  to  prevent  a  separation  of  the  sand  from  the  cement. 

A  mortar  box  should  be  provided  for  each  10  or  12  feet  of  width 
of  pavement. 

998.  Mixing    Machine.    Recently    a    small    mechanical    mixer 


FIG.  187. — GROUT-MIXING  MACHINE. 

been    introduced    for    preparing    the    grout.      Fig.    187   and    188 
show  such  a  machine.     In  using  the  machine,  it  is  important  that 


*  The  National  Paving  Brick  Manufacturers  Association  publish  complete  specifications 
and  working  drawings  for  such  a  box,  which  may  be  had  gratuitously  of  the  Secretary,  Cleve- 
land, Ohio, 


524 


BRICK   PAVEMENTS 


[CHAP,  xvii 


the  mixing  be  not  hurried;  and  that  the  conditions   stated  in  the 
second  paragraph  of  §  996  be  observed. 

Some  engineers  permit  the  use  of  the  machine  for  mixing  the  first 
application  of  grout,  but  not  for  the  second,  as  the  latter  should  be 
stiffer.  However,  if  carefully  manipulated,  the  machine  will  make 
as  good  grout  as  can  be  made  in  a  box;  and  besides  the  mixing  is  less 
likely  to  be  slighted,  and  the  cost  of  mixing  is  less. 


FIG.  188. — GROUT-MIXING  MACHINE  AT  WORK. 

999.  Applying  the  Grout.  Before  the  grout  is  applied  to  the 
pavement,  the  brick  should  be  thoroughly  wetted  by  being  gently 
sprayed.  A  strong  stream  is  likely  to  displace  the  mortar.  The 
grout  should  be  applied  to  the  pavement  in  small  quantities,  and 
should  be  quickly  swept  into  the  joints  with  an  ordinary  brush  broom. 
The  strokes  of  the  broom  should  be  mostly  lengthwise  of  the  brick 
to  most  effectively  get  the  grout  into  the  joints.  It  is  better  that 
the  joints  should  be  only  about  half  or  two  thirds  filled  at  the  first 
application,  since  then  there  is  a  less  depth  of  grout  in  the  joints  and 
consequently  less  liability  of  the  separation  of  the  sand,  the  cement, 
and  the  water. 

Fig.  189  shows  the  process  of  making  the  first  application  of 
cement-grout  filler  using  mixing  boxes.  Notice  that  the  filler  is 
dipped  out  of  the  box  and  poured  upon  the  pavement  with  scoop 
shovel;  and  that  the  grout  is  spread  with  brush  brooms. 

In  applying  the  cement  filler  it  is  very  important  that  the  grout 
shall  not  bridge  across  or  dam  up  a  joint;  and  care  should  be  taken 
to  see  that  the  grout  really  reaches  the  bottom  of  all  joints. 


ART.   2] 


CONSTRUCTION 


525 


1000.  If  a  grout  filler  is  to  be  used  with  a  sand  cushion,  the  first 
application  of  filler  should  be  made  very  thin,  so  that  it  will  pene- 
trate the  sand  that  is  pushed  up  into  the  joint  by  the  rolling  and 
convert  the  sand  into  mortar.  If  the  sand  in  the  bottom  of  the  joint 
is  not  thus  converted  into  mortar,  the  bricks  are  likely  to  spall  on  the 
top  owing  to  concentrated  pressure  at  the  top  of  the  joint,  due  to  the 
expansion  of  the  pavement. 

In  making  this  very  thin  grout,  it  is  necessary  first  to  mix  the  sand 
and  cement  dry,  and  then  to  add  water  gradually  and  stir  the  mix- 
ture vigorously,  until  a  good  stiff  mortar  is  produced,  and  next  to 


FIG.  189. — FIRST  APPLICATION  OF  GROUT  FILLER. 

continue  the  gradual  addition  of  water  and  the  mixing  until  a  very 
thin  mortar  results.  If  all  the  water  is  added  at  once,  or  if  the 
water  is  added  too  rapidly,  or  the  mortar  is  not  thoroughly  and  con- 
tinuously mixed  while  the  water  is  being  added,  it  is  nearly  impos- 
sible to  keep  the  sand,  cement  and  water  from  segregating. 

1001.  After  the  first  application  has  been  carried  forward  40  or 
50  feet,  and  after  it  has  settled  but  before  the  initial  set  has  begun, 
a  second  application  should  be  made  in  the  same  manner  as  the  first, 
except  that  the  grout  should  be  somewhat  thicker. 
f-  Fig.  190  shows  the  process  of  making  the  second  application  of 
grout  filler,  when  the  mixing  bases  are  used.  The  grout  is  dipped 


526 


BRICK   PAVEMENTS 


[CHAP,  xvii 


with  a  scoop  shovel  and  spread  with  a  squeegee,  a  wooden  scraper 
having  a  rubber  edge. 

1002.  After  the  second  application  has  settled  but  before  initial 
set  has  begun,  all  surplus  grout  on  the  surface  should  be  forced  along 


FIG.  190. — SECOND  APPLICATION  OF  GROUT  FILLER. 

over  the  pavement  with  a  squeegee.  The  squeegee  should  always 
move  at  an  angle  with  the  joints,  thus  leaving  them  level  full. 

In  making  the  final  finish,  the  squeegee  should  be  drawn,  not 
pushed,  over  the  surface;  or  in  other  words,  the  workmen  should  not 
track  up  the  finished  surface,  for  doing  so  raises  little  projections  on 
the  surface  of  the  grout  which  when  hardened  are  very  destructive 
upon  automobile  tires.  This  precaution  was  not  observed  in  building 
the  automobile  brick  race-track  at  Indianapolis,  even  though  rec- 
ommended by  the  engineer;  but  after  the  track  was  completed,  it  was 
necessary  to  spend  considerable  time  and  money  to  remove  these  pro- 
jections. Of  course,  on  a  public  highway  these  projections  will 
ultimately  be  worn  off  by  steel-tired  vehicles;  but  in  the  meantime 
much  damage  will  be  done  to  auto  tires,  and  it  costs  practically  noth- 
ing to  prevent  the  projections. 

Fig.  191  shows  a  vertical  view  of  a  perfectly  grouted  brick  pave- 
ment. Fig.  192  shows  a  well-grouted  Ohio  rural  brick  road. 

Fig.  193  shows  a  sand-cushion  cement-grouted  brick  pavement 
on  South  Sixth  Street,  Terre  Haute,  Indiana,  when  28  years  old. 
This  is  one  of  the  first,  if  not  the  first,  cement-grouted  brick  pave- 


ART.   2] 


CONSTRUCTION 


,527 


ment;    and  it  is  in  a  remarkably  good  condition.     The  view  was 
taken  two  blocks  from  the  main  business  street.     When  this  pave- 


FIG.   191. — VERTICAL  VIEW  OF  A  PERFECTLY  GROUTED  BRICK  PAVEMENT. 

ment  was  laid  Terre  Haute  had  a  population  of  30,217,  and  in  1910 
it  had  58,157.  The  joint  filler  and  the  brick  have  worn  down  to- 
gether, and  the  surface  is  as  smooth  as  a  marble  mosaic.  The  top 


FIG.   192. — A  WELL-GROUTED  OHIO  RURAL  ROAD. 


faces  of  the  brick  are  flat,  and  the  joints  are  level  full  of  cement  grout. 
Scarcely  a  single  chipped  or  broken  brick  can  be  found;   and  the 


528  BRICK   PAVEMENTS  [CHAP.    XVII 

general  wear,  in  the  middle  third  of  the  street,  has  been  only  about  ^ 
to  TG  of  an  inch  of  depth,  with  a  very  few  holes  |  inch  deep  caused 
by  soft  brick.  The  brick  are  not  as  good  as  those  made  at  the  present 
time;  but  the  pavement,  particularly  for  that  time,  was  unusually 
well  constructed.  It  was  provided  with  an  adequate  foundation, 


FIG.  193. — SAND-CTTSHION  CEMENT-GROUTED  BRICK  PAVEMENT  28  YEARS  OLD. 

the  brick  were  well  burned,  and  were  carefully  and  thoroughly  rolled, 
and  the  joints' were  entirely  filled  with  good  portland-cement  grout, 
and  consequently  this  pavement  has  worn  exceedingly  well.  Of 
course  other  pavements  constructed  with  as  good  material  and  with 
the  same  care  would  wear  equally  well. 

1003.  With  hill-side  brick  (§  936)  the  grout  should  be  swept  from 
the  grooves  before  it  sets. 

1004.  After  the  joints  have  thus  been  filled,  and  after  the  grout 
has  set  so  that  a  coating  of  sand  or  earth  will  not  absorb  moisture 
from  the  joint  filler,  a  half  inch  of  fine  sand  should  be  spread  over  the 
entire  surface  of  the  pavement;    and  if  the  weather  is  very  hot  or 
dry,  the  sand  should  be  sprinkled  at  intervals  for  two  or  three  days, 
to  insure  that  the  cement  does  not  lose  by  vaporization  the  water 
necessary  for   chemical    combination   in    setting.     Some  engineers 
prefer  hay  or  straw  instead  of  sand  or  loam,  since  they  can  be  moved 
ahead  and  used  a  second  time. 

Travel  should  be  kept  off  the  pavement  from  seven  to  ten  days, 
or  at  least  until  the  cement  has  fully  set,  and  it  is  much  better  if 


ART.    2]  CONSTRUCTION  529 

travel  is  kept  off  longer.  Some  engineers  specify  three  weeks  in 
warm  weather,  and  longer  for  cold  weather.  If  the  cement  filler  is 
disturbed  before  it  is  firmly  set,  it  is  practically  no  better  than  sand. 
If  the  cement  filler  is  put  in  as  described  above  and  allowed  to  set 
firmly  before  travel  is  admitted,  the  filler  will  wear  no  faster  than  the 
best  paving  blocks  and  will  prevent  spalling  and  chipping  of  the 
bricks  at  the  edges  and  corners. 

1005.  To  separate  the  grouted  section  from  the  ungrouted  por- 
tion, a  row  of  metal  strips  yr  by  6  by  36  inches  should  be  inserted  in 
a  transverse  joint  of  the  pavement.    By  this  means  the  grouting  will 
end  in  a  transverse  joint.     These  metal  strips  should  be  removed 
when  the  grout  has  become  stiff,  but  before  initial  set. 

1006.  Cost.    The  amount  of  grout  required  will  vary  with  the 
openness  of  the  joints,  and  also  with  the  quantity  of  sand  of  the 
cushion  course  that  works  up  into  the  lower  part  of  the  joints  while 
the  bricks  are  being  rolled. 

"  If  a  1  :  1  portland-cement  grout  is  used,  the  area  filled  with 
one  barrel  of  cement  will  be  as  follows :  With  4-inch  brick  and  a  sand 
cushion,  32  square  yards  of  re-pressed  brick,  and  24  square  yards 
of  wire-cut  lug  brick;  and  with  4-inch  brick  on  a  f-inch  mortar  bed, 
30  square  yards  of  re-pressed  brick,  and  22  square  yards  of  wire-cut 
lug  brick."*  It  is  quite  common  to  estimate  f  of  a  barrel  of  cement 
per  square  yard  for  1  :  1  grout.  Of  course,  the  cost  of  the  cement 
for  the  filler  will  vary  with  the  market  price  of  cement.  The  grout 
will  require  about  0.2  cubic  foot  of  sand  per  square  yard  of  pavement; 
and  its  cost  at  $1.00  per  cubic  yard,  will  be  about  0.7  cent  per  square 
yard  of  pavement.  The  cost  of  applying  the  grout  filter  will 
vary  considerably  with  the  details  of  doing  the  work,  i.  e.,  the  number 
of  applications,  and  whether  the  mixing  is  done  in  a  box  or  a  machine; 
but  the  cost  will  usually  be  2  or  3  cents  per  square  yard.  The  total 
cost  of  grout  filler  was  formerly  8  to  10  cents  per  square  yard,  but  in 
1917  was  usually  10  to  12  cents  per  square  yard. 

1007.  Merits.  The  advantage  of  the  cement  filler  is  that  it  pro- 
tects the  edges  of  the  bricks  from  chipping,  and  thus  adds  to  the 
durability  of  the  pavement.  When  the  joints  are  filled  with  sand, 
the  edges  of  the  brick  chip  off,  the  upper  faces  wear  round,  the  pave- 
ment becomes  rough,  and  the  impact  of  the  wheels  in  jolting  over  the 
surface  tends  to  destroy  the  brick;  while  with  a  good  cement  filler, 
the  edges  do  not  chip,  the  whole  surface  of  the  pavement  is  a  smooth 

*  H.  E.  Bilger,  Road  Engineer,  Illinois  Highway  Commission,  in  Engineering  and  Contract- 
ing,  Vol.  46  (1916),  p.  502. 


530  BRICK   PAVEMENTS  [CHAP.    XVII 

mosaic  over  which  the  wheels  roll  without  jolt  or  jar,  and  conse- 
quently the  life  of  the  pavement  is  materially  increased.  Fig.  193, 
page  528,  shows  a  grout-filled  pavement  28  years  old  upon  which  there 
have  been  practically  no  repairs.  See  the  last  paragraph  of  §  1002 
for  a  description  of  its  present  condition. 

An  objection  to  the  cement  filler  is  that  it  does  not  take  up  the 
expansion  of  the  pavement  due  to  increase  of  temperature,  and  that 
consequently  the  pavement  is  likely  to  rise  from  the  foundation  and 
give  out  a  rumbling  noise  as  vehicles  go  over  it.  This  rumbling 
can  be  eliminated  by  inserting  longitudinal  expansion  joints  as 
described  in  §  1017. 

Another  objection  to  the  cement  filler  is  that  in  making  repairs 
it  is  difficult  to  remove  the  bricks  without  breaking  many,  and  it  is 
difficult  to  clean  the  bricks  so  that  they  may  be  used  again.  This 
is  an  advantage,  if  it  will  in  any  degree  prevent  the  tearing  up  of  the 
pavement;  and  at  best  this  objection  ought  not  to  have  much  weight 
against  durable  construction. 

A  third  objection  is  that  the  street  can  not  be  used  while  the 
cement  is  setting.  Often  the  cement  is  not  allowed  to  set  fully 
before  throwing  the  street  open  to  travel,  and  consequently  the  chief 
advantage  of  the  rigid  filler  is  lost.  The  semi-monolithic  and  the 
monolithic  types  of  construction  are  free  from  this  objection  (§  979 
and  §  982). 

1008.  Bituminous  Filler.  Both  asphalt  and  tar  are  used  as  a 
filler  for  the  joints  of  a  brick  pavement. 

i  1009.  Asphalt  Filler.  The  various  producers  and  refiners  of 
asphalt  -prepare  a  grade  of  asphalt  particularly  for  use  as  a  filler  for 
brick,  stone-block,  and  wood-block  pavements.  For  the  specifica- 
tions of  such,  see  §  544. 

1010.  Tar  Filler.    For  the  specifications  of  a  tar  suitable  for  a 
joint  filler  for  brick  pavements,  see  §  576-77. 

1011.  Applying  Bituminous  Filler.     The  bricks  should  be  dry, 
and  the  bituminous  filler  should  be  hot  enough  to  flow  freely  and 
adhere  to  the  brick.     The  asphalt  fillers  should  be  applied  at  a  tem- 
perature of  350  to  450°  F.,  and  the  tar  fillers  between  300  and  350° 
F.     If  either  filler  must  be  heated  hotter  than  this  to  make  it  pour 
freely,  then  it  will  be  so  hard  as  to  chip  out  of  the  joint  in  cold  weather; 
and  if  it  can  be  poured  much  colder  than  this,  it  will  be  so  soft  as  to 
run  out  of  the  joints  in  hot  weather.     However,  manufacturers  vary 
the  temperature  of  pouring  to  fit  extreme  climates. 

The  bituminous  filler  is  poured  into  the  joints  through  the  point 


ART.    2]  CONSTRUCTION  531 

of  a  cone-shaped  pouring  can.  The  point  of  the  can  has  a  cast  iron 
tip  with  an  opening  in  it  about  J  inch  in  diameter.  The  tip  is  opened 
and  closed  by  a  valve,  which  is  operated  by  a  handle  projecting  at  the 
top  of  the  can.  The  cast  iron  tip  is  placed  in  a  joint,  the  valve  is 
opened,  and  the  can  is  drawn  along  as  the  joint  becomes  filled.  A 
helper  fills  the  can  as  it  is  emptied. 

As  soon  as  the  joints  in  a  short  section  of  the  pavement  have 
been  filled,  and  while  the  bituminous  cement  is  still  soft,  a  light 
layer  of  sand  should  be  spread  over  the  pavement,  but  only  enough 
to  prevent  the  cement  from  sticking  to  passing  wheels.  In  cold 
weather  the  sand  should  be  heated  so  as  to  bond  rqfidily  with  the 
pitch. 

Particular  care  should  be  taken  in  applying  the  filler  around  man- 
holes, at  the  gutter,  etc.,  to  prevent  leakage  of  water  into  the  sub* 
grade. 

1012.  A  pouring  can,  or  rather  tank,  having  multiple  spouts 
has  recently  been  put  upon  the  market.     The  tank  is  mounted  upon 
wheels,  and  somewhat  flexible  spouts  project  below.     The  tips  of 
the  spouts  are  placed  in  the  joints,  and  the  tank  is  drawn  along  by 
hand  as  the  joints  are  filled. 

Contractors  claim  that  it  is  materially  more  difficult  to  fill  the 
joints  of  a  pavement  made  of  wire-cut  lugs  than  of  re-pressed  brick, 
as  with  the  former  it  is  more  difficult  to  keep  the  tip  of  the  pouring 
can  in  the  joint.  Recently  bituminous  filler  has  been  successfully 
applied  with  a  squeegee.  The  only  objection  to  this  method  is  that 
the  cement  may  be  chilled  by  contact  with  the  brick  and  fail  to 
penetrate  to  the  full  depth  of  the  joint. 

1013.  Cost.    The  cost  depends  upon  the  locality,  the  closeness 
of  the  joints,  and  the  amount  of  bituminous  material  left  upon  the 
surface  of  the  pavement.     A  tar  filler  usually  costs  8  to  10  cents  per 
gallon  and  asphalt  about  10  to  12;   and  1  to  If  gallons  is  generally 
required  for  a  square  yard  of  pavement.     The  labor  of  heating  and 
pouring  is  usually  about  5  to  7  cents  per  square  yard.     The  total 
cost  of  a  tar  filler  is  therefore  about  13  to  15  cents  per  square  yard, 
and  of  asphalt  about  15  to  19  cents. 

1014.  Merits  and  Defects.     A  bituminous  filler  is  superior  to 
sand  in  that  it  makes  a  perfectly  water-tight  pavement,  and  better 
protects  the  edges  of  the  bricks.     Bituminous   filler   is    preferable 
to  cement  grout  in  that  the  pavement  can  be  opened  to  travel  as 
soon  as  it  is  laid;  but  bituminous  filler  does  not  protect  the  edges  of 
the  brick  as  well  as  grout. 


532 


BRICK   PAVEMENTS 


[CHAP,  xvii 


1015.  Tar-sand  Filler.  Recently  a  mixture  of  tar  and  sand, 
usually  called  tar  mastic  or  pitch  mastic,  has  been  employed  as  a 
joint  filler  for  brick  pavements.  The  tar  pitch  should  conform  to  the 
specifications  in  §  576-77.  The  filler  is  made  of  pitch  and  as  much 
fine  clean  sand  as  the  pitch  tar  will  carry,  usually  about  1:1;  but 
in  no  case  should  the  volume  of  the  sand  exceed  that  of  the  tar. 
The  coarser  the  sand,  the  smaller  the  proportion  of  it  should  be  used. 
The  sand  when  mixed  with  the  tar  should  be  at  a  temperature  be- 
tween 300°  and  400°  F. ;  and  the  tar  shall  be  heated  to  250°  to  325°. 
The  mixing  is  most  easily  done  with  a  hoe  in  a  wheelbarrow  or  a 
concrete  bugg}i  The  mastic  is  poured  on  the  pavement  and  pushed 
into  the  joints  with  a  squeegee. 

Fig.   194  shows  the  method  of  applying  tar-mastic  filler.     The 
smoke  indicates  that  the  sand  was  too  hot. 

This  filler  has  been  used  for  brick  pavements  in  a  few  cases  with 
every  evidence  of  success;  but  the  experience  is  too  limited  in  both 


FIG.  194. — APPLYING  TAR-SAND  FILLER. 


extent  and  time  to  establish  the  merits  of  the  method.  A  tar-sand 
filler  protects  the  edges  of  the  bricks  better  and  is  less  susceptible 
to  temperature  changes  than  tar  alone;  and  the  only  question  is 


ART.   2]  CONSTRUCTION  533 

whether  or  not  the  tar-sand  filler  can  be  made  so  as  to  flow  satis- 
factorily into  the  joints  of  a  brick  pavement.  Since  the  joints  of 
stone-block  pavements  (Chapter  XVIII)  are  wider  than  those  of 
brick  pavements,  such  a  filler  is  more  needed  and  can  be  more  sat- 
isfactorily applied  to  the  former  than  to  the  latter.  For  specifica- 
tions for  a  tar-sand  filler,  and  for  further  details  concerning  its  use, 
see  §  110. 

1016.  EXPANSION    JOINTS.    Expansion  joints   may  be  either 
longitudinal  or  transverse. 

1017.  Longitudinal  Joints.    With  a  sand  filler  (§  995)  there  is 
little  or  no  need  for  expansion  joints,  since  the  sand  in  the  joints 
will  yield  enough  to  compensate  for  the  expansion  or  contraction 
due  to  changes  of  temperature.     For  much  the  same  reason,  expan- 
sion joints  are  not  necessary  with  a  bituminous  filler  (§  1008).     But 
with  a  rigid  grout  filler  and  sand  bedding-course,  it  is  necessary  to 
construct  longitudinal  expansion  joints  next  to  the  curb  or  gutter  on 
each  side,  to  provide  for  the  expansion  and  contraction  due  to  changes 
of  temperature  in  the  wearing  course.     To  be  perfectly  safe  the 
expansion  joint  should  extend  to  the  bottom  of  the  concrete  base; 
although  often  it  reaches  only  to  the  top  of  the  concrete  base.     The 
omission  of  longitudinal  expansion  joints  is  likely  to  cause  the  ex- 
pansion of  the  wearing   course  to  lift  the  brick  from  the  bedding 
course  and  to  cause   the  pavement  to  give  out  a  deafening  noise 
when  a  heavy-laden  steel-tired  vehicle  goes  over  it  at  any  consid- 
erable speed  (§  1027). 

The  longitudinal  expansion  joint  may  be  made  by  placing  a 
\-  to  1-inch  board  on  edge  against  the  curbs;  and  then  after  the 
bricks  are  set  withdraw  the  plank  and  fill  the  space  with  tar  or 
asphalt.  A  close  examination  of  Fig.  171,  page  483,  will  show  such 
a  plank  in  position  with  wedges  between  it  and  the  curb  to  facilitate 
the  removal  of  the  board.  An  objection  to  the  poured  expansion 
joint  is  that  it  is  liable  to  get  blocked  by  a  pebble  or  a  brick  spall 
getting  into  the  space  before  the  bituminous  material  is  poured. 
Instead  of  using  the  plank  as  described  above,  a  much  better  way  is 
to  place  pre-moulded  strips  of  mastic  next  to  the  curbs  before  laying 
the  brick.  There  are  several  forms  of  these  strips  on  the  market. 
The  strips  are  usually  f  of  an  inch  thick  for  a  pavement  20  to  30 
feet  wide,  and  proportionally  thicker  for  wider  pavements.  The 
strips  are  comparatively  short,  and  should  fit  closely  end  to  end; 
and  should  extend  the  full  depth  of  the  brick,  and  should  be  stiff 
enough  to  stand  alone  in  place  until  the  bricks  are  placed  against 


534  BRICK   PAVEMENTS  [CHAP.    XVII 

them.     The  material  should  be  pliable  at  32°  F.,  and  should  not 
melt  or  flow  at  125°  F. 

A  true  monolithic  pavement  needs  no  longitudinal  expansion 
joints,  although  they  are  sometimes  provided. 

1018.  Transverse  Joints.     Transverse  expansion  joints  are  not 
needed  with  either  a  sand  or  a  bituminous  filler.     Opinions  differ 
as  to  the  need  of  such  joints  with  grout  filler;  but  the  best  practice 
seems  to  be  to  omit  them. 

There  are  two  forms  of  transverse  expansion  joints  in  use.  In 
one  method  two  or  three  or  four  of  the  transverse  joints  between  the 
courses  of  brick  are  filled  with  bituminous  cement.  In  the  other 
method  a  J-inch  plank  is  inserted  between  courses  of  brick  at  inter- 
vals of  25  to  50  feet;  and  then  after  the  brick  are  laid  and  grouted, 
the  plank  is  removed  and  the  space  is  filled  with  bituminous  cement. 
Or,  instead  of  the  plank,  a  pre-moulded  sheet  of  mastic  (§  1017)  is 
used. 

1019.  There  are  several   objections   to  'transverse   contraction 
joints  in  a  pavement  having  a  grout  filler. 

1.  The  expansion  joint  is  weaker  than  other  joints,  and  hence 
the  weight  of  passing  wheels  is  likely  to  break  the  bond  of  a  brick 
next  to  the  joint,  and  then  the  bond  of  one  brick  after  another  fails 
in  succession. 

2.  The  contraction  joint  concentrates  all  of  the  shortening  of  a 
section  of  the  pavement  at  one  line,  and  opens  the  contraction  joint 
so  as  to  permit  water  to  enter  the  sand  cushion.     The  water  acts  as 
a  lubricant  and  causes  the  sand  to  shift,  and  often  permits  a  brick 
to  settle;  and  then  the  impact  of  a  passing  wheel  breaks  the  bond  of 
another  brick,  and  the  defect  gradually  extends.     The  water  may 
freeze  and  lift  the  pavenent,   which  rarely  returns  to  its  former 
position,  for  the  game  reason  that  the  continued  action  of  frost  lifts 
loose  stones  to  the  top  of  the  ground.     If  there  are  no  contraction 
joints,  the  contraction  is  likely  to  open  many  narrow  cracks  which 
are  less  harmful  than  a  few  wider  ones. 

3.  The  filler  in  the  expansion  joint  becomes  more  rigid  at  the 
top  than  at  the  bottom,  partly  by  vaporization  and  oxidation,  and 
partly  by  the  pounding  in  of  street  dirt;    and  consequently  the 
expansion  of  the  pavement  concentrates  pressure  at  the  top  of  the 
joint,  and  the  adjoining  brick  are  spalled.     This  roughens  the  pave- 
ment and  increases  the  effect  of  impact,  which  breaks  the  bond  and 
causes  the  sand  cushion  to  shift.     If  there  are  no  transverse  expan- 
sion joints,  the  expansion  simply  produces  compression  in  the  pave- 


ART.    2] 


CONSTRUCTION 


535 


ment  and  does  no  harm.  This  conclusion  is  in  harmony  with  expe- 
rience with  concrete  pavements  (§  466-68),  namely,  that  trans- 
verse expansion  joints  are  not  only  not  needed,  but  are  a  positive 
detriment. 

1020.  Fig.  195  shows  two  examples  of  failures  due  primarily  to 
a  contraction  joint.  In  the  left-hand  view  the  wheel-track  crosses 
the  joint  near  the  middle  of  the  picture;  and  doubtless  the  damage 


FIQ.  195. — FAILURES  AT  TRANSVERSE  CoNTRAcriONLJoiNTS. 


started  at  this  point  and  gradually  progressed  in  all  directions,  for 
each  of  the  three  reasons  explained  in  §  1019.  In  the  right-hand 
view  the  contraction  joint  slopes  up  and  to  the  right  across  the 
picture.  For  some  reason  the  damage  is  more  to  the  right  of  the 
joint  than  to  the  left,  perhaps  because  of  defective  grouting  (§  1059). 

1021.  Expansion  Joints  at  Anchors.    An  expansion  joint  |  to 
f  of  an  inch  wide  should  be  provided  around  nlanhole  covers,  water 
boxes,  etc.,  which  might  act  as  anchors  to  prevent  the  expansion  of 
the  pavement.     Frequent  examples  are  seen  where  the  pavement 
buckles  at  such  points  owing  to  the  lack  of  adequate  provision  for 
expansion. 

1022.  HEADERS.     A  header  is  a  wood  or  stone  or  concrete  curb 
or  protection  placed  at  the  end  of  the  pavement  or  at  an  alley  and 
street  intersection,  to  protect  the  edge  of  the  pavement  from  vehicle 
wheels  bumping  against  it  in  getting  on  the  pavement.     A  wood 
plank  2  to  4  inches  thick,  held  in  position  by  posts,  is  sometimes 
used;    but  stone  or  concrete  are  more  durable,  and  are  not  much 
higher  in  first  cost.     A  4-inch  hard  limestone  or  a  6-inch  concrete 
slab  is  usual. 


536  BRICK  PAVEMENTS  [CHAP.    XVII 

With  a  monolithic  brick  pavement  the  header  is  not  absolutely 
necessary,  as  the  brick  will  stand  a  good  deal  of  bumping  without 
being  dislodged;  but  even  in  this  case  the  use  of  a  substantial  header 
is  true  economy. 

1023.  COMPARISON  OF  TYPES  OF  BRICK  PAVEMENTS.    Brick 
pavements  differ  chiefly  as  to  the  nature  of  the  bedding  course  and 
the  character  of  the  joint  filler.     The  different  types  will  be  com- 
pared as  to  durability,  smoothness,  noisiness,  thickness,  time  in 
construction,  and  cost. 

1024.  Durability.     The  durability  depends  chiefly  upon  the  ma- 
terial of  the  joint  filler.     The  merits  of  the  several  joint  fillers  have 
already  been  considered;    and  hence  little  need  be  said  here.     How- 
ever, it  may  be  repeated   that  a   cement-grout  filler  protects   the 
edges  of  the  brick  best,  and  that  such  a  filler  makes  the  most  durable 
pavement. 

1025.  Smoothness.     It  has  been  abundantly  proved  by  expe- 
rience in  the  field  that  it  is  easier  to  get  a  smooth  surface  with  a 
mortar   bedding-course    than    with    a    sand    cushion.     Smoothness 
promotes  durability;    and  besides  the  smoother  the  pavement  the 
less  noisy  it  is. 

1026.  A  grout  or  a  bituminous  filler  is  not  retained  in  the  joints 
of  re-pressed  brick  as  well  as  of  those  not  re-pressed,  since  the  former 
have  a  rounded  edge  while  the  latter  have  a  square  edge.     With  a 
rounded  edge,  if  the  joint  is  filled  level  full,  the  filler  feathers  out  at 
its  edges  and  is  easily  crumbled  off;   and  then  the  next  wheel  drops 
into  the  depression,  and  breaks  out  more  of  the  filler.     Soon  the 
filler  is  broken  out  to  a  considerable  depth,  and  then  the  joint  be- 
comes a  groove  into  which  each  passing  wheel  drops  with  a  bump  that 
disintegrates  the  edge  of  the  brick.     With  a  square-edged  brick  the 
joint  filler  wears  away  only  as  fast  as  the  face  of  the  brick. 

1027.  Noisiness.    A  brick   pavement   may  produce   noise  from 
two  causes.     One  of  these  is  the  roughness  of  the  surface,  which  has 
just  been  considered  in  the  preceding  paragraph. 

The  second  occurs  only  with  a  sand  bedding-course  and  grout- 
filled  joints,  and  is  due  to  the  fact  that  the  wearing  course  is  sepa- 
rated from  the  bedding  course,  which  causes  the  pavement  to  give 
out  a  rumbling  or  roar  when  a  steel-tired  wheel  goes  over  it.  This 
separation  may  be  due  to  the  drying  out  of  the  sand  cushion  in  spots, 
which  causes  it  to  shrink  away  from  the  brick  wearing  coat.  Many 
brick  pavements  rumble  from  this  cause.  When  this  occurs  the 
pavement  gives  out  a  rumbling  when  a  steel-tired  wheel  goes  over 


ART.   2]  CONSTRUCTION  537 

one  of  these  hollow  spots.  Sometimes  a  high  temperature  lifts  the 
whole  wearing  coat  up  from  the  bedding  course,  when  the  noise  is 
very  marked.  Something  like  the  same  result  occurs  in  cold  weather, 
possibly  owing  to  the  expansive  action  of  freezing  water  in  the  soil 
behind  the  curb  crowding  the  curbs  inward  and  thus  lifting  the 
wearing  coat  up  from  the  bedding  course.  If  each  curb  of  a  40-foot 
pavement  is  forced  inward  ^V  of  an  inch,  the  crown  of  the  pavement 
will  be  lifted  from  the  foundation  more  than  an  inch.  This  result 
will  occur  only  when  the  subsoil  outside  of  the  curbs  freezes  while  it 
is  at  least  nearly  saturated  with  water. 

Bgth  the  semi-monolithic  and  the  monolithic  types  of  brick  pave- 
ment are  free  from  any  rumbling  noise. 

1028.  Thickness.  Nominally  there  are  three  forms  of  bedding 
courses;  but  really  there  are  only  two,  viz.:  sand,  and  cement 
mortar.  A  brick  pavement  has  thickness  primarily  to  enable  it 
to  distribute  the  concentrated  load  of  a  wheel  over  sufficient  area 
of  the  subgrade  to  enable  the  native  soil  to  support  the  load.  The 
pavement  distributes  the  load  mainly,  if  not  wholly,  by  its  strength 
as  a  beam,  which  enables  it  to  bridge  over  any  soft  spot  and  also  to 
resist  the  lifting  action  of  frost  in  the  subgrade. 

The  thickness  <3f  a  brick  pavement  having  a  sand  cushion  is 
about  as  follows:  foundation,  6  inches;  sand  cushion,  2  inches; 
and  wearing  coat,  4  inches, — a  total  of  12  inches;  but  considered 
as  a  beam,  the  effective  thickness  of  such  a  pavement  is  only  that 
of  the  concrete  base,  i.  e.,  6  inches.  The  thickness  of  a  pavement 
having  a  cement  mortar  bedding-course  is  as  follows:  foundation, 
6  inches;  bedding  course,  1  inch,  and  wearing  coat,  4  inches, — a  total 
of  11  inches;  but  considered  as  a  beam,  the  effective  thickness  of  such 
a  pavement  is  1 1  inches.  The  strength  of  a  beam  varies  as  the  square 
of  its  depth,  and  therefore  the  relative  beam  strength  of  the  two 
pavements  as  above  is  as  36  to  121;  or  the  monolithic  pavement 
considered  as  a  beam  is  3.35  times  the  stronger.*  Even  though 
there  may  not  be  a  perfect  union  between  the  foundation  and  the 
wearing  coat,  the  above  ratio  is  nearly  correct,  for  the  bedding 
course  is  nearly  at  the  center  of  the  beam  and  hence  there  is  little 
or  no  longitudinal  shear  upon  it,  and  hence  the  pavement  acts  nearly 
as  a  solid  beam.  Laboratory  experiments  show  that  a  well-grouted 
layer  of  brick  has  as  great  transverse  strength  as  a  concrete  slab 
of  equal  depth. 

*  For  data  on  the  strength  of  slabs  of  monolithic  brick  pavements,  see  Engineering  Record, 
Vol.  73  (1916),  p.  86;   and  Engineering  News-Record,  Vol.  79  (1917),  p.  820-23. 


538  BRICK   PAVEMENTS  [CHAP.    XVII 

Since  the  practicability  of  laying  the  brick  in  a  bed  of  cement 
mortar  has  been  demonstrated,  it  has  often  been  proposed  to  reduce 
the  total  depth  of  a  brick  pavement  having  a  mortar  bedding-course 
and  cement-filled  joints.  In  support  of  the  possibility  of  making 
thinner  pavements  when  the  wearing  coat  is  cement-grouted,  it  is 
often  cited  that  4-inch  concrete  roads  in  California  give  at  least  fair 
satisfaction;  and  that  many,  perhaps  most,  concrete  roads  in  the 
Mississippi  Valley  are  only  6  inches  thick.  Some  engineers  have 
reduced  the  thickness  of  the  concrete  foundation,  and  others  have 
reduced  the  thickness  of  the  wearing  coat. 

The  extreme  of  the  former  practice  is  perhaps  in  Stockton  -Town- 
ship, Vermilion  County,  Illinois,  in  which  in  1916  6J  miles  of  rural 
brick  roads  were  constructed  with  a  brick  wearing  coat  4  inches 
thick  and  a  concrete  base  only  1  inch  thick.  Another  striking 
example  is  in  Polo,  Illinois,  where  in  1917  4-inch  bricks  were  laid  on 
2  inches  of  concrete.  The  purpose  of  either  the  1-inch  or  2-inch 
concrete  in  the  above  examples  is  not  to  act  as  a  foundation  to  sup- 
port either  the  brick  or  the  load  upon  the  pavement,  but  to  make  a 
smooth  surface  on  which  to  set  the  brick  and  also  to  prevent  the  grout 
filler  from  penetrating  the  subgrade.  In  both  of  the  above  examples 
the  subsoil  is  clay  or  loam. 

An  example  of  the  practice  of  reducing  the  depth  of  the  brick  is 
to  be  found  in  many  cities  in  the  Mississippi  Valley  west  of  the  river, 
in  which  many  pavements  were  built  between  1912  and  1917  using 
vertical-fiber  brick  2J  inches  deep.  Another  example  of  the  use  of 
2j-inch  brick  is  the  driveway  entrance  to  the  Pennsylvania  Passenger 
Station  in  New  York  City  (§  981).  The  wear  on  a  grout-filled  brick 
pavement  is  very  small  (see  Fig.  193,  page  528) ;  and  hence  a  brick 
2|  inches  deep  will  last  nearly  as  long  as  a  brick  4  inches  deep. 

An  example  of  reducing  the  thickness  of  both  the  foundation  and 
the  wearing  coat,  is  over  50  miles  of  rural  brick  roads  in  Florida 
consisting  of  3J-inch  grouted  brick  laid  on  puddled  native  sand; 
and  similar  pavements  are  laid  on  the  streets  of  several  southern 
cities.  Some  of  these  pavements  have  been  in  service  two  years, 
and  have  carried  10-ton  motor  trucks  without  any  signs  of  distress. 
Of  -course,  these  pavements  are  not  subjected  to  frost  action. 

1029.  It  is  impossible  to  compute  or  otherwise  determine  in 
general  the  permissible  minimum  thickness  for  any  particular  form 
of  construction,  since  the  required  thickness  varies  greatly  with  the 
character  of  the  subsoil  and  the  climate;  but  it  is  certain  that  under 
conditions  where  a  sand-cushion  brick  pavement  gave  fairly  satis- 


ART.    2]  CONSTRUCTION  539 

factory  service,  a  thinner  pavement  may  be  used  if  it  is  built  mono- 
lithic. Only  considerable  experience  will  determine  the  safe  and  not 
extravagant  thickness  of  a  pavement. 

1030.  Of  course,   the  cost  will  vary  with  the  thickness;    and 
whether  it  is  cheaper  to  diminish  the  thickness  of  the  concrete  base 
or  that  of  the  brick-wearing  course  will  depend  upon  conditions. 
There  are  localities  where  it  is  economical  to  decrease  the  depth  of 
the  brick,  as  for  example  where  bricks  are  expensive  and  materials 
for  concrete  are  cheap;   and  on  the  other  hand,  there  may  be  con- 
ditions under  which  it  is  wiser  to  decrease  the  thickness  of  the  con- 
crete. 

1031.  Time  under  Construction.     One  of  the  most  important 
advantages  of  the  monolithic  type  of  brick  pavement  is  the  length 
of  time  required  for  construction.     All  parts  of  it   (the  concrete 
foundation,  the  bedding  course,  and  the  grout  filler)  are  constructed 
and  seasoned  simultaneously.     With  a  concrete  foundation,  sand 
cushion,    and   grout-filled   joints,    the   concrete   foundation   should 
be  allowed  to  set  for  about  20  days  before  the  sand  cushion  is  spread 
and  the  brick  set  (see  §  464) ;  and  another  20  days  should  be  allowed 
for  the  cement  grout  to  harden.     This  is  one  reason  why  a  grout 
filler  is  not  used  more  frequently. 

1032.  Cost.     The  cost  of  construction  of  the  monolithic  type  is 
10  to  12  cents  per  square  yard  less  than  that  of  the  sand-cushion 
grout-filled  type.     The  reasons  for  this  difference  of  cost  is  as  fol- 
lows:   1.  All  parts  of  the  work  are  done  at  substantially  the  same 
time,  and  hence  so  much  care  is  not  required  in  protecting  and  caring 
for  the  work  while  the  cement  sets.     2.  A  lighter  roller  is  used  in 
rolling  the  brick.     3.  Less  sand  is  used  for  the  bedding  course. 
4.  There  is  less  risk  of  having  to  take  up  and  re-lay  brick  on  account 
of  the  sand  cushion  having  been  rained  on.     5.  It  is  possible  to  use 
either  a  thinner  concrete  foundation  or  a  shallower  brick.     6.  For 
a  rural  road  the  monolithic  construction  does  away  with  the  need 
of  a  curb  or  edging. 

1033.  Conclusion.     In   all   points  the  monolithic   pavement  is 
superior  to  any  other  type  of  brick  pavement. 

1034.  PAVEMENT  ADJACENT   TO   TRACK.    It  is  exceedingly 
difficult  to  construct  any  pavement  adjacent  to  a  street-railway 
track  that  will  not  need  frequent  and  extensive  repairs.     A  large 
part  of  the  difficulty  is  due  to  the  foundation  of  the  track,  which 
subject  has  been  considered  in  Art.  3,  of  Chapter  XV — Foundations 
for  Street-railway  Tracks.     Another  difficulty  is  in  keeping  a  water- 


540 


BRICK   PAVEMENTS 


[CHAP,  xvn 


tight  joint  between  the  head  of  the  rail  and  the  pavement.     Fig.  196 
shows  the  standard  practice  of  laying  brick  in  the  track  area  when 


9"Raif       ^Mortarbect 


;:;^te^.fc^g  I  ye^g^f!±fS     %m$jj$M$$j§ 


FIG.  196. — STANDARD  PRACTICE  IN  BALTIMORE,  MD. 

a   grooved   rail   is  used;    and   Fig.   197   shows   the   corresponding 
arrangement  when  a  T  rail   is   used.     Most  brick   manufacturers 

f  /£  "Pawng  Sand 


3andand  ^ 
Cement 


"*  7-0"  Wh,fc  Oak  Tie 


a^g^vlJJ^J,]. -ImU ,\ ,\*L \  \  -Ml •  I  -LI      L^L 


/// 


,    brich- 


Fia.  197. — STANDARD  PRACTICE  OF  FORT  WAYNE  AND  WABASH  VALLEY  TRACTION  Co. 

make  the  "  bull-nose  J>  brick  for  placing  next  to  the  inside  of  the 
rail  as  shown  in  the  upper  view  in  Fig.  197.  Notice  in  the  lower 
part  of  Fig.  197  that  the  paving  between  the  rails  is  level  with 
the  top  of  the  rails;  but  often  it  is  made  level  a  little  below  the  top 
of  the  rail.  Notice  also  that  the  lower  half  of  Fig.  197  has  a  longitu- 
dinal concrete  beam  under  each  rail. 

Bricks  are  much  used  for  paving  the  railway  area,  particularly 
between  the  rails,  because  of  their  low  first  cost  and  of  the  ease  with 
which  they  can  be  laid.  *| 

It  is  practically  impossible  to  maintain  a  brick  pavement  with  a 
sand  cushion  adjoining  a  railway  track,  since  the  cushion  will  shift 
under  travel  and  since  water  will  leak  into  the  cushion  and  freeze. 

1035.  MAXIMUM  PERMISSIBLE  GRADE.  The  Committee  of 
the  American  Society  of  Civil  Engineers  recommends  12  per  cent 
as  the  permissible  maximum  grade  for  a  brick  pavement  with  a 


ART.    2]  CONSTRUCTION  541 

bituminous  filler,  and  6  per  cent  for  a  grout  filler— see  Table  15, 
page  58.  The  report  impliedly  recommends  the  use  of  a  plain  brick 
and  bituminous  filler  on  steep  grades;  but  this  is  not  in  accordance 
with  accepted  good  practice.  The  best  practice  employs  grout  on  all 
grades;  and  uses  plain  brick  on  grades  up  to  5  or  6  per  cent,  and 
hill-side  brick  (§  936)  for  grades  up  to  10  or  12  per  cent.  Hill-side 
brick  and  grout  filler  have  given  fair  satisfaction  on  grades  as  high 
as  15  and  18  per  cent. 

1036.  STREETS  PAVED  WITH  BRICK.     The  preceding  discussion 
of  brick  pavements  has  been  without  special  reference  to  either  city 
streets  or  country  roads.     A  few  differences  resulting  from  the  dif- 
ferent locations  of  the  pavement  remain  to  be  considered. 

The  chief  difference  in  the  construction  of  a  brick  pavement  on  a 
city  street  and  on  a  rural  road  is  due  to  the  fact  that  usually  the 
paved  portion  is  wider  on  the  former  than  the  latter.  This  neces- 
sitates either  the  use  of  a  longer  template  in  striking  the  concrete 
foundation  and  the  bedding  course,  or  the  placing  of  screeds  and  the 
use  of  a  shorter  template.  The  construction  of  the  concrete  founda- 
tion has  been  discussed  in  Chapters  VII  and  XIV.  The  template 
employed  in  striking  the  concrete  is  described  in  §  460,  and  that  used 
for  the  bedding  course  in  §  972.* 

The  bricks  are  usually  transported  from  the  parking  to  the  setter 
either  by  hand  with  pallets  or  tongs,  or  by  a  roller  gravity  conveyor 
—usually  the  latter. 

Longitudinal  expansion  joints  are  always  required  for  street 
pavements. 

1037.  ROADS  PAVED  WITH  BRICK.     The  foundation  for  brick 
pavements  on  rural  roads  more  frequently  than  on  city  streets  is  an 
old  macadam  road  (§  437). 

Formerly  brick  roads  usually  had  a  sand  cushion,  but  recently 
the  semi-monolithic  or  monolithic  construction  is  ordinarily  em- 
ployed— generally  the  latter.  Fig.  198  shows  two  typical  views  of 
the  construction  of  a  monolithic  brick  road.  The  left-hand  view 
shows  the  bricks,  the  fine*  and  coarse*  aggregate,  and  the  cement 
delivered  ready  for  work,  and  also  the  side  forms  in  place.  Notice 
that  the  materials  have  been  transported  to  the  job  on  an  industrial 
railway.  The  right-hand  view  shows  work  in  progress.  In  the  lat- 
ter notice  the  steel  side-forms,  and  the  tamping  template.  The 
double  template  (§  982)  is  shown  between  the  tamping  template 

j*  For  an  illustrated  account  of  the  laying  of  a  monolithic  brick  pavement  33  feet  wide  on  a 
city  street,  see  Engineering  News,  Vol.  76  (1916),  p.  978-79. 


542 


BRICK   PAVEMENTS 


[CHAP,  xvii 


and    the    concrete    mixer,  and    is    weighted    down    with    bags    of 
cement. 

Formerly  the  sand-cushion  brick'  road  was  built  with  an  inde- 
pendent curb  (§  729-34),  or  with  an  integral  curb  or  an  edging 
(§  771).  Fig.  199  shows  the  concrete  foundation  for  a  sand-cushion 


FIG.  198. — CONSTRUCTION  OF  MONOLITHIC  BRICK  ROAD. 


FIG.  199. — CONCRETE  FOUNDATION  WITH  EDGING. 


brick  road  with  edging,  in  process  of  construction.  The  concrete  is 
being  spread  to  grade  with  shovels  and  smoothed  with  the  back  of  a 
shovel.  The  surface  on  the  concrete  slab  is  not  first-class;  but  is 
good  for  the  method  used,  and  is  good  enough  in  consideration  that  a 
sand  cushion  is  later  to  be  used.  The  combined  curb  and  gutter 


AET.   2] 


CONSTRUCTION 


543 


(of  the  shallow  V  type)  is  completed  on  the  far  side,  and  the  form  for 
the  integral  edging  is  in  position  in  the  foreground  of  the  left  side. 
The  trussed  scantling  or  template  is  being  used  in  testing  the  crown 
or  elevation  of  the  concrete,  the  ends  of  the  projecting  strip  indicating 
the  height  the  concrete  should  have. 

Fig.  200  shows  four  views  of  a  semi-monolithic  brick  road 
with  edging,  in  process  of  construction.  In  view  1,  notice  the 
low  spot  in  the  sand-cement  bed.  In  view  2,  notice  that  the  men 
are  walking  on  the  ungrouted  brick,  which  is  objectionable,  but  less 
so  with  a  cement-sand  bed  than  with  a  sand-cushion  (§971).  In 


1.  Striking  the  Sand-cement  Bed. 


2.  Laying  the  Bricks. 


3.  Sprinkling  the  Brick.  4.  Second  Application  of  Grout. 

FIG.  200. — CONSTRUCTION  OF  MONOLITHIC  BRICK  ROAD  WITH  EDGING. 

view  3,  the  stream  of  water  is  so  solid  or  heavy  as  to  wash  out  the 
cement  in  the  bedding  layer,  although  the  laborer  says  the  stream 
does  no  harm  as  he  always  plays  upon  the  center  of  a  brick.  View  4 
shows  the  mush-like  consistency  of  the  last  coat  of  grout,  which  is 
dangerously  near  being  too  thick  to  run  into  the  joints  well. 

1038.  Since  the  introduction  of  the  monolithic  brick  pavement, 
the  brick  wearing-coat  for  rural  roads  is  laid  without  any  curb  or 
edging  (§  471),  experience  having  shown  that  the  bricks  at  the  edge 
of  the  pavement  are  not  dislodged  by  traffic. 


544  BRICK   PAVEMENTS  [CHAP.   XVII 

The  edge  or  corner  of  a  monolithic  brick  road  is  rather  rough  and 
ragged,  and  very  destructive  of  automobile  tires  in  turning  off  and 
onto  the  pavement,  if  the  earth  is  not  kept  well  filled  up  against  the 
pavement.  It  is  nearly  impossible  to  keep  the  earth  shoulders  full, 
and  gravel  or  broken-stone  shoulders  are  seldom  used;  and  hence 
this  ragged  edge  is  an  objection  to  omitting  the  concrete  edging. 
This  objection  could  be  eliminated  by  laying  a  bull-nose  brick  (see 
§  1034)  at  the  end  of  a  course;  but  it  is  not  known  that  this  has  ever 
been  done.  The  outer  corner  of  the  concrete  edging  can  be  readily 
rounded  off  with  an  edging  tool,  although  it  is  not  often  so  done. 

1039.  COST  OF  BRICK  PAVEMENTS.  The  cost  will  vary  with  the 
locality  and  the  details  of  construction,  and  consequently  any  gen- 
eral statement  of  cost  will  be  only  approximately  true  for  any  par- 
ticular case. 

The  grading  is  usually  done  by  the  cubic  yard;  and  the  cost 
varies  with  the  character  of  the  soil,  the  depth  to  be  removed,  the 
length  of  haul,  etc.  The  cost  of  grading  ranges  from  15  to  50  cents 
per  cubic  yard;  but  in  easy  soil  and  moderate  cuts,  it  generally 
varies  from  25  to  35  cents.  It  usually  costs  3  to  5  cents  a  square 
yard  to  dress  off  the  subgrade  after  it  has  been  graded  with  drag 
or  wheel  scrapers,  and  to  throw  the  material  into  wagons. 

The  cost  of  rolling  the  subgrade  will  depend  upon  whether  it  is 
rolled  longitudinally  only,  or  both  longitudinally  and  transversely. 
With  a  self-propelled  roller  the  cost  of  rolling,  both  transversely  and 
longitudinally,  will  be  about  0.6  cent  a  square  yard,  exclusive  of 
interest,  storage,  and  depreciation  of  the  roller. 

The  cost  of  the  concrete  foundation  will  vary  with  the  price  of 
cement,  the  proximity  of  broken  stone  or  gravel,  the  character  of  the 
concrete,  etc.  Ordinarily  the  materials  for  a  6-inch  course  will  cost 
50  to  60  cents  per  square  yard,  and  the  labor  6  to  8  cents  per  square 
yard. 

The  price  of  bricks  will  vary  with  their  size,  the  locality,  and  the 
freight  rate.  Previous  to  1916  there  was  no  uniformity  in  size, 
common  sizes  for  the  wearing  face  being  3|  by  8J  inches,  3f  by  8, 
and  3  by  9;  and  some  brands  requiring  42  for  a  square  yard,  some  40, 
and  some  38.  Since  the  beginning  of  1916  there  has  been  a  vigorous 
attempt  to  have  all  paving  bricks  of  standard  size,  or  rather  to  have 
the  wearing  face  3J  by  8i  inches,  of  which  40  make  a  square  yard  of 
pavement.  In  1915,  before  the  disturbance  of  prices  due  to  the 
Great  European  War,  the  average  price  at  the  plant  for  standard 
blocks  4  inches  deep  was  about  $15.00  per  thousand,  or  60  cents  per 


ART.    2]  CONSTRUCTION  545 

square  yard;  but  in  1917  for  various  reasons  the  price  was  practically 
50  per  cent  more.  There  is  no  difference  in  price  between  wire-cut 
lug  and  re-pressed  bricks.  In  estimating  the  freight  it  may  be 
helpful  to  know  that  a  brick  2£  X  4  X  8J  inches  weighs  about  7 
lb.,  and  a  block  3J  X  4  X  8J  inches  about  9.75  Ib.  In  estimating 
freight,  the  fact  should  not  be  overlooked  that  for  one  reason  or 
another  a  considerable  number  of  bricks  are  rejected.  With  careful 
grading  at  the  kiln  the  broken  and  rejected  brick  is  likely  to  be  1  to  2 
per  cent. 

In  the  early  history  of  brick  paving  it  was  customary  for  the 
contractor  to  buy  brick  by  the  thousand;  but  the  contractor  claimed 
that  the  manufacturer  did  not  cull  the  brick  sufficiently  carefully 
at  the  kiln,  and  consequently  the  rejections  on  the  job  were  unduly 
great.  For  a  time  it  became  customary  to  buy  the  brick  f.o.b. 
destination  at  a  stated  price  per  square  yard  in  place  in  the  pave- 
ment; but  under  this  plan,  the  manufacturer  claimed  that  the  con- 
tractor did  not  use  proper  care  in  handling  the  bricks,  did  not  keep 
them  clean,  used  good  brick  instead  of  chipped  or  broken  brick  in 
making  closures,  and  left  good  bricks  along  the  finished  pavement. 
At  present  it  is  customary  to  sell  the  brick  by  the  thousand  f.o.b. 
destination.  The  usual  price  is  $25.00  per  thousand  f.o.b.  destina- 
tion. 

The  cost  of  hauling  and  piling  on  the  side  of  the  street  is  about 
$1.50  per  thousand  for  a  haul  of  1  mile,  of  which  sum  about  half  is 
the  cost  of  loading  and  unloading,  and  half  the  cost  of  team  and 
driver;  but  this  cost  for  team  and  driver  necessitates  the  use  of  three 
wagons  with  each  team. 

The  cost  of  setting  blocks  of  which  40  make  a  square  yard  varies 
from  4  to  6  cents  per  square  yard. 

The  cost  of  turning  the  chipped  brick  and  replacing  the  rejected 
ones  will  depend  mainly  upon  the  severity  of  the  inspection  and  upon 
the  degree  of  care  employed  in  culling  the  brick  before  they  are  laid. 
In  a  particular  case,  80  hours  were  required  to  turn  the  chipped 
blocks  and  to  replace  the  rejected  blocks  with  good  ones,  in  1,633 
square  yards  of  pavement,  or,  say,  1  hour  for  each  20  square  yards. 
The  blocks  were  3X4X9  inches,  and  about  2  per  cent  were 
turned  and  about  2  per  cent  were  rejected. 

For  data  concerning  the  cost  of  sand  filler,  see  §  995;  for  cost  of 
cement  filler,  see  §  1006;  and  for  cost  of  bituminous  filler,  see  §  1013. 

Examples  of  the  actual  cost  of  brick  pavements,  are  given  in 
§  1041-48. 


546 


BRICK    PAVEMENTS 


CHAP.   XVII 


1040.  In  this  connection  it  should  not  be  overlooked  that  the 
cost  of  the  pavement  proper  is  usually  not  the  only  cost  of  improving 
the  street  when  it  is  paved.     For  details  see  §  878. 

1041.  Examples  of  Cost.     In  the  following  examples  an  attempt 
has  been  made  to  present  the  data  in  such  detail  as  to  make  clear 
the  form  of  construction;  but  unfortunately  it  is  not  always  possible 
to  present  information  concerning  important  economic  conditions,  as 
freight  rates,  the  condition  of  the  wagon  roads  over  which  material 
is  hauled,  efficiency  of  labor,  etc. 

1042.  Sand-cushion    Asphalt-filled   Brick    Pavement.     Table    56 
gives  the  details  of  laying  a  brick  pavement.     The  excavation  was 
done  with  a  Maney  4- wheel  scraper  (§  154)  and  a  20-H.P.  tractor. 
Wages  were  as  follows:   Common  labor  20  cents  per  hour;   brick 
setters,    55   and  40   cents;   engine  runner  for  concrete  mixer,   30 
cents;  team,  wagon,  and  driver,  50  cents  per  hour. 


TABLE  56 

COST  OF  SAND-CUSHION  ASPHALT-FILLED  BRICK  PAVEMENT 
In  Central  Illinois  in  1916 


ITEMS. 


COST  PER  SQ.   YD. 


Partial. 


Total. 


SUBGRADE : 

Rough  grading  at  29.8  cts.  per  cubic  yard $0 . 137 

Surfacing  and  rolling  with  10-ton  roller .040       $0.217 

CONCRETE  FOUNDATION,  6  inches  of  1  :  3  :  5 : 

Labor  at  20  cts.  per  hour 056 

Cement  at  $1.48  per  barrel  (net)  on  job 227 

Gravel  at  $1.40  per  cubic  yard  on  job .  156 

Sand  at  $1.50  per  cubic  yard  on  job 122 

Coal  and  water 006  . 568 

SAND  CUSHION,  1|  inches  thick 0. 640 

BRICK,  wire-cut  lug,  3|X4X8|  inches: 

Purchase  price,  f.o.b.  destination 807 

Handling  in  car 027 

Hauling  to  street— average  2,200  feet 020 

Laying,  contract  price .050  .904 

JOINT  FILLER: 

Asphalt,  12  Ib.  per  square  yard 095 

Labor,  heating  and  pouring 060  . 155 

MISCELLANEOUS  EXPENSE .052 

Total  cost,  exclusive  of  administration,  tools  and  profits $1 . 960 


AET.   2] 


CONSTRUCTION 


547 


1043.  3-inch  Brick  Pavement.  Table  57  shows  the  cost  of  a 
brick  pavement  having  a  4-inch  concrete  foundation,  IJ-inch  sand 
cushion,  3-inch  vertical-fiber  brick  wearing  coat,  and  asphalt  joint- 
filler. 

TABLE  57 
COST  OF  3-iNCH  BRICK  PAVEMENT  IN  FALLS  CITY,  NEB.,  IN  1914* 


COST  PE 

R    SQ.  YD. 

Partial. 

Total. 

SUBGRADE  I 

Rough  grading                 

$0  03 

Surfacing  and  rolling  

015 

$0  045 

CONCRETE  FOUNDATION,  4  inches  thick: 
Cement  at  $1.63  per  barrel  

0  185 

Stone  at  $2.25  per  cubic  yard  

232 

Sand  at  $1  10  per  cubic  yard. 

060 

]Vlixing  and  placing 

050 

527 

SAND  CUSHION,  1^  inches,  at  $1.10  per  cubic  yard  

.046 

BRICK,  vertical-fiber,  3  inches  deep,  f.o.b  destination.  .  .  . 
Unloading  and  hauling  

0.70 
.05 

Preparing  cushion  and  setting  brick 

04 

Rolling 

006 

796 

JOINT  FILLER: 
Bituminous  material     .  .        

0  12 

ADolvinff 

03 

150 

INCIDENTAL  EXPENSES  

017 

Total,  exclusive  of  administration,  depreciation,  and  profits. 

$1.581 

*  Engineering  News,  Vol.  73  (1915),  p.  223. 

1044.  Brick  Roads  in  New  York.     Table  58,  page  548,  shows  the 
average  cost  of  a  great  number  of  brick  roads  built  in  New  York 
by  the  State  Highway  Commission  in  1912  and  1913.*    The  roads 
were  15  feet  wide,  had  5-inch  concrete  base,  6-inch  concrete  edging, 
sand  cushion,  and  grout  filler.     The  cost  below  includes  engineering 
expenses. 

1045.  Monolithic  Pavement.     Table  59,  page  548,  shows  the  labor 
and  materials  required  for  a  monolithic  brick  pavement  33  feet  wide, 
with  4-inch  brick,  and  1-inch  special  expansion  joints  every  82  feet. 
Table  60,  page  548,  shows  the  labor  cost  of  this  job. 

*  Engineering  News,  Vol.  70  (1913),  p.  1149. 


548 


BRICK  PAVEMENTS 


[CHAP.  XVII 


TABLE  58 
AVERAGE  COST  OF  BRICK  ROADS  IN  NEW  YORK  IN  1912  AND  1913 

TOTAL  COST. 


ITEMS. 

Per  Cent. 

Per  Mile. 

Excavation  

9.0 

$2200 

Drainage  structures.                   

2.8 

700 

Subgrade  and  foundation  

25.9 

6300 

Brick  wearing-course  and  edging  

60.1 

14700 

Minor  expenses  

2.0 

500 

Total   . 

100  0 

$24  400 

TABLE  59 

AMOUNT  OF  LABOR  AND  MATERIALS  FOR  MONOLITHIC  BRICK  PAVEMENT* 
Central  Illinois,  1916 


Labor. 

Hours  per 
Sq.  Yd. 

Materials. 

Cu.  Ft.  Per 
Sq.  Yd. 

BASE  AND  BED: 
laborers  
engine  runner  

0.2305 
.0154 

CONCRETE  BASE,  4  inches  : 
1  :  6  cement  
gravel  

0.5 
3.000 

sub-foreman 

0138 

BEDDING  MORTAR  ^-inch  * 

BRICK  SETTING: 

1  :  2  cement  

.094 

laborers 

1772 

sand 

188 

setters 

0227 

GROUT  1:1* 

filling  joints. 

0887 

cement. 

144 

spreading  sand 

0048 

sand 

144 

OVERHEAD  : 
timekeeper  

0171 

TOP  COVERING: 
sand 

188 

foreman.  

0171 

water  boy  

.0171 

TABLE  60 

COST  OF  LABOR  FOR  MONOLITHIC  BRICK  PAVEMENT* 
Central  Illinois,  1916 


'm                                 ITEMS. 

COST 
PER.  SQ.  YD. 

Concrete  base,  4  inches  of  1  :  6  gravel  

$0  0649 

Setting  brick,  3|  X4  X8|  inches  

0525 

Filling  joints  with  1  :  1  grout  

0244 

Spreading  top  covering  of  sand  

0011 

Total  cost  of  labor  

$0  1429 

*" Engineering  News,  Vol.  76  (1916),  p.  1219." 


AET.    2]  CONSTRUCTION  549 

1046.  Brick  on  Old  Macadam.  Table  61  shows  the  cost  of 
laying  a  new  brick  pavement  on  an  old  macadam  base  by  the  Street 
Department  of  Carlisle,  Pa.  The  macadam  was  spiked  with  a  13-ton 
3-wheel  self-propelled  roller,  excavated  to  subgrade,  surfaced  with 
hand  picks,  and  rolled.  The  bricks  were  rolled  with  a  5-ton  roller 
drawn  by  12  men. 

TABLE  61 
COST  OF  NEW  BRICK  PAVEMENT  ON  OLD  MACADAM  FOUNDATION* 


ITEMS. 

COST 
PER  SQ.  YD. 

Grading  and  rolling  subgrade    

$0   1126 

Placing  5  inches  of  concrete  in  pipe  trenches  

0848 

Cushion  course,  —  limestone  screenings  

0543 

Brick  f.o.b.  destination  

8600 

Unloading  and  hauling  brick. 

0777 

Laving  brick 

0376 

Rolling  brick  by  manual  labor 

0069 

Grouting  joints 

0845 

Expansion  joints,  longitudinal  and  transverse  

0353 

Top  coating  of  sand      .        

0047 

Total               

$1  3587 

*  Engineering  News,  Vol.  72  (1914),  p.  1263. 

1047.  Semi-monolithic  Brick  on  Old  Macadam.     Table  62,  page 

550,  shows  the  cost  of  laying  a  semi-monolithic  brick  pavement  on 
an  old  macadam  road.     A  2-inch  dry  1  :  4  mixture  of  cement  and 
sand  was  used  for  the  bedding  course.     The  water  cost  nothing 
except  the  piping.     The  self-propelling  roller  used  on  the  subgrade 
was  loaned,  and  no  charge  therefor  is  included.     Common  labor 
received  20  cents  per  hour,  and  team  and  driver  $6.00  per  day. 

1048.  Cost  in  Various  Cities.     Table  63  and  64,  page  550  and 

551,  shows  the  cost  and  several  details  of  3-inch  and  4-inch  brick 
pavements  in  various  cities.     A  few  cities  use  3j-inch  brick,  and  a 
few  2 J-inch ;  but  none  of  these  are  given  here. 

1049.  MERITS  OF  BRICK   PAVEMENTS.    Bricks  as. a  paving 
material  have  some  attractive  features.     1.  They  may  be  had  in 
small  units  of  practically  uniform  size.     2.  They  may  be  had  in 
large  or  small  quantities.     3.  They  may  be  laid  rapidly  without 
special  expert  labor.     4.  When  ailing  pipes  or  other  causes  neces- 
sitate the  disturbance  of  the  pavement,  ordinary  tools  and  intelli- 
gence can  restore  the  original  surface.     5.  Brick  pavements  give  a 
good  foothold  for  horses.     6.  They  do  not  wear  slippery,     7.  They 


550 


BRICK    PAVEMENTS 


[CHAP,  xvn 


TABLE  62 

COST  OF  SEMI-MONOLITHIC  BRICK  PAVEMENT  ON  MACADAM* 
Alton,  Illinois,  1915 


ITEMS. 


COST  PEK  SQ.  YD. 


Partial. 


Total. 


LABOR : 

Grading,  setting  forms,  building  barricade $0 . 084 

Placing  mortar  bed  and  laying  brick 087 

Mixing  and  applying  1  :  1  grout  filler 027       $0. 198 

BRICK:  rattler  loss  25% 560 

MORTAR  BED: 

Sand— 0.083  ton  at  $0.82 068 

Cement— 0.079  barrel  at  $1.35 105         0.173 

GROUT  FILLER: 

Sand— 0.008  ton  at  $0.82 007 

Cement— 0.036  barrel  at  $1.35 047  .054 

MATERIALS  FOR  FORMS: 

Lumber  at  $20  per  M— salvage  at  67% 009 

Template 008 

Stakes  and  nails 002         0.020 

MISCELLANEOUS: 

Depreciation  on  small  tooh  and  water  line,  teaming,  etc 0 . 038 

CLEANING  UP .  017 

Total,  exclusive  of  administration  and  profits $1 .060 


*  Engineering  Record,  Vol.  73  (1916),  p.  414. 

TABLE  63 
COST  OF  3-lNCH  BRICK  PAVEMENTS  IN  VARIOUS  CITIES  IN  1916  * 


LOCALITY. 


State. 


City. 


Sq.  Yd 
Laid  in 
1916. 


FOUNDATION. 


Thick- 
ness, 
inches. 


Propor- 
tions. 


BED  COURSE. 


Thick- 
ness, 
inches. 


Kind,  t  Kind.  J 


FILLER. 


Propor- 
tions. 


Cost 

Per 

Sq.Yd. 


Kansas. 


Missouri .  .  .  . 
Nebraska. .  .  . 

Oregon 

Texas 


Arkansas  City 

Ottawa 

Parsons 

Salina 

Topeka 

Sedalia 

Fremont 

Lincoln 

Astoria 

Houston 

San  Antonio .  . 


26  132 
14000 
6208 
27461 
46000 
12  345 
20809 

"3418 
17500 
41  571 


:  5 


$1.81 
1.57 
1.63 
1.92§ 
1.77H 
1.66 
2.04 
1.95 
2.70|| 
2.45 
2.50 


*  Municipal  Engineering,  Vol.  52  (1917),  p.  128-30. 
t  S  =  sand;    S-C  =  sand-cement;     M  =  mortar, 
t  A  =  asphalt;    G  =  grout. 


§  $2.15  with  4-inch  re-pressed  brick. 
f  $1.97  with  4-inch  re-pressed  brick. 
||  2f-inch  brick. 


ART.    2] 


CONSTRUCTION 


551 


TABLE  64 
COST  OF  4-lNCH  BRICK  PAVEMENTS  IN  VARIOUS  CITIES  IN  1916 


LOCALITY 

Sq.Yd. 
Laid  in 
1916. 

FOUNDATION. 

BED  COURSE 

FILLER. 

Cost 
Per 
Sq.Yd 

$2.52 
1.65 
1.81 
1.72 
1.76 
1.63 
1.60 
1.30 
1.79 
2.03 
1.60 
2.25 
1.80 
1.81 
2.77 
2.14 
1.90 
1.94 
2.50 
2.08 
2.16 
1.60 
1.74 
1.82 
2.04 
1.80 
2.10 
1.95 
2.55 
1.99 
2.15 

State. 

City. 

Thick- 
ness. 

Propor- 
tions. 

Thick- 
ness. 

Kind.f 

Kind.J 

Propor- 
tions. 

California.  .  . 
Illinois 

Los  Angeles.  .  . 
Alton  
Champaign.  .  . 
Danville  
Galesburg.  .  .  . 
Mattoon  
Peoria  
Crawf  ordville  . 
Ft.  Wayne  
Muncie  
New  Castle.  .  . 
Wabash  
Leavenworth.  . 

1  800 
51  510 
30000 
42000 
17760 
32269 
36536 
4  108 
21005 
12000 
19408 
2575 
4  717 
59758 
13844 
71  800 
34  512 
2572 
4310 
19947 
25000 
50  120 
34637 
22223 
63  121 
42000 
42333 
15000 
112581 
14488 
39326 

4" 
4 
6 
5 
4 
5 
4-5 
4 
6 
6 
5 
6 
5 
6 
5 
6 
5 
5 
6 
5 
5 
5 
6 
6 
6 
5 
6 
4 
5-6 
5 
5 

1:3:6 
1:3:6 
1:3:5 
1:3:5 
1:3:6 
1  :  4*  :  8 
1:3:5 
1:3:5 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:5 
1:3:6 
1:2:4 
1:3:6 
1  :3i  :7 
1:2:5 
1:3:6 
1  :  6 
1:3:6 
1:3:5 
1:3:5 
1:3:6 
1  :  3i  :  6 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:6 
1:3:5 

.5-2" 
1 

i! 

1 

1-2 
U 
2 
2 

ii 

1* 
!J 

1 
li 

!» 

2 
2 

M 

..... 

S 
S-C 

s 
s-c 

s 
s 
s 
s 
s 
s 
s-c 
s 
s 
s 
s 
s 
s 

'  M' 

s 

M 

s 
s 
s 

M 

s 
s 
s 
s-c 

s 
s 

G 

G 
A 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 
G 

"  G' 
G 
A 
G 
G 
G 
G 
A 
G 

i    i 
i    i 

1      1 
1      1 

Indiana  

Kansas  
Kentucky.  .  . 
Mass  
Michigan.  .  .  . 

New  York  .  .  . 
Ohio  

S.  Carolina.  . 
Washington. 
Wisconsin.  . 

i    i 

1     2 

1    1 
1    1 
1    1 

i    i 

1      2 

Fitchburg.  .  .  . 
Detroit 

Grand  Rapids  . 
Port  Huron.  .  . 
Amsterdam.  .  . 
Jamestown.  .  . 
Poughkeepsie  . 
Canton  
Findlay  
Lakewood  .  .  . 
Toledo  
Warren 

1    1 
1    1 

"i  i1 

i    i 
i    i 
i    i 
i    ii 

Youngstown  . 
Greenwood  .  . 
Seattle 

Beloit  
Racine  

*  Municipal  Engineering,  Vol.  52  (1917),  p.  128-30. 
t  S  =  sand;   S-C  =  sand-cement;    M  =  mortar, 
t  A  =  asphalt;   G  =  grout. 

are  adapted  to  all  grades.  8.  They  have  low  tractive  resistance, 
particularly  if  the  joints  are  filled  with  cement  grout.  9.  They  are 
not  specially  noisy  when  properly  laid.  10.  Brick  pavements  yield 
no  mud  or  dust.  11.  They  are  easily  cleaned.  12.  If  the  joints 
are  filled  with  sand,  they  are  only  slightly  absorbent;  and  if  filled 
with  hydraulic  or  bituminous  cement,  they  are  non-absorbent.  13. 
Brick  pavements  have  a  pleasing  appearance.  14.  They  are  very 
durable,  particularly  if  the  joints  are  filled  with  cement  grout.  15. 
They  are  cheap  in  consideration  of  their  small  cost  of  maintenance 
and  long  life. 

1050.  SPECIFICATIONS.  The  American  Society  for  Testing 
Materials  publish  specifications  for  the  standard  rattler,  the  standard 
rattler  test,  and  paving  brick,  copies  of  all  of  which  can  be  had  for  a 
nominal  sum  of  the  Secretary.  The  American  Society  of  Municipal 
Improvements  publish  complete  specifications  for  brick  street  pave- 
ments having  sand  bedding  course,  and  grout  or  tar  or  asphalt  filler, 


552  BRICK    PAVEMENTS  CHAP.    XVII 

copies  of  which  may  be  had  for  a  nominal  sum  of  the  Secretary. 
The  National  Paving*  Brick  Manufacturers  Association  publish  com- 
plete specifications  for  all  varieties  of  brick  paving  for  both  streets 
and  roads,  using  wire-cut  lug  or  re-pressed  brick,  copies  of  which 
may  be  had  gratuitously  of  the  members  of  the  Association  and  of  the 
Secretary,  Brotherhood  Building,  Cleveland,  Ohio.  The  Western 
Paving  Brick  Manufacturers  Association  publish  specifications  for 
all  varieties  of  brick  paving  including  the  use  of  vertical-fiber  brick, 
copies  of  which  may  be  had  gratuitously  of  members  of  the  Asso- 
ciation, or  the  Secretary,  D wight  Building,  Kansas  City,  Missouri. 
The  Dunn  Wire-cut  Lug  Brick  Co.,  Conneaut,  Ohio,  an  organization 
to  promote  the  use  of  wire-cut  lug  brick  by  securing  good  and  eco- 
nomical brick  pavements,  publish  a  complete  set  of  specifications 
for  wire-cut  lug  brick-paving,  which  may  be  had  gratuitously. 
Many  of  the  State  Highway  Commissions  publish  complete  speci- 
fications for  brick  paving,  which  can  doubtless  be  had  gratuitously 
by  residents  of  the  respective  states. 

ART.  3.     MAINTENANCE 

1052.  The  maintenance  of  pavements  has  never  received  atten- 
tion in  proportion  to  either  its  economic  importance  or  the  comfort 
of  the  user.     This  is  particularly  true  of  brick  pavements,  partly 
because    they  are    comparatively   new,    and     partly    because   the 
material  not  being  subject  to  decay  the  need  of  maintenance  has 
been  over-looked.     There  has  been  no  comprehensive  diagnosis  of 
the  diseases  of  brick  pavements,  nor  have  any  effective  remedies 
been  developed. 

1053.  REPAIRS  OF  BRICK   PAVEMENTS.    The  more  common 

matters  that  need  attention  in  the  maintenance  of  brick  pavements 
are:  (1)  holes  due  to  soft  brick,  (2)  depressions  due  to  shrinkage  or 
flow  of  the  sand  cushion,  (3)  depressions  due  to  sinking  of  founda- 
tion, (4)  depressions  due  to  settlement  of  trenches,  (5)  defects  at 
transverse  expansion  joints,  (6)  defective  grouting,  (7)  bulges, 
(8)  longitudinal  cracks,  (9)  re-laying  pavements  in  patches  and  cuts, 
and  (10)  re-surf acing,  worn  pavements. 

1054.  Soft  Brick.    A  soft  brick  in  the  pavement  wears  away  and 
makes  a  hole.     Each  wheel,  particularly  a  steel-tired  one,  dropping 
into  the  hole  crushes  and  chips  the  adjoining  bricks  even  though 
they  are  hard,  and  the  hole  gradually  increases  in  size  and  depth. 
When  such  a  brick  shows  itself  it  should  be  cut  out  and  be  replaced 


ART.   3]  MAINTENANCE  553 

with  a  good  one.  The  defective  brick  must  be  cut  out  with  a  chisel 
and  hammer,  the  joints  adjacent  to  it  must  be  cleaned,  and  the  bed- 
ding course  must  be  removed.  If  the  bedding  course  is  sand,  it 
must  be  carefully  compacted ;  and  if  it  is  mortar,  a  new  bed  must  be 
laid.  After  the  new  brick  has  been  placed  and  firmly  settled  into  the 
bedding  course,  and  found  to  conform  to  the  general  surface  of  the 
pavement,  the  joints  should  be  filled.  If  the  joint  filler  is  grout, 
the  new  brick  should  be  dampened  before  the  grouting  is  done,  and 
care  must  be  taken  to  see  that  the  sand  cushion  has  not  pushed  up 
into  the  joint;  and  after  the  joint  is  filled,  the  spot  should  be  bar- 
ricaded until  the  cement  has  set. 

The  patch  should  be  barricaded  for  at  least  a  day  or  two.  Theo- 
retically, this  time  is  far  too  short  for  the  cement  grout  to  gain  its 
full  strength;  but  experience  seems  to  show  that  a  day  or  two  is 
reasonably  safe.  The  explanation  of  this  anomalous  result  is  that 
the  spot  being  small,  there  is  not  much  probability  that  a  wheel 
carrying  a  maximum  load  will  go  over  the  spot  until  the  cement 
grout  has  gained  sufficient  strength  to  hold  it.  Obviously  then, 
the  denser  the  traffic  or  the  heavier  the  loads,  the  longer  the  barri- 
cades should  remain. 

If  the  brick  are  not  quite  uniform  in  quality,  there  may  be  so 
many  soft  bricks  as  to  make  it  impracticable  to  remove  each  soft  one, 
in  which  case  the  pavement  must  continue  in  service  until  it  is  re- 
surfaced as  described  in  §  1063,  1068,  or  1069,  or  until  it  is  replaced 
with  a  new  pavement.  The  possibility  of  this  condition  arising  is 
the  reason  for  making  the  rattler  test  in  such  a  way  as  to  secure 
information  as  to  the  uniformity  of  the  brick;  and  is  also  a  reason 
for  carefully  inspecting  the  bricks  after  they  are  laid. 

1055.  Shrinkage  of  Sand  Cushion.  A  common  defect  of  brick 
pavements  having  a  sand  cushion  is  the  shrinking  of  the  cushion 
away  from  the  brick,  due  probably  to  the  sand  being  wet  when  the 
bedding  course  was  spread.  If  the  joints  are  filled  with  sand,  the 
first  evidence  of  the  shrinkage  of  the  cushion  is  a  shallow  saucer-like 
depression  of  the  surface  of  the  pavement.  The  remedy  is  to  take 
up  the  low  spot  and  re-lay  the  sand  cushion.  If  the-  spot  is  not 
repaired,  the  depression  is  likely  to  be  enlarged  by  the  impact  of 
passing  wheels  and  the  flow  of  the  sand  cushion.  The  flow  of  the 
sand  cushion  sometimes  gives  rise  to  broad  shallow  ruts,  particularly 
on  a  street  having  a  street-car  track,  since  then  the  travel  is  con- 
centrated on  a  narrow  strip. 

If  the  joints  between  the  bricks  are  filled  with  cement  grout,  the 


554  BRICK    PAVEMENTS  [CHAP.    XVH 

first  evidence  that  the  sand  cushion  has  shrunk  away  from  the 
brick  is  a  rumbling  noise  when  a  steel-tired  wheel  goes  over  the 
spot.  Tapping  the  surface  of  such  a  pavement  with  a  hammer  will 
reveal  many  such  hollow  spots.  Such  spots  finally  break  down  by 
the  'shearing  of  the  joint  filler.  After  this  is  done,  water  enters 
through  the  broken  joints,  and  freezing  lifts  the  pavement;  and 
sometimes  breaks  other  joints,  and  perhaps  tilts  a  portion  of  the 
pavement  and  thereby  roughens  the  surface.  The  examination  of 
any  sand-cushion  pavement  after  it  has  been  in  service  a  year  or 
two,  will  show  a  number  of  such  breaks.  The  only  remedy  for  this 
defect  is  to  cut  out  the  low  spot  and  re-lay  it.  Many  old  pave- 
ments have  so  many  such  breaks  that  it  is  impracticable  to  repair 
them.  The  adequate  preventive  is  not  to  use  a  sand  cushion,  or  at 
least  not  a  thick  one.  For  a  discussion  of  the  method  of  resurfac- 
ing a  brick  pavement,  see  §  1063,  1068,  or  1069. 

1056.  Settlement  of  Foundation.    Sometimes  a  round  depression 
is  due  to  the  settlement  of  a  spot  of  the  subgrade.     It  can  not  always 
be  determined  from  a  surface  examination  whether  the  defect  is  due 
to  the  shrinkage  of  the  sand  cushion  or  the  settlement  of  the  sub- 
grade.     To  determine  the  cause,  remove  the  wearing  coat  and  the 
bedding  course,  and  examine  the  top  of  the  foundation  for  a  crack 
near  the  edge  of  the  depression.     If  the  foundation  is  broken,  it 
should  be  taken  out,  and  the  cause  of  the  settlement  be  sought  for. 
Perhaps  it  is  due  to  lack  of  consolidation  at  the  time  of  construction, 
or  perhaps  it  is  due  to  a  leaking  pipe  or  a  spring.     If  it  is  due  to  the 
first,  the  remedy  is  thorough  tamping;  if  to  a  leaky  pipe,  the  remedy 
is  obvious;  and  if  to  a  spring,  then  adequate  drainage  must  be  pro- 
vided.   After  the  causes  have  been  removed,  the  pavement  must  be 
replaced  as  described  in  §  1061. 

1057.  Settlement  of  Trench.     The  settlement  of  the  back-filling 
of  a  trench  is  one  of  the  most  common  and  most  serious  defect  in 
pavements.     The  settlement  over  a  longitudinal  trench  is  more 
noticeable  but  less  damaging  than  that  over  a  transverse  trench. 
The  prevention  during  construction  is  discussed  in  §  764-70. 

When  a. pavement  sinks  over  a  trench,  it  is  difficult  to  eliminate 
the  defect.  The  first  step  is  to  remove  in  succession  the  wearing 
coat,  the  bedding  course,  and  the  foundation.  The  second  step  is 
to  consolidate  the  back-filling;  but  this  is  not  easily  done.  During 
construction,  if  the  trench  is  not  too  deep  and  if  it  was  not  back- 
filled under  very  unfavorable  conditions,  the  back-filling  can  usually 
be  consolidated  fairly  well  by  rolling  (§  763) ;  but  this  is  impracticable 


ART.    3]  MAINTENANCE  555 

after  the  pavement  is  laid.  Under  some  conditions  flooding  (§  766) 
may  be  worth  trying.  The  most  feasible,  way  is  to  dig  out  the  trench 
and  replace  the  material  with  adequate  tamping  (§  767-69).  The 
usual  method  is  simply  to  fill  the  trench,  generally  a  little  more  than 
full,  and  re-lay  the  pavement,  allowing  it  to  be  a  little  high  with  the 
expectation  that  it  will  settle  to  place.  But  sometimes  the  pave- 
ment does  not  settle,  and  sometimes  it  settles  too  much;  and  then 
in  either  case,  another  trial  is  needed  to  secure  a  good  surface. 

Sometimes  an  attempt  is  made  to  remedy  such  a  defect  by  laying 
a  thicker  concrete  foundation  over  the  trench  and  allowing  the  slab 
to  extend  laterally  beyond  the  trench;  but  usually  such  a  remedy 
is  not  effective  except  perhaps  where  the  trench  is  comparatively 
narrow  and  where  the  banks  of  the  trench  are  fairly  solid.  A  radical 
remedy  would  be  to  dig  out  the  trench  and  fill  it  with  sand,  or  gravel, 
or  a  very  lean  concrete  (§  770).  The  difficulty  of  removing  this 
defect  is  a  reason  in  favor  of  good  original  construction. . 

For  a  discussion  of  the  method  of  replacing  the  brick  in  making 
such  a  repair,  see  §  1061. 

1058.  Transverse    Contraction    Joints.    Transverse   contraction 
joints  are  a  common  cause  of  deterioration  in  a  brick  pavement 
having  grout  filler.     The  action  of  the  joint  in  damaging  the  adja- 
cent pavement  has  been  explained  in  §  1019-20,  and  illustrated  in 
Fig.  195,  page  535.     The  only  remedy  in  such  cases  is  to  remove 
the  expansion  joint  and  re-lay  the  pavement.     For  some  hints  con- 
cerning the  method  of  re-laying  the  pavement,  see  §  1061. 

1059.  Defective   Grouting.     Defective   grouting   is   a   cause   of 
serious  deterioration  in  a  brick  pavement.     In  a  pavement  with  a. 
rigid  filler,  it  is  essential  that  the  filler  extend  from  top  to  bottom  of 
the  joints,  as  otherwise  the  pressure  due  to  the  expansion  of  the  pave- 
ment will  be  concentrated  on  one  portion  of  the  joint,  which  will 
crush  the  filler  and  break  the  bond,  and  cause  the  pavement  to  wear 
as  though  it  were  sand-filled ;  or  the  pressure  may  cause  the  brick  jto 
spall  or  crush.     There  are  two  possible  reasons  for  such  defects:  ,1. 
The  sand  of  the  cushion  course  may  have  pushed  up  into  the  bottom 
of  the  joint  when  the  bricks  are  rolled;   and  consequently  the  filler 
at  the  bottom  of  the  joint  will  not  be  as  rigid  as  that  at  the  top.     2. 
The  grout,  particularly  that  of  the  second  coat,  may  have  been  so 
thick  as  to  bridge  the  joint  and  fill  only  the  top  of  the  joint.    Fig.  201, 
page  556,  shows  a  spot  in  which  the  grouting  was  defective.     Notice 
that  a  rut  has  formed  at  the  right  of  the  defective  spot,  that  a  brick 
at  the  top  of  the  spot  is  badly  shattered,  and  that  several  bricks  have 


556  BKICK  PAVEMENTS  [CHAP.   XVH 

spalled.  These  defective  spots  are  usually  small  in  the  beginning, 
but  they  gradually  enlarge  owing  to  the  increased  effect  of  impact 
and  also  to  the  effect  of  the  water  that  penetrates  such  spots. 


FIG.  201. — DEFECTIVE  GROUTING. 


These  spots  may  be  cut  out  by  hand  with  chisel  and  hammer, 
or  with  a  pneumatic  chisel  or  chipping  hammer  as  shown  in  Fig.  202. 


PIG.  202. — PNEUMATIC  CHISEL  CUTTING  OUT  DEFECT. 

In  Baltimore  the  cost  of  toothing  out  a  brick  pavement  having 
grout  filler  (see  Fig.  204,  page  559)  was  18  cents  per  lineal  foot  by 
hand,  and  2.76  cents  per  lineal  foot  with  a  pneumatic  chisel.* 

*  Municipal  Engineering,  Vol.  52  (1917),  p.  6. 


ART.   3]  MAINTENANCE  557 

In  cutting  out  such  a  spot  no  half  brick  should  be  left,  as  other- 
wise there  will  not  be  sufficient  bond  between  adjacent  bricks.  The 
joint-filler  should  be  thoroughly  cleaned  from  the  edges  of  the  toothed 
bricks,  care  being  taken  not  to  break  the  bond  of  the  remaining 
bricks.  The  bedding  course  should  be  adjusted  so  as  to  bring  the 
tops  of  the  new  bricks  to  the  proper  elevation.  Care  should  be 
taken  that  the  brick  to  be  used  in  filling  the  patch  are  of  exactly  the 
same  size  as  the  old  ones.  The  brick  should  be  laid,  tamped,  and 
grouted  as  in  the  original  construction. 

1060.  Bulge.  A  bulge  is  a  buckling  or  heaving  of  the  wearing 
coat  of  a  brick  pavement  due  to  its  expansion.  Sometimes  the  bulge 
is  simply  a  wave,  sometimes  a  ridge  having  a  crack  at  its  crest, 
and  occasionally  an  explosion  or  "  blow-up  "  in  which  a  number 
of  bricks  are  thrown  into  the  air  with  considerable  force.  Fig.  203 


FIG.  203. — Two  BULGES  IN  BRICK  PAVEMENT. 

is  a  view  of  two  upheavals.  The  right-hand  example  is  a  mild  up- 
heaval, and  the  left-hand  one  is  a  moderate  explosion.  A  bulge 
usually  occurs  at  a  crowned  foot-way  crossing  or  a  street  intersec- 
tion. Notice  that  both  of  the  bulges  shown  in  Fig.  203  occurred  at  a 
crowned  foot-way  crossing. 

If  a  main  street  is  paved  and  its  crown  is  carried  through  the 
intersection  with  another  street  and  later  the  latter  is  paved,  the 
pressure  due  to  the  expansion  of  the  pavement  of  the  cross  street 
against  the  abutments  of  the  crown  of  the  main  street  may  cause 
the  crown  of  the  arch  to  rise  with  or  without  an  explosion.  The 
mildest  form  of  this  phenomena  is  simply  lifting  the  pavement  from 
the  sand  cushion,  in  which  case  the  pavement  will  give  out  a  rumbling 
sound  as  a  wheel  passes  over  it,  and  it  will  come  back  to  place  when 
the  pavement  cools.  Or  the  pavement  may  be  cracked  near  the 


558  BRICK   PAVEMENTS  [CHAP.    XVII 

middle  of  the  main  street,  in  which  case  the  pavement  must  be  re- 
laid  along  the  crack,  since  the  pavement  will  not  of  itself  return  to 
its  former  position.  Upheavals  or  cracks  from  this  cause  can  be 
prevented  by  putting  in  an  expansion  joint  where  the  pavement  of 
the  cross  street  abuts  against  that  of  the  main  street. 

Sometimes  a  lateral  pressure  on  the  pavement  will  lift  the  crown 
from  the  sand  cushion,  as  is  shown  by  a  rumbling  when  a  vehicle 
goes  over  it;  and  in  extreme  cases  the  pavement  will  crack  near  the 
middle  of  the  street.  This  lateral  thrust  may  be  due  to  an  inadequate 
or  obstructed  expansion  joint  next  to  the  curbs.  A  poured  longi- 
tudinal expansion  joint  may  become  obstructed  by  pebbles  or  brick 
spalls  dropping  into  the  space  or  by  the  sand  cushion's  running  into 
it,  before  the  mastic  is  poured.  The  crown  of  the  pavement  may  be 
lifted  also  by  the  expansion  of  freezing  water  in  the  soil  behind  the 
curbs. 

An  upheaval  may  occur  in  a  level  stretch  of  pavement  due  to 
defective  grouting.  If  only  the  top  of  the  transverse  joints  are 
rilled,  the  pressure  of  the  expansion  is  concentrated  at  the  top  of 
the  joints;  and  if  the  joints  over  a  considerable  area  are  in  like  con- 
dition, they  may  all  fail  at  once — usually  with  a  loud  report  and  a 
general  upheaval  of  the  affected  area. 

1061.  Re-laying  Pavement.  If  a  brick  pavement  is  to  be  main- 
tained in  a  fair  condition,  it  will  frequently  be  necessary  to  re-lay 
the  pavement  over  patches  and  also  over  openings  made  to  lay  or 
repair  sewers,  water  or  gas  pipes,  electric  conduits,  etc.,  for  with  the 
utmost  care  and  foresight  many  such  openings  will  be  made  (§  656- 
57).  The  typical  case  is  re-laying  a  pavement  over  a  trench. 

In  making  the  cut  in  the  pavement  alternate  brick  will  some- 
times be  broken  in  the  middle,  thus  leaving  three  bricks  with  their 
ends  in  line,  which  would  prevent  a  good  bond  of  the  new  pavement 
with  the  old;  and  therefore  these  broken  bricks  must  be  "  toothed 
out  "  so  that  only  whole  brick  remain.  This  may  be  done  by  hand 
with  a  stout  long-handled  chisel  and  hammer,  or  with  the  pneu- 
matic chisel  (Fig.  202,  page  556).  After  the  broken  brick  are 
"  toothed  out,"  the  cut  will  have  the  general  appearance  shown  in 
Fig.  204.  Next  the  joint-filler  should  be  chiseled  off  from  the  brick 
that  are  "  toothed  out,"  care  being  taken  not  to  break  the  bond  of 
these  bricks  with  those  adjoining. 

Before  re-laying  the  concrete  foundation,  the  soil  in  the  trench 
should  be  thoroughly  compacted;  and  particularly  the  soil  that  has 
run  out  into  the  trench  from  under  the  edge  of  the  foundation,  should 


ART.    3]  MAINTENANCE  559 

be  replaced  and  rammed  laterally  to  give  a  firm  bearing.  The  con- 
crete should  then  be  laid  and  tamped  as  in  the  original  construction. 
If  the  bedding  course  is  sand,  great  care  is  necessary  in  packing  or 
ramming  it  under  the  edge  of  the  undisturbed  pavement. 

As  far  as  they  are  available,  the  old  brick  should  be  cleaned  and 
used,  for  they  will  match  the  others  in  size  and  color.  If  new 
brick  are  used,  they  should  be  of  exactly  the  same  width  as  the  old 


Fia.  204. — PATCH  PROPERLY  TOOTHED  OUT. 

ones  so  that  the  courses  will  match.  The  brick  should  be  bedded 
in  the  sand  or  mortar  bed-course  so  their  upper  faces  conform  to  the 
surface  of  the  pavement;  and  then  the  joints  should  be  filled.  If 
grout  is  used,  two  or  three  applications  should  be  added  as  described 
in  §  999-1004;  and  after  the  grout  has  taken  an  initial  set,  the  patch 
should  be  covered  with  sand  or  straw  which  is  kept  wet  for  3  or  4 
days.  In  this  connection,  see  the  latter  portion  of  the  second  para- 
graph of  §  1054. 

With  care  in  making  a  re-placement,  the  patch  can  be  made  so 
that  the  original  surface  and  strength  of  the  pavement  is  fully  re- 
stored; and  if  the  work  is  well  done,  the  patch  will  hardly  be  visible. 
Fig.  205,  page  560,  shows  such  a  patch  over  a  trench  in  a  pavement 
in  dleveland,  Ohio.  This  patch  is  visible  only  because  of  the  differ^ 


560  BRICK   PAVEMENTS  [CHAP.   XVII 

ence  in  color  of  the  old  and  the  new  brick.  In  that  city  are  many 
such  repairs  which  are  scarcely  visible. 

Part  of  the  reasons  for  this  good  work  is  that  the  back-filling  of 
the  trench  is  inspected  and  the  pavement  is  re-laid  by  employees  of 
the  division  of  street  repairs. 

1062.  Cracks.  The  only  cracks  requiring  consideration  are  lon- 
gitudinal ones,  which  are  usually  near  the  center  of  the  road.  The 
transverse  cracks  are  usually  narrower,  and  are  less  harmful.  A 


FIG.  205. — BRICK  PAVEMENT  RE-LAID  OVER  A  TRENCH. 

longitudinal  crack  is  likely  to  develop  into  a  rut.  A  longitudinal 
crack  may  be  due  to  any  one  of  several  causes.  1.  It  may  be  due 
to  the  heaving  action  of  frost  under  the  edges  of  the  foundation.  2. 
It  may  be  due  to  the  settlement  of  the  edges  of  the  foundation  when 
the  frost  is  out  under  the  edges  of  the  foundation  and  not  under  the 
center.  3.  It  may  be  due  to  the  shrinkage  of  the  soil  under  the 
edge  of  the  foundation  due  to  the  soil's  drying  out.  4.  It  may  be  due 
to  expansion  as  explained  in  the  last  two  paragraphs  of  §  1060. 

It  is  sometimes  impossible  to  explain  the  cause  of  such  cracks. 
They  seem  to  be  more  common  in  the  North  than  in  the  Sou.th; 
and  hence  it  is  concluded  that  frost  is  a  common  cause.  They  seem 


ART.    3]  MAINTENANCE  561 

to  be  less  frequent  the  flatter  the  crown  of  the  pavement;  and  they 
seem  to  be  less  frequent  with  a  flat  than  with  a  crowned  subgrade. 

The  only  practicable  thing  that  can  be  done  with  a  longitudinal 
crack  is  to  clean  and  fill  it  with  a  bituminous  cement  as  described  for 
concrete  roads — see  §  482. 

1063.  RE- SURFACING.   Re-surfacing  is  a  method  of  radical  repairs. 
There  are  many  sand-filled  pavements  so  badly  worn  and  so  full  of 
depressions  as  to  be  of  but  little  value  as  a  pavement,  which  may 
be  used  as  a  foundation  for  a  new  wearing  surface.     There  are  three 
methods  that  may  be  employed  in  re-surfacing  such  pavements, 
viz.:    (1)  covering  the  brick  with  a  bituminous  surface;    (2)  turning 
the  brick  upside  down  and  re-laying  them;    and  (3)  laying  a  new 
monolithic  surface  on  the  old  pavement. 

The  bituminous  surface  may  consist  of  either  asphalt  or  tar. 

1064.  Asphalt  Top.     The  asphaltic  surface  may  be  either  asphalt 
concrete  or  a  binder  course  and  a  wearing  coat  similar  to  that  of  a 
sheet  asphalt  pavement.     When  the  work  is  well  done  in  every  par- 
ticular, the  result  is  very  satisfactory;    but  there  have  been  many 
failures,  probably  owing  to  the  failure  to  meet  one  or  more  of  the 
conditions  necessary  for  success.     There  are  several  essential  con- 
ditions to  be  fulfilled.     1.  The  original  pavement  must  be  absolutely 
rigid,  and  not  show  any  vibration  or  settlement  when  a  heavy  load 
goes  over  it.     The  lack  of  rigidity  in  the  original  pavement  or  in 
portions  of  it  which  are  re-laid  preparatory  to  re-surfacing  it,  is  one 
of  the  most  common  causes  of  failure  of  a  bituminous  top.     2. 
The  surface  of  the  old  pavement  must  be  leveled  up  by  filling  the 
depressions  with  concrete,  so  that  the  asphalt  will  be  of  nearly  uni- 
form thickness,  as  otherwise  it  will  creep  and  loosen  from  the  brick. 
Depressions  deeper  than  2  inches  should  be  filled  with  hydraulic 
concrete;  but  depressions  less  than  2  inches  deep  may  be  filled  with 
the  mixture  for  the  asphalt  binder  course  (§  812-24).     3.  The  asphalt 
must  be  of  good  quality  and  proper  consistency.     4.  The  pavement 
must  be  perfectly  clean.     The  dirt  must  not  only  be  removed  from 
the  surface  but  also  from  the  cracks  for  at  least  J  an  inch.     This 
can  be  done  with  wire  brooms,  but  it  requires  great  care.     The  dirt 
can  be  removed  more  effectively  with  a  fire  hose  than  by  sweeping. 

5.  The  brick  should  be  perfectly  dry  when  the  asphalt  is  applied. 

6.  The  old  brick  should  not  be  cold  when  the  asphalt  is  applied.     If 
a  surface  heater  (Fig.  161,  page  450)  is  available,  it  is  wise  to  warm 
the  old  brick  before  applying  the  asphalt.     7.  Apply  a  paint  course 
of  asphaltic  cement  thinned  with  naphtha,  at  the  rate  of  not  more 


562  BRICK   PAVEMENTS  [CHAP.    XVII 

than  a  gallon  to  the  square  yard.  Dust  or  dampness  or  cold  will 
prevent  the  paint  coat  from  adhering  perfectly.  8.  As  soon  as  the 
naphtha  has  fully  evaporated  from  the  paint  coat,  and  while  it  is 
still  perfectly  clean,  the  sheet  asphalt  should  be  laid  and  rolled. 

The  sheet  asphalt  may  be  either  a  binder  course  and  a  wearing 
coat  of  a  total  thickness  of  2  inches,  or  a  wearing  coat  alone  having  a 
thickness  of  2  inches.  The  binder  course  and  also  the  wearing  coat 
are  to  be  mixed  and  laid  as  described  for  the  corresponding  opera- 
tions for  a  sheet  asphalt  pavement  (§  812-24  and  §  825-45,  respec- 
tively). Experience  has  shown  that  if  it  is  possible,  the  asphalt 
surface  should  not  be  less  than  2  inches  thick  at  -any  point,  as  it  is 
likely  to  creep  and  form  humps.  This  is  due  to  the  fact  that  in  rolling 
the  binder  course  or  the  wearing  coat,  the  wide  roller  will  be  sup- 
ported on  the  high  points  and  not  compress  the  asphalt  in  the  holes; 
and  later  narrow  tires  will  compress  the  asphalt  in  the  holes  and 
make  a  depression,  and  wheels  dropping  into  these  depressions  will 
displace  the  asphalt  and  loosen  it  from  the  bricks. 

1065.  One  of  the  most  difficult  questions  encountered  in  plan- 
ning to  add  a  new  surface  to  an  old  pavement  is  the  matter  of  drain- 
age. The  curb  or  gutter  that  was  only  deep  enough  for  the  original 
pavement  will  be  too  shallow  if  a  new  2-  or  3-inch  surface  is  put  on 
top  of  the  old.  This  difficulty  is  not  as  serious  on  streets  having 
considerable  longitudinal  grade  as  on  nearly  level  streets.  On  the 
latter,  either  of  two  things  may  be  done,*  viz.:  1.  Take  up  the  old 
brick  and  the  foundation  next  to  the  curb  (or  next  to  the  combined 
curb  and  gutter)  for  a  width  of  3  or  4  feet,  and  lay  a  new  concrete 
foundation  at  such  a  height  that  when  the  old  brick  are  re-laid  and 
the  asphalt  is  placed  thereon,  the  gutter  will  be  of  suitable  depth 
(or  the  top  of  the  asphalt  will  be  even  with  the  top  of  the  concrete 
gutter).  2.  Take  up  the  old  brick  for  3  or  4  feet  next  to  the  curb 
(or  next  to  the  combined  curb  and  gutter)  and  lay  concrete  in  this 
space  making  its  upper  surface  next  to  the  gutter  of  such  a  height 
that  the  asphalt  when  laid  thereon  will  given  sufficient  depth  of 
gutter  (or  will  come  even  with  the  top  of  the  concrete  gutter),  and 
making  the  top  of  its  edge  next  to  the  undisturbed  brick  level  with 
the  top  of  the  brick.  Of  course,  both  of  these  methods  increase  the 
transverse  slope  of  the  pavement  near  the  curb,  but  usually  this  is 
not  serious. 

At  street  intersections  that  are  not  to  receive  an  asphalt  top,  a 

*  Thomas  H.  Brannan,  Superintendent  Asphalt  Streets,  Columbus,  Ohio,  in  Proc.  Amer. 
Soc.  Municip.  Improvements,  1916,  p.  107-8. 


ART.    3]  MAINTENANCE  563 

somewhat  similar  adjustment  is  necessary  where  the  new  asphalt 
top  meets  the  pavement  of  the  side  street. 

1066.  Tar  Top.     Tar  has  not  been  used  for  this  purpose  as  much 
as  asphalt,  but  it  has  been  employed  enough  to  show  that  it  can  be 
made  to  give  reasonably  satisfactory  results.     The  method  of  apply- 
ing the  tar  is  as  follows:   1.  The  brick  surface  should  be  rigid,  clean, 
dry,  and  warm  as  described  in  items  1,  4,  5,  and  6  of  §  1064.     2. 
All  depressions  more  than  1  inch  deep  should  be  filled  with  hydraulic 
concrete.     3.  All  depressions  less  than  1  inch  deep  should  be  thor- 
oughly painted  with  tar,  and  then  be  filled  with  J-  to  ^-inch  broken 
stone  perfectly  free  from  dust.     4.  The  tar  is  then  applied  sub- 
stantially as  described  for  bituminous  carpets  (§  591-93),  except  that 
a  greater  quantity  is  applied,  depending  upon  the  degree  the  bricks 
are  worn.     Enough  should  be  applied  to  have  a  layer  of  about  f  of 
an  inch  thick  on  the  face  of  the  brick.     The  tar  should  be  well 
brushed  or  rubbed  into  the  joints  with  a  wire  broom.     5.  As^soon 
as  the  tar  is  rubbed  into  the  joints,  a  half-inch  layer  of  stone  chips 
is  applied  as  described  in  §  594-95. 

When  the  surface  is  finished,  it  will  have  the  appearance  of  a 
bithulithic  pavement  (§  893),  and  will  give  good  service  for  several 
years,  depending  upon  the  character  and  amount  of  traffic.  When 
worn  through,  the  surface  can  be  renewed  by  the  addition  of  a  new 
coat  of  tar  and  screenings. 

Water  injures  a  tar  surface,  and  hence  this  treatment  is  more 
permanent  the  better  the  drainage  of  the  surface. 

1067.  Turning  the  Bricks.     In  Champaign,  Illinois,  in  1916,  two 
experiments  were  tried  of  laying  a  monolithic  brick  pavement  on  the 
existing  concrete  base,  turning  and  using  the  old  bricks.     The  old 
pavement  was  laid  on  a  2-inch  sand  cushion,  and  the  joints  were 
filled  with  sand.     The  brick  were  badly  worn. 

The  repair  work  was  carried  out  as  follows:  1.  The  concrete  base 
was  cleaned,  and  on  it  was  laid  a  2-inch  layer  of  1  :  3  :  2  gravel  con- 
crete for  a  bedding  course.  2.  The  brick  were  thoroughly  cleaned 
with  wire  brushes,  and  laid  on  the  bedding  course  before  the  con- 
crete had  set.  3.  The  brick  were  rolled  and  grouted  in  the  usual 
way  (§991  and  §996-1002).  4.  The  street  was  barricaded  for  15 
days. 

In  one  section,  the  brick  were  very  badly  worn,  and  varied  in 
depth  from  2J  to  4  inches.  About  10  per  cent  had  to  be  replaced 
with  new  ones.  No  attempt  was  made  to  size  them,  and  it  was 
difficult  to  get  a  smooth  surface.  If  the  brick  had  been  sorted,  and 


564  BRICK   PAVEMENTS  [CHAP.   XVII 

the  most  worn  ones  placed  next  to  the  curb  and  the  least  worn  at  the 
crown,  it  is  believed  a  better  surface  would  have  been  obtained  with 
less  labor.  On  the  other  section,  the  brick  were  not  worn  so  much, 
and  a  good  surface  was  obtained  with  much  less  trouble.  The  sur- 
face of  this  section  compares  favorably  with  that  of  a  new  mono- 
lithic pavement.  The  cost  was  80  cents  per  square  yard,  which 
could  doubtless  be  reduced  with  greater  experience, 

1068.  MONOLITHIC  BRICK  TOP.  In  1917  near  Danville,  111.,  a 
new  method  of  re-surfacing  an  old  brick  pavement  was  tried.*  The 
pavement  was  on  a  suburban  road  which  carries  a  dense  and  heavy 
traffic;  and  after  considering  asphalt,  tar,  and  concrete  as  materials 
for  the  new  surface,  a  monolithic  brick  surface  with  a  thin  concrete 
base  was  adopted.  The  concrete  was  one  part  of  cement  to  four 
parts  of  fine,  well-graded  gravel;  and  its  thickness  varied  from  1  to  5 
inches  according  to  the  depth  of  the  holes  in  the  old  pavement.  Part 
of  the  new  brick  were  4  and  part  3  inches  deep.  The  concrete, 
the  bedding  course,  and  the  brick  were  laid  as  described  under  mono- 
lithic pavements  (§  969,  §  982,  and  §  996-1002,  respectively).  Fig. 
206  shows  two  views  of  this  work  in  progress.  The  cost  was  $1.65 
per  square  yard;  and  the  saving  was  substantially  the  cost  of  a 
new  concrete  base. 


<&. 

fet. 

•    ^ 

FIG.  206. — PUTTING  A  BRICK  TOP  ON  AN  OLD  BRICK  ROAD. 

Obviously  this  method  of  re-surfacing  would  be  inapplicable  to  a 
city  street,  on  account  of  the  difficulty  of  maintaining  proper  drainage. 

1069.  COST  OF  MAINTENANCE.  As  stated  in  §  868,  the  City 
of  Buffalo,  N.  Y.,  is  noted  for  the  completeness  of  its  records  of  the 
cost  of  construction  and  maintenance  of  pavements.  The  following 
is  a  summary  for  the  cost  of  maintenance  of  brick  pavements  for 

-   v  *  Harlan  H.  Edwards,  Engineering  News-Record,  Vol.  79  (1917),  p.  830-32. 


ART.    3]  MAINTENANCE  565 

the  year  ending  June  30,  1916.  Of  the  twenty  brick  pavements  over 
twenty  years  old,  two  short  streets  have  required  no  repairs;  and 
the  repairs  on  the  other  eighteen  streets  cost  from  0.22  to  4.92  cents 
per  square  yard  per  year,  all  but  six  costing  less  than  0.85  cent  per 
square  yard  per  year.  Of  the  fifty  brick-paved  streets  from  ten  to 
twenty  years  old  and  out  of  guaranty,  twenty  have  required  no 
repairs;  and  the  repairs  on  the  other  thirty  have  ranged  from  0.04 
to  1.58  cents  per  square  yard  per  year,  only  three  of  these  costing 
more  than  an  average  of  1  cent  per  square  yard  per  year.*  The 
average  cost  of  repairs  during  1915-16  on  231,355  square  yards  was 
2.9  cents  per  square  yard.f 

*  Report  of  Dept.  of  Public  Works— Bureau  of  Engineering,  1915-16,  p.  481-511. 
t  Ibid.,  p.  70. 


CHAPTER  XVIII 
STONE-BLOCK  PAVEMENTS 

1073.  Stone-block   pavements   rank   third   in   area   among   the 
permanent   pavements,  being  exceeded  by  sheet  asphalt  and  brick 
— see  the  tabular  statement  on  page  320. 

1074.  CLASSIFICATION.      The     earliest    pavements    of    ancient 
times  consisted  of  irregular  shaped  blocks  of  stone  more  or  less 
accurately  fitted  together.     The  form  and  size  of  the  blocks  have 
varied  greatly  from  time  to  time,    a   fact  which  has  given  rise  to 
different   classes   of  pavements.    A   few   of   these   will  be  briefly 
described. 

1075.  Roman  Roads.     The  Roman  roads  so  frequently  referred 
to  by  modern  writers  are  the  earliest  examples  of  stone-block  pave- 
ments.    The  details  of  construction   varied  somewhat,   but  as   a 
rule  they  were  about  as  follows:  The  foundation  was  laid  in  a  trench 
about  3  feet  deep,  with  no  attempt  at  underdrainage.     The  base 
was  formed   of   one   or  sometimes  two  courses  of  large  flat  stones 
laid  in  lime  mortar,  and  was  usually  about  15  inches  thick.     Upon 
this  was  laid  a  9-inch  course  of  small  fragments  of  stone  imbedded 
in  lime  mortar,  the  intention  of  this  course  apparently  being  to  bind 
together  the  tops  of  the  large  stones  in  the  course  below.     Next 
was  laid  a  6-inch  layer  of  concrete,  apparently  to  make  a  smooth 
bed  to  receive  the  stones  of  the  top  course.     The  wearing  surface 
consisted  of  closely-jointed,  irregular-shaped  stones,  about  6  inches 
thick.     The  total  thickness  of  the  road  was  about  36  inches.     In  and 
near  the    cities,  the  top  course  was  formed  of  irregular  blocks  of 
basalt,  porphyry,  or  lava,  which  had  a  top  area  of  4  or  5  square  feet 
and  a  thickness  of   12  to  15  inches.     These   blocks    were  dressed 
and   fitted   together  with  extreme  accuracy,   and  were  imbedded 
in  cement.     These  ancient  pavements  have  aptly  been  described 
as  "  masonry  walls  laid  on  their  sides." 

The  Romans  seem  to  have  located  their  roads  in  straight  lines, 

566 


CLASSIFICATION 


567 


running  them  toward  prominent  land-marks  without  much  regard 
to  the  topography  or  to  natural  obstacles.  They  were  wasteful  of 
materials  and  labor,  which,  however,  cost  nothing  but  the  lives 
of  captives  who  were  forced  to  build  these  roads  for  the  armies  of 
their  captors.  The  results  were  roads  which  are  remarkable  chiefly 
for  their  cost,  and  which  were  inferior  to  modern  pavements  costing 
only  one  eighth  to  one  quarter  as  much.  The  durability  of  these 
roads  does  not  seem  so  remarkable  when  it  is  remembered  that  the 
traffic  was  light,  and  consisted  mostly  of  footmen,  unshod  horses, 
and  ox-carts  having  wooden  wheels,  and  also  that  probably  the 
surface  of  the  road  was  kept  covered  with  earth  two  or  three  inches 
deep. 

1076.  Cobble-stone  Pavement.  A  cobble-stone  pavement  con- 
sists of  cobble  stones  or  small  bowlders  placed  side  by  side  upon  a 
bed  of  sand  or  upon  the  natural  soil.  The  stones,  usually  somewhat 
kidney-shaped,  are  selected  with  some  relation  to  size,  set  on  end 
side  by  side  in  holes  dug  in  the  sand  or  unconsolidated  native  soil 
by  a  laying  tool,  one  end  of  which  serves  as  a  scoop  and  the  other  as  a 
hammer  to  settle  the  stones  in  place,  and  lastly  sand  or  fine  gravel 
is  spread  over  the  surface  to  fill  the  spaces  between  the  stones.  Fig. 
207  shows  a  transverse  section  of  a  cobble-stone  pavement;  and 
Fig.  208,  page  568,  shows  the  only  tool  used  in  laying  it. 


FIG.  207. — SECTION  OF  COBBLE-STONE  PAVEMENT. 


The  earliest  pavements  in  many  of  the  older  cities,  both  American 
and  European,  were  of  this  type;  and  until  about  the  beginning  of 
this  century  on  account  of  their  comparatively  low  first  cost  were 


568  STONE-BLOCK    PAVEMENTS  [CHAP.    XVIII 

quite  common.  In  1884,  93  per  cent  of  all  the  pavements  in 
Philadelphia  were  made  of  cobble  stones;  but  in  1901  less  than  6  per 

cent  were  of  this  kind.  In  1902  Baltimore 
had  321  miles  of  cobble-stone  pavement, 
— more  than  any  other  city  in  the  United 
States, — over  90  per  cent  of  the  pave- 
ments being  cobble  stones.  In  Septem- 
ber, 1901,  New  York  City  still  had  229 
FIG.  208.-COBBLE-8TONE  miles  of  streets  paveci  with  cobble  stones, 

HAMMER 

but  nearly  all  of  them  have  been  replaced 

with  better  pavements.  Since  the  introduction  of  asphalt,  brick,  and 
bituminous  pavements,  and  since  the  decrease  in  the  cost  of  stone- 
block  pavement  by  the  introduction  of  improved  methods  of  quarry- 
ing and  manufacture,  there  is  no  excuse  for  the  construction  of 
cobble-stone  pavements,  and  little  excuse  for  their  continuance.  The 
construction  of  such  pavements  has  been  practically  abandoned,  and 
in  some  cities  it  has  been  prohibited  by  law — like  theft  and  murder. 

1077.  Rubble  Paving.     In  some  cities  having  no  cobble  stones 
but   having   comparatively   plenty   of  even   bedded   sandstone   or 
limestone,  the  streets  were  paved  by  laying  rough  rubble  stones 
flatwise,  the  stones  being  4  to  6  inches  thick  and  having  a  top  sur- 
face of  4  to  6  square  feet.     The  irregular  joints  between  the  stones 
were  filled  with  spalls.     The  blocks  chipped  on  the  edges,  wore  round 
on  top,  and  got  out  of  place,  thus  making  an  exceedingly  rough 
pavement. 

1078.  Belgian-block    Pavement.     This  is   a    stone-block  pave- 
ment made  of  blocks  nearly  cubical  in  form,  from  5  to  7  inches  on  a 
side.     For  a  time  this  form  of  pavement  was  very  common  in  both 
Europe  and  America.     The  abjections  to  the  Belgian  pavement  are: 
1,  On  account  of  the  size  and  form  of  the  blocks,  it  is  difficult  to 
keep  them  in  place;  2,  the  blocks  are  of  such  a  form  as  to  give  a  poor 
foothold  to  horses;   and  3,  there  is  always  a  considerable  length  of 
joints  parallel  to  the  line  of  travel,  which  causes  ruts  to  form  in  the 
pavement.     Belgian  blocks  have  usually  been  laid  with  their  sides 
perpendicular  and  parallel  to  the  sides  of  the  street;  but  if  a  square 
block  is  to  be  used,  it  should  be  laid  in  courses  diagonal  to  the  street, 
so  that  no  joints  shall  be  parallel  to  the  line  of  travel,  a  method  which 
would  add  some  extra  expense.     The  Belgian  block  has  been  dis- 
carded in  this  country  for  the  oblong  block. 

1079.  Oblong    Block    Pavement.    At    present    practically   the 
only  stone  paving-blocks  employed  are  about  3J  to  4J  inches  wide, 


ART.    1]  THE    STONE  569 

8  to  12  inches  long,  and  nominally  4  or  5  inches  deep.  They  are  laid 
on  a  concrete  base  with  their  longest  dimension  perpendicular  to 
the  line  of  the  street.  This  is  the  form  of  stone-block  pavement 
that  will  be  considered  in  detail  in  this  chapter. 

1080.  Durax  Pavement.     This  form  consists  of  granite  cubes 
from  2|  to  4  inches  on  a  side.     This  form  of  pavement  will  be  con- 
sidered only  briefly — see  §  1117. 

ART.  1.     THE  STONE 

1081.  As  stone-block  pavements  are  employed  only  where  the 
travel  is  heavy,  the  material  of  which  the  blocks  are  made  should 
be  hard  enough  to  resist  the  abrasive  action  of  the  travel,  and  tough 
enough    to  prevent  being  broken  by  the  impact  of  loaded  wheels. 
The  hardest  stones  will  not  necessarily  give  the  best  results  in  the 
pavement,  since  a  very  hard  stone  usually  wears  smooth  and  becomes 
slippery,  and  the  edges  of  the  block  chip  off  and  the  upper  face 
becomes  rounded,  thus  making  the  pavement  very  rough.     A  hard 
stone  may  be  necessary  under  a  heavy  traffic;   but  under  medium 
traffic  a  softer  stone  may  give  more  satisfactory  results. 

The  stone  could  be  tested  to  determine  its  strength  and  dura- 
bility much  as  paving  bricks  are  tested,  but  it  is  not  known  that  any 
such  tests  have  been  made.  An  examination  of  a  stone  as  to  its 
structure,  the  closeness  of  its  grain,  its  homogeneity,  etc.,  may 
assist  in  forming  an  opinion  as  to  its  value  for  use  in  a  pavement; 
but  in  the  present  state  of  our  knowledge,  a  service  test  in  the  pave- 
ment is  the  only  certain  guide. 

Granite,  trap,  sandstone,  and  limestone  have  been  used  for 
paving  blocks. 

Granite  paving  blocks  are  much  the  most  com  neon,  and  ordinarily 
the  term  granite-block  pavement  is  employed  as  being  synonymous 
with  stone-block  pavement. 

1082.  GRANITE.      This  is  a  massive,  unstratified,  granular  rock 
composed   essentially  of  quartz   and  feldspar;  but  almost  always 
containing  other  components,  such  as  mica,  hornblende,  and  tour- 
maline in  varying  proportions.     The  quartz  and  the  feldspar  are 
called  essential  ingredients,   since  their   presence  is  necessary  to 
form  a  granite;   while  the  other  constituents  are  called  accessories, 
since  they  merely  determine  the  variety  of  the  granite.     The  term 
granite  is  popularly  applied  to  any  feldspathic  granular  rock,  and 
includes   gneiss,    syenite,    and   porphyry,    or   any   crystalline   rock 


570  STONE-BLOCK   PAVEMENTS  [CHAP.   XVIII 

whose  uses  are  the  same  as  granite.  Gneiss  is  a  rock  of  granitic 
composition  that  has  a  decided  banding  or  parallel  arrangement 
of  its  mineral  constituents.  Syenite  is  a  granitic  rock  containing 
no  quartz.  Porphyry  is  popularly  any  fine-grained  compact  rock 
having  large  crystals  scattered  throughout  its  mass. 

Granite  varies  in  texture  from  very  fine  and  homogeneous  to 
coarse  porphyritic  rocks  in  which  the  individual  grains  are  an  inch 
or  more  in  length.  The  color  may  be  red,  dark  mottled,  light  to 
dark  gray,  or  almost  black.  The  durability  is  closely  related  to  the 
accessory  minerals  present;  and  although  granite  is  popularly 
regarded  as  the  hardest  and  most  durable  stone,  there  are  some  nota- 
ble exceptions.  A  quartoze  granite,  one  in  which  quartz  predom- 
inates, is  too  brittle  for  paving  purposes;  a  feldspathic  granite,  one 
containing  an  excess  of  feldspar,  is  too  easily  decomposed;  and  a 
micaceous  granite,  one  containing  considerable  mica  in  parallel 
laminas,  is  too  easily  split  for  use  in  paving  blocks.  Gneiss  is  usually 
too  much  stratified  to  make  a  good  paving  material.  Syenite  is 
one  of  the  gest  materials  for  paving  blocks,  and  usually  the  darker 
the  color  the  better  the  stone. 

The  crushing  strength  of  granite  usually  lies  between  15,000  and 
20,000  Ib.  per  square  inch.  It. is  customary  to  specify  that  the 
granite  shall  have  a  toughness  of  not  less  than  9  (§  342),  and  a  French 
coefficient  of  wear  of  not  less  than  11  (§  343). 

A  most  important  property  possessed  by  all  granitic  rocks  is 
that  of  splitting  in  three  planes  at  right  angles  to  each  other,  so 
that  paving  blocks  may  readily  be  formed  with  nearly  plane  faces 
and  square  corners.  So  far  as  discovered,  this  valuable  property  is 
possessed  only  by  the  granitic  and  trappean  rocks.  This  property 
is  called  rift  or  cleavage,  and  was  caused  by  pressure  before  the  rock 
was  consolidated.  The  fine-grained  granites  possess  the  most  perfect 
rift,  and  it  decreases  as  the  size  of  the  grains  increase,  so  that  a  coarse- 
grained variety  is  likely  to  require  considerable  dressing  to  bring  the 
face  of  the  block  to  a  plane  surface. 

1083.  Granite  paving-blocks  are  produced  in  large  quantities 
in  Wisconsin,  Maine,  New  Hampshire,  Massachusetts,  North  Car- 
olina, Georgia,  and  Minnesota.  The  order  in  the  above  list  is  that 
of  the  number  of  blocks  produced  in  1916,  the  first  two  states  pro- 
ducing more  than  all  the  others.  In  recent  years  the  production  of 
granite  paving-blocks  has  greatly  fallen  off,  apparently  more  than 
one  half,  probably  owing  to  the  substitution  of  asphalt  and  brick 
for  stone  blocks  for  paving  purposes;  but  the  quality  has  greatly 


ART.    1]  THE   STONE  571 

improved,  partly  in  response  to  a  demand  for  smoother  block- 
pavements;  and  partly  by  using  smaller  blocks,  which  can  be  cut 
more  accurately;  and  partly  by  abandoning  the  use  of  the  hardest 
granites  and  using  the  softer  and  finer-grained  varieties,  which  split 
more  easily  and  regularly  and  make  a  better  wearing  surface.* 

1084.  TRAP.     This   is   a   popular   term   applied   to   any   dark- 
colored,  massive,  igneous  rock.     Owing  to  the  difficulty  of  making 
them,  trap  is  not  much  used  for  paving  blocks. 

1085.  SANDSTONE.     Sandstones  are  rocks  made  up  of  grains  of 
sand  which  are  cemented  together  by  siliceous,  ferruginous,  calca- 
reous, or  argillaceous  material.     The  texture  of  the  stone  varies 
according  to  the  sizes  of  the  sand  grains,  of  which  there  are  all  gra- 
dations from  those  that  are  so  fine  as  to  be  barely  discernible  to  those 
that  are  very  coarse.     The  hardness,  strength,  and  durability  of  the 
stone  is  dependent  upon  the  character  of  the  cementing  material. 
Only  the  harder  and  tougher  sandstones,  generally  those  in  which  the 
cementing  material  is  siliceous,   are  used  for  paving.     Sandstone 
paving-blocks  are  common  in  the  Lake  and  Western  cities.     The 
principal  quarries  from  which  sandstone  paving-blocks  are  obtained 
will  be  briefly  described. 

1086.  Medina  Sandstone.     This  stone  is  found  in  the  state  of 
New  York,  extending  from  Oneida  and  Oswego  counties  on  the 
east  along  the  shores  of  Lake  Ontario  westerly  to  the  Niagara  river. 
It  is  generally  a  deep  brownish  red  in  color,  though  sometimes  light 
and  yellowish,  and  in  a  few  localities  gray.     The  stone  is  evenly 
bedded,  and  the  beds  are  divided  into  blocks  by  systems  of  vertical 
joints,  generally  at  right  angles  to  each  other,  an  arrangement  which 
greatly  facilitates  the  work  of  quarrying.     It  absorbs  2J  to  3^  per 
cent  of  water,  but  it  is  not  materially  affected  by  alternate  freezing 
and  thawing. 

This  stone  is  much  used  for  paving  in  the  Lake  cities,  where  it 
is  often  preferred  to  granite,  since  it  does  not  wear  slippery. 

1087.  Potsdam    Sandstone.     This    formation    is    worked    at    a 
number  of  places  in  the  state  of  New  York,  the  largest  quarries  being 
near  Potsdam.     That  quarried  at  Potsdam  is  hard  and  compact, 
evenly  grained,  and  reddish  in  color.     It  is  largely  used  as  a  building 
stone  and  to  a  considerable  extent  for  pavements. 

1088.  Colorado  Sandstone.     In  Boulder  County,  Colorado,  are 
several  deposits  of  sandstone  that  furnish  stone  for  paving  purposes. 

*  For  an  interesting  and  elaborately  illustrated  article  on  the  Manufacture  of  Granite  Paving 
Blocks,  see  Engineering  News,  Vol.  73  (1915),  p.  376-81. 


572  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

It  splits  easily,  and  breaks  readily  at  right  angles,  so  that  it  is  formed 
into  flagging,  curb  stones,  and  paving  blocks  without  difficulty. 
It  is  hard  and  tough,  and  wears  well  in  a  pavement.  It  is  never  slip- 
pery; and  after  a  little  wear  forms  a  smooth  and  pleasing  pave- 
ment, very  similar  to  one  made  of  Medina  stone. 

1089.  Sioux  Falls  Quartzite.     This  is  a  metamorphic  sandstone 
quarried  at  Sioux  Falls,  South  Dakota.     The  stone  is  almost  pure 
silica  with  only  enough  iron  oxide  to  give  it  color,  which  varies 
from  light  pink  to  jasper  red.     It  is  very  close  grained,  and  will 
take  a  polish  almost  like  glass.     It  is  said  to  be  the  hardest  stone 
in  this  country.     Its  crushing  strength  is  about  25,000  Ib.  per  square 
inch.     It   possesses   a   remarkably  good   rift   and   grain,   although 
not  so  perfect  as  that  of  granite.     It  is  used  considerably  as  a  paving 
material,  being  shipped  as  far  east  as  Chicago;  but  it  wears  smooth 
with  a  glassy  surface. 

1090.  Kettle   River   Sandstone.     This  is   a   fine-grained,   light- 
pink  sandstone,   found  in  large   quantities   at   Sandstone,   Minn., 
about  a  hundred  miles  north  of  Minneapolis,  which  has  been  used 
for  paving  purposes  in  Wisconsin  and  Minnesota.     The  stone  wears 
flat,  does  not  polish,  and  approaches  granite  in  its  resistance  to 
crushing. 

1091.  LIMESTONES.      These  differ  greatly  in  structure,  from  a 
light  friable  variety  highly  charged  with  fossils  to  a  hard  compact 
rock  denser  and  heavier  than  granite.     The  thin  bedded  varieties 
are  easily  broken  into  paving  blocks.     Although  some  varieties  of 
limestone  are  very  dense  and  strong,  it  wears  unevenly  when  used  as 
a  paving  material,  and  the  blocks  are  speedily  shivered  by  traffic 
and  split  by  frost,  owing  to  the  fact  that  the  lamination  is  vertical, 

ART.  2.     CONSTRUCTION 

1093.  Fig.  209  shows  a  transverse  section  of  the  better  form  of 
stone-block  pavements. 

1094.  FOUNDATION.     The  method  of  preparing  the  subgrade 
has  already  been  discussed — see  Art.   1,  Chapter  XV.     Formerly 
the  foundation  always  consisted  of  a  bed  of  sand  upon  the  natural 
soil  (§  966),  but   at  present  it  is  nearly  always  a  layer  of  concrete 
(Art.  2,  Chapter  XV).     Stone-block  paving  is  laid  only  on  streets 
subject  to  heavy  travel. 

1095.  BEDDING   COURSE.     On   the    concrete   foundation  must 
be  spread  some  material  to  even  up  the  surface  of  the  concrete  and 


ART.    2] 


CONSTRUCTION 


573 


to  give  a  good  bed  for  the  blocks.  The  smoother  the  surface  of  the 
concrete  and  the  less  the  variation  in  the  depth  of  the  blocks,  the 
thinner  can  be  the  cushion  coat.  Sand  has  generally  been  used  for 
the  bedding  course,  but  recently  cement  mortar  has  been  employed 
tentatively. 


,vc«l:y>:alS^ypfee?:.5a:.^g^Q^^^j 
FIG.  209.— SECTION  OF  STONE-BLOCK  PAVEMENT. 

1096.  Sand  Cushion.  The  sand  should  be.  fine,  clean  and  dry. 
The  finer  the  sand  the  better.  It  should  contain  no  pebbles 
greater  than  J  inch  in  diameter.  The  sand  should  be  clean  so  as  to 
compress  uniformly;  and  it  should  be  dry  so  it  will  not  shrink  away 
from  the  blocks  in  drying  out  (§  1055). 

In  spreading  the  sand  cushion  for  brick  pavements,  great  care  is 
taken  to  secure  a  bed  of  uniform  thickness  and  density  (§  972),  so 
that  when  the  pavement  is  rolled,  the  surface  will  be  smooth;  but 
stone-blocks  are  not  as  uniform  in  size  as  bricks,  and  hence  they  must 
be  settled  to  place  by  ramming  each  individual  block,  and  therefore 
it  is  not  necessary  to  spread  the  sand  for  stone-blocks  with  as  much 
care  as  for  bricks.  However,  for  the  best  results,  it  is  wise  to  spread 
the  sand  with  a  shovel  as  uniformly  as  possible,  and  then  rake  it  to 
loosen  up  any  spots  that  have  been  consolidated  by  throwing  down 
a  shovelful  of  sand  and  to  level  it  off  and  secure  a  layer  of  uniform 
thickness  and  density.  After  the  sand  is  leveled  off  it  should  not  be 
stepped  upon;  and  in  laying  the  blocks  the  men  should  stand  upon 
those  already  placed.  However,  these  precautions  are  seldom  taken; 
and  usually  the  blocks  are  deposited  on  the  sand  cushion  and  the 
man  who  sets  them  stands  upon  the  sand  cushion  while  at  work. 


574  STONE-BLOCK    PAVEMENTS  [CHAP.    XVIII 

The  thickness  of  the  bed  should  vary  with  the  accuracy  of  the 
dressing  of  the  blocks.  If  the  more  inaccurately  dressed  blocks 
(paragraph  1,  §  1100)  are  employed,  a  depth  of  2  inches  may  not  be  too 
much;  but  if  the  most  accurately  dressed  blocks  (paragraph  2, 
§  1100)  are  used  and  if  the  top  of  the  concrete  bed  is  reasonably 
smooth,  the  sand  cushion  need  not  be  more  than  1  inch.  The  sand 
cushion  should  be  no  thicker  than  is  necessary  to  give  a  good  bed  for 
the  blocks. 

1097.  For  a  statement  of  the  objections 'to  a  sand  cushion  for 
brick  pavements,  see  §  977-78.     These  objections  apply  with  nearly 
equal  force  to  stone-block  pavements. 

1098.  Mortar  Bedding  Course.     In  view  of  the  success  of  the 
monolithic  brick  pavement,  particularly  for  rural  roads  (§  979-82), 
it  has  been  proposed  to  lay  granite  blocks  in  a  mortar  bedding-course; 
but  there  has  been  only  a  little  experience  with  this  form  of  con- 
struction.    The  mortar  bedding  course  could  be  a  dry  mixture  of 
cement  and  sand  (§  979)  on  a  concrete  base  partially  set,  or  a  coat 
of  green  mortar  on  a  concrete  base  which  has  not  taken  initial  set 
(§  982). 

Some  claim  that  with  a  monolithic  stone-block  pavement  on 
city  streets  it  would  be  too  difficult  to  make  openings  to  lay  or  repair 
pipes,  conduits,  etc.;  but  this  might  be  an  advantage,  if  such  a 
pavement  would  cause  greater  care  in  laying  the  pipes  in  the  begin- 
ning. 

1099.  THE  BLOCKS.     The  blocks  should  be  made  of  sound  and 
durable  stone,  free  from  seams,  and  should  be  of  uniform  hardness, 
since  the  pavement  will  wear  unevenly  if  hard  and  soft  blocks  are 
laid  together.     For  the  appearance  of  the  pavement,  it  is  desirable 
that  blocks  of  only  one  color  be  laid  together. 

Fig.  210  shows  four  stages  in  the  manufacture  of  modern 
granite  paving-blocks. 

1100.  Dressing.     The  blocks  should  be  split  and  dressed  so  as 
to  have  as  nearly  as  possible  plane  rectangular  faces  and  square 
corners.     The  more  regular  the  blocks  the  thinner  the  joints,  and 
consequently  the  smoother  and  more  durable  the  pavement.     In  a 
general  way  there  may  be  said  to  be  three  standards  in  dressing  stone 
paving-blocks. 

1.  Formerly  the  blocks  were  roughly  dressed;  and  would  lay 
v/ith  joints  f  to  1  inch  wide  or  perhaps  more,  and  would  show  de- 
pressions of  1  inch  under  a  3-foot  straight  edge  laid  parallel  to  the 
curb.  The  joints  were  filled  with  pea  gravel  and  sand  or  tar. 


ART.    2]  CONSTRUCTION  575 

2.  Recently  there  has  been  a  demand  for  a  less  noisy  and  more 
sanitary  pavement,  and  hence  the  blocks  have  been  more  accurately 
dressed,  and  the  joints  have  been  filled  with  bituminous  cement  and 
sand  or  portland-cement  grout.  In  this  case  the  blocks  are  dressed 
to  conform  to  specifications  about  as  follows:  "  The  blocks  shall  be 


FIG.  210. — FOUR  VIEWS  OP  THE  MANUFACTURE  or  GRANITE  PAVING-BLOCKS. 

approximately  rectangular  on  top  and  sides,  and  uniform  in  width. 
They  shall  be  so  cut  that  the  joints  between  individual  blocks  when 
laid  shall  average  not  more  than  f  of  an  inch.  The  head  of  the  block 
shall  have  no  depression  greater  than  J  inch  from  a  straight  edge 
laid  in  any  direction  and  parallel  to  the  general  surface  of  the  block."* 
Fig.  211,  page  576,  shows  the  two  types  of  pavements  described 
above. 

*  Specifications,  Borough  of  Manhattan,  New  York  City,  1917, 


576  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

3.  It  is  very  difficult  to  meet  the  above  specification  even  when 
the  best  splitting  granites  are  available,  and  it  is  practically  impos- 
sible with  the  harder  granites;  and  further,  if  the  joints  are  filled 
with  cement  grout,  there  is  little  need  to  require  joints  as  thin  as 


Old  style  in  foreground;   new  style  in  background. 
FIG.  211. — OLD  AND  NEW  TYPES  OF  STONE  BLOCK  PAVEMENTS. 

f  inch.  Therefore  some  good  authorities  specify  that  the  blocks  shall 
be  dressed  so  as  to  lay  side  joints  not  more  than  f  of  an  inch  wide, 
end  joints  not  more  than  \  inch,  and  that  the  top  shall  not  depart 
more  than  \  of  an  inch.fyom  a  true  plane. 

1101.  Re-cutting,  formerly  the  blocks  were  made  larger,'  especi- 
ally deeper,  than  is  now  considered  good  practice;  and  besides  the 
blocks  were  not  dressed  as  accurately  as  is  the  custom  at  present. 
Consequently  there  are  many  stone-block  pavements,  particularly  in 
the  older  cities,  that  are  very  rough  and  composed  of  comparatively 
large  blocks;  and  hence  in  recent  years  many  of  these  old  blocks 
have  been  taken  up,  re-cut,  and  re-laid.  Different  methods  are 
employed  in  breaking  the  old  blocks  depending  upon  their  size. 
For  example,  blocks  12  X  8  X  4  inches  may  be  broken  into  four 
new  ones  6X4X4  inches.  But  blocks  either  much  smaller  or 
much  larger  receive  different  treatment.  For  example,  with  a 
smaller  block,  sometimes  a  new  block  is  taken  from  the  end,  and  two 
new  ones  are  made  from  the  remainder  of  the  old  block  by  dividing 
the  depth;  and  for  larger  blocks  sometimes  four  new  blocks  may  be 
taken  side  by  side  successively  from  the  end.*  The  blocks  are 

*  For  a  liberally  illustrated  account  of  the  method  of  re-cutting  granite  paving-blocks,  see 
Engineering  News,  Vol.  73  (1915),  p.  1020-23, 


ART.    2]  CONSTRUCTION  577 

usually  dressed  to  conform  to  the  specifications  in  paragraph  2  or  3 
of  §  1100.  The  depth  of  the  re-cut  or  napped  blocks  is  usually  less 
than  that  of  the  old  ones;  and  therefore  the  re-cut  blocks  generally 
make  a  greater  area  of  pavement  than  the  original  ones,  the  excess 
sometimes  being  nearly  100  per  cent.  The  cost  of  taking  up  and 
re-cutting  ranges  from  J  to  f  of  the  cost  of  new  blocks.  * 

1102.  Size  of  Blocks.     For  an  ideal  pavement  the  blocks  should 
be  of  one  size;   but  if  it  were  necessary  to  cut  the  blocks  to  exact 
dimension,  the  expense  would  be  unreasonably  great.     It  is  con- 
sidered good  practice  to  allow  variations  in  length  from  8  to  12  inches, 
in  width  from  3J  to  4J  inches,  and  in  depth  from  4J  to  5J  inches. 
It  is  customary  to  require  that  the  blocks  shall  be  sorted  according 
to  width,  and  be  laid  in  courses  of  practically  uniform  width. 

The  above  specifications  are  for  a  block  nominally  5  inches  deep; 
but  a  few  cities  use  blocks  nominally  4  inches  deep.  When  stone 
blocks  were  usually  set  in  the  native  soil  or  in  a  thick  sand  cushion, 
it  was  customary  to  make  the  depth  7  or  8  inches;  but  when  a  con- 
crete foundation  was  introduced,  the  depth  was  generally  reduced  to 
5  inches.  It  is  probable  that  with  the  more  accurate  cutting 
now  customary,  with  a  concrete  base,  a  mortar  cushion-coat 
and  a  grout  joint-filler,  a  4-inch  granite  block  meeting  the  spe- 
cifications of  §  1100  will  be  more  durable  than  either  a  5-inch  block 
with  a  sand  cushion  and  a  gravel  filler,  or  a  7-  or  8-inch  block  with  a 
thick  sand  foundation  and  a  gravel  filler.  The  reduction  of  a  block 
in  depth  by  wear  under  the  heaviest  travel  is  inappreciable,  partic- 
ularly with  a  rigid  filler.  For  information  concerning  the  use  of 
granite  blocks  less  than  4  inches  deep,  see  §  1117-18. 

1103.  Measuring.     Usually  the  contractor  buys  the  blocks  by 
the  thousand,  but  gets  paid  for  them  by  the  square  yard;  and  there- 
fore it  is  to  his  financial  advantage  to  use  as  many  large  blocks  as 
possible.     Again,  the  man  who  sets  the  blocks  is  usually  paid  by  the 
square  yard;   and  therefore  it  is  to  his  financial  advantage  to  make 
the  joints  as  wide  as  he  may.     It  is  very  undesirable  that  it  should 
be  to  the  financial  interests  of  the  contractor  and  of  the  paver  to 
secure  a  poor  pavement,  i.  e.,  one  having  large  blocks  and  wide 
joints.     An  excess  in  the  width  of  the  block  is  more  important  than 
in  the  length,  since  it  is  proportionally  a  larger  matter,  and  also 
since  it  has  a  more  important  influence  upon  the  quality  of  the  pave- 

*  For  an  account  of  the  history  of  re-cutting  granite  paving-blocks  with  examples  of  the 
saving  in  a  number  of  cases,  see  Proc.  Amer.  Soc.  Municipal  Improvements,  1914,  p.  321-35, 
and  p.  336-42. 


578  STONE-BLOCK   PAVEMENTS  [CHAP.   XVII 1 

ment;  and  therefore  special  care  should  be  taken  to  prevent  either 
an  excessive  width  of  blocks  or  too  thick  side-joints.  This  precau- 
tion was  more  important  formerly  than  at  present,  since  then  the 
joints  were  filled,  or  rather  partially  filled,  with  pebbles,  and  con- 
sequently wide  joints  were  more  destructive  than  now;  but  never- 
theless the  principle  is  still  worth  considering.  To  identify  as  far 
as  possible  the  interests  of  the  contractor  with  those  of  the  city,  the 
following  method  of  measuring  a  stone-block  pavement  has  been 
proposed.* 

"The  blocks  must  be  substantially  smooth  and  square  on  all  their  faces,  and 
within  the  limits  of  the  following  dimensions:  Not  less  than  3^  inches  nor  more 
than  4^  inches  wide  across  their  upper  and  lower  faces;  not  less  than  7  nor 
more  than  8  inches  deep;  and  not  less  than  8  nor  more  than  14  inches  long,  except 
where  shorter  stones  are  necessary  to  fill  out  courses. 

"The  sum  to  be  paid  per  square  yard  shall  be  ascertained  as  follows:  The 
number  of  blocks  per  square  yard  upon  which  the  bid  of  the  contractor  is  based 
shall  be  22  £.  The  actual  average^  number  of  blocks  laid  per  square  yard  shall 
be  determined  as  follows:  The  City  Engineer  shall  from  time  to  time,  during  the 
progress  of  the  work,  measure  the  width  of  50  to  100  courses,  and  from  this  deduce 
the  average  width  of  a  course.  The  average  length  of  the  blocks  is  hereby  fixed 
for  the  purpose  of  computing  the  number  of  blocks  laid  per  square  yard,  at  12^ 
inches,  f 

"For  each  block  or  fractional  part  thereof,  that  the  average  number  laid  per 
square  yard  shall  exceed  22|,  there  shall  be  added  to  the  contractor's  bid  per 
square  yard  an  amount  computed  at  the  rate  of  9|  cents  per  block.  For  each 
block  or  fractional  part  thereof,  that  the  average  number  laid  per  square  yard 
shall  fall  short  of  22^,  there  shallibe  deducted  from  the  contractor's  bid  per  square 
yard  an  amount  computed  at  the  rate  of  9|  cents  per  block." 

According  to  this  method,  if  the  contractor  uses  narrow  blocks 
and  thin  joints,  the  price  per  squard  yard  is  proportionally  increased; 
but  if  he  uses  thick  blocks  and  wide  joints,  the  price  per  yard  is 
decreased.  To  meet  the  case  in  which  a  contractor  should  buy  large 
blocks  at  a  considerable  reduction,  it  might  be  wise  to  make  the 
amount  per  block  to  be  deducted  greater  than  that  added.  For 
convenience  in  applying  the  above  method,  a  table  is  computed 
which  gives  in  one  column  the  width  of  50  courses  and  in  a  second 
column  the  corresponding  number  of  blocks  per  square  yard.  Of 
course,  the  number  of  blocks  to  a  square  yard  would  vary  with  the 
specified  dimensions  of  the  blocks  and  with  the  width  of  joints, 


*  By  Horace  Andrews,  City  Engineer  of  Albany,  N.  Y.,  in  1890  in  Engineering  Record,  Vol. 
21,  p.  314  and  329;  Vol.  25,  p.  110-11. 

t  This  value  was  determined  by  measuring  a  number  of  blocks  in  pavements  laid  with  blocks 
of  the  size  stated  above. 


ART.    2]  CONSTRUCTION  579 

which  latter  would  vary  with  the  different  kinds  of  stone  and  even 
with  the  same  kind  from  different  quarries,  and  could  be  deter- 
mined in  any  particular  case  only  by  measuring  the  combined  width 
of  a  number  of  courses  of  blocks  in  the  pavement.  The  normal  or 
contract  number  of  blocks  per  square  yard  should  be  stated  according 
to  the  quality  of  work  desired. 

Some  cities  buy  the  blocks  and  contract  for  laying  them,  a  method 
which  eliminates  the  interest  of  the  contractor  in  using  large  blocks. 
In  some  cities  it  is  the  custom  for  the  contractor  to  buy  the  blocks  by 
the  square  yard  in  the  pavement,  in  which  case  the  contractor  pays 
only  for  the  blocks  accepted,  and  has  no  financial  interest  in  the  size 
of  the  blocks  or  the  thickness  of  the  joints.  In  Great  Britain  it  is 
customary  to  buy  the  blocks  by  weight,  a  method  which  eliminates 
any  interest  of  the  contractor  in  the  size  of  the  blocks. 

1104.  Some  cities  require  the  blocks  to  be  inspected  and  sorted 
to  sizes  before  being  piled  on  the  street.     The  advantages  of  this 
are:    (1)  After  being  stacked  upon  the  street  it  is  nearly  impossible 
to  inspect  them,  since  only  the  outside  blocks  of  the  pile  can  be 
seen;    (2)  when  the  blocks  are  being  laid,  the  inspector  has  enough 
to  do  to  watch  the  quality  of  the  workmanship  without  having  also 
to  inspect  the  blocks;    (3)  removing  rejected  blocks  from  the  pave- 
ment delays  the  opening  of  the  street;  and  (4)  if  the  blocks  are  sorted 
before  being  piled  upon  the  street,  different  sizes  are  not  so  likely 
to  get  into  the  same  course,  and  therefore  the  joints  will  be  narrower. 

In  Cleveland,  Ohio,  where  the  specified  width  of  the  stone  paving- 
block  was  from  3J  to  5  inches,  the  blocks  were  sorted  into  three 
classes.  Class  No.  1  included  blocks  from  3J  to  3J  inches,  Class 
No.  2  blocks  from  3f  to  4J  inches,  and  Class  No.  3  blocks  from  4J 
to  5  inches.  Blocks  in  Class  No.  1  were  marked  with  red  paint, 
blocks  in  Class  No.  2  with  blue  paint,  and  those  in  Class  No.  3  with 
black  paint,  so  that  when  the  blocks  were  delivered  on  the  street 
each  class  could  be  easily  recognized  and  laid  by  itself. 

1105.  SETTING  THE  BLOCKS.     In  placing  the  blocks,  the  work- 
man should  stand  upon  the  finished  work,  that  the  sand  cushion 
may  not  be  disturbed;    but  he  usually 

stands  on  the  sand  cushion,  the  blocks 

being  piled  on  the  sand  bed  behind  him. 

The  workman  with  the  pointed  end  of 

the  hammer  shown  in  Fig.  212  excavates  FlG'  ™-~s™™  PAVER'S  HAMMEB. 

a  hole,  if  need  be,  into  which  to  set  the  block. 

To  secure  the  proper   form  to  the  surface  of  the   pavement,  a 


580  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

chalk  line  is  made  upon  each  curb  or  a  string  is  stretched  in  each 
gutter  to  indicate  the  top  of  the  blocks,  and  a  row  of  blocks  20  to  25 
feet  apart  is  set  in  the  center  of  the  street  with  their  tops  to  grade 
as  determined  by  measuring  down  from  a  string  stretched  from 
curb  to  curb.  If  the  street  is  wide,  one  or  more  rows  of  blocks 
are  placed  between  the  curb  and  the  crown.  Ordinarily  the  sur- 
face of  the  pavement  is  brought  to  grade  between  the  guide  blocks 
with  the  unaided  eye;  but  in  the  best  work,  a  straight  edge  or  string 
is  placed  parallel  to  the  line  of  the  street  on  the  guide  blocks,  by  which 
to  grade  the  surface,  and  between  these  lines  the  blocks  are  brought 
to  the  surface  indicated  by  a  straight  edge  parallel  to  the  line  of 
the  street  resting  upon  the  pavement  already  completed. 

The  blocks  should  be  set  with  their  long  dimension  across  the 
street,  except  at  street  intersections ;  and  should  be  placed  in  straight 
rows  with  as  close  joints  as  possible.  Each  course  should  be  formed 
of  blocks  of  uniform  width  and  depth;  and  the  bond  should  be  ap- 
proximately half  the  length  of  a  blocks  or  at  least  3  inches.  As  the 
blocks  are  of  uneven  lengths,  the  securing  of  the  proper  bond  requires 
careful  attention.  The  paver  is  instructed  to  secure  thin  joints,  and 
consequently  has  a  tendency  to  set  the  block  with  the  larger  end  up ; 
but  when  set  in  this  way  the  block  will  surely  sink  under  traffic. 
Placing  the  large  end  of  the  block  down  makes  a  wide  joint,  which  is 
objectionable  if  the  joints  are  to  be  filled  only  with  sand  and  pebbles 
(§  1108),  but  is  no  serious  objection  if  the  joints  are  to  be  filled  with 
hydraulic-cement  grout  (see  §  1112). 

The  courses  at  street  intersections  are  arranged  substantially 
as  in  brick  pavements  (§  988).  The  work  should  progress  up  grade 
and  from  the  gutter  towards  the  crown,  so  that  the  blocks  may  have 
no  tendency  to  settle  away  from  each  other  and  thus  increase  the 
width  of  the  joints. 

Fig.  213  shows  four  views  of  the  laying  of  stone-block  paving. 

1106.  RAMMING  THE  BLOCKS.  After  the  blocks  have  been 
placed,  they  should  be  thoroughly  rammed  until  they  come  to  a 
firm  bearing.  As  a  rule  the  workman  is  more  interested  in  secur- 
ing a  uniform  surface  than  in  bringing  the  blocks  to  an  unyielding 
bearing.  Each  block  should  receive  at  least  three  hard  blows — 
one  near  each  end  and  one  in  the  middle.  The  rammer  employed, 
Fig.  214,  page  582,  weighs  from  50  to  90  lb.,  ordinarily  60  to  75  Ib. 

If,  after  being  rammed,  a  block  does  not  conform  to  the  general 
surface  of  the  pavement,  it  should  be  lifted  out,  and  sand  should 
be  added  to  the  sand  bed  or  extracted  from  it  to  bring  the  top  of  the 


ART.   2] 


CONSTRUCTION 


581 


582 


STONE-BLOCK    PAVEMENTS 


[CHAP,  xvin 


FIG.  214. — STONE-BLOCK 
RAMMER. 


block  to  the  proper  elevation.  Any  imperfect  or  broken  blocks 
should  be  removed  and  be  replaced  with  perfect  ones.  Finally 
each  block  should  be  adjusted  so  that  it 
stands  perpendicular  to  the  sand  bed  and 
has  its  top  face  conforming  to  the  surface 
of  the  pavement.  The  quality  of  the  pave- 
ment depends  largely  upon  the  care  with 
which  this  adjustment  is  made. 

The  ramming  is  likely  to  be  slighted 
unless  closely  watched.  The  man  who  does 
the  ramming  is  likely  to  tap  lightly  a  block 
which  if  thoroughly  rammed  would  be 
driven  below  the  general  surface  of  the 
pavement;  and  subsequent  travel  will  force 
the  stone  down  and  make  a  depression  in 
the  surface.  The  important  thing  is  to 
have  each  block  equally  and  sufficiently 
rammed  to  bring  it  to  a  solid  bearing  on 
the  bedding  course  and  at  the  same  time 
bring  its  top  to  the  proper  elevation. 

To  secure  a  thorough  ramming  of  the  pavement,  it  is  sometimes 
specified  that  there  shall  be  one  rammer  to  each  paver,  and  occa- 
sionally one  rammer  to  two  pavers.  No  ramming  should  be  allowed 
within  20  or  25  feet  of  the  course  last  laid,  to  prevent  the  tipping 
of  the  block  out  of  the  vertical  position ;  but  all  the  blocks  set  should 
be  rammed  before  work  ceases  for  the  day. 

Fig.  215  is  a  near  view  of  a  granite-block  pavement. 

1107.  FILLING  THE  JOINTS.     Four  materials  are  in  common  use 
for  filling  the  joints  of  stone-block  pavements,  viz.:    (1)  pea  gravel, 
(2)  tar  and  sand,  (3)  asphalt  and  sand,  and  (4)  cement  grout.     The 
first  is  used  with  joints  that  are  f  to  1  inch  wide,  and  the  others  with 
joints  f  to  f  of  an  inch  wide. 

1108.  Pea  Gravel.     Where  the  joints  are  J  to  1  inch  wide,  it  is 
customary  to  fill  them  with  pea  gravel  or  pea  gravel  and  tar.     It 
is  usually  specified  that  the  pea  gravel  shall  pass  a  J-inch  mesh  and 
be  retained  on  a  J-inch  mesh.     If  tar  is  to  be  poured  upon  the  peb- 
bles, they  should  not  be  too  small,  or  they  will  not  permit  the  tar 
to  flow  freely  to  the  bottom  of  the  joint.     Since  the  joints  are  wide 
and  the  blocks  are  roughly  cut,  the  joints  are  usually  partly  filled 
(say,  1|  to  2  inches  deep)  with  pebbles  before  the  blocks  are  rammed, 
to  keep  them  in  place  during  ramming.     After  the  joints  have  been 


ART.  2] 


CONSTRUCTION 


583 


partially  filled  and  the  blocks  have  been  rammed,  the  pebbles  in  the 
joints  are  tamped  with  a  bar  having  a  chisel-shaped  end.  The  joints 
are  next  swept  full  of  hot  pebbles  and  again  tamped. 

1109.  There  are  two  methods  in  more  or  less  common  use  for 
completing  the  filling  of  the  joints. 

1.  The  filling  is  completed  by  spreading  fine  sand  over  the  pave- 
ment to  a  depth  of  J  to  1  inch,  and  allowing  travel  to  work  it  into  the 


Joints  in  foreground  not  filled;   joints  in  background  filled  with  tar. 
FIG.  215. — NEAR  VIEW  OF  GRANITE-BLOCK  PAVING. 

joints.  Until  recently  this  was  the  only  method  employed,  and  even 
yet  it  is  quite  common.  When  filled  in  this  way,  the  joints  are  not 
impervious;  and  the  filling  does  not  aid  much  in  keeping  the  blocks 
in  position. 

2.  Recently  it  has  become  the  custom  with  the  better  class  of 
stone-block  paving  to  complete  the  filling  of  the  joints  by  pouring 
hot  tar  over  the  pebbles.  The  tar  is  applied  in  substantially  the 
same  way  as  in  the  case  of  brick  pavements — see  §  1011-12.  The 


584  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

pebbles  should  be  perfectly  dry,  for  an  almost  inappreciable  amount 
of  water  will  cause  the  tar  to  foam  and  will  prevent  it  from  adhering 
to  the  pebbles  and  from  forming  a  water-tight  joint.  It  may  be 
necessary  to  dry  the  pebbles  artificially.  The  tar  must  not  be 
applied  when  the  pebbles  are  very  cold.  The  joints  should  be 
entirely  filled  with  the  tar,  to  secure  which  it  is  usually  necessary 
to  pour  the  joints  twice.  To  keep  the  contractor  from  having  a 
financial  interest  in  not  filling  the  joints  entirely  full,  it  is  sometimes 
specified  that  there  shall  be  brought  upon  the  ground  not  less 
than  a  stated  number  of  gallons  of  paving  cement  for  each  square 
yard  of  pavement,  and  that  whatever  remains  after  the  completion 
of  the  work  is  the  property  of  the  city. 

In  some  cases  it  is  specified  that  the  pebbles  and  tar  shall  be 
applied  alternately  in  three  stages. 

The  quantity  of  tar  required  to  fill  the  joints  varies  from  1  to  3| 
gallons  per  square  yard,  according  to  the  width  of  the  joints,  which 
varies  with  the  quality  of  the  stone  and  the  workmanship. 

The  tar  in  the  joints  makes  the  pavement  impervious,  and 
therefore  more  sanitary.  The  tar  also  assists  in  keeping  the  blocks 
in  position,  and  therefore  adds  to  the  durability  and  smoothness  of 
the  pavement. 

Fig.  216  shows  the  process  of  filling  the  joints  of  a  stone-block 
pavement  with  tar.  Incidentally  this  figure  also  shows  the  differ- 
ence between  the  new  and  old  types  of  pavements. 

1110.  Tar  and  Sand.  When  the  joints  are  nominally  f  of  an  inch 
wide,  they  are  often  filled  with  tar  and  sand,  which  is  sometimes  called 
tar-pitch  mastic  or  pitch-sand  mastic.  The  following  are  the  speci- 
fications for  this  form  of  joint  filler  adopted  by  the  American  Society 
of  Municipal  Improvements.  * 

"The  joint  filler  shall  be  the  paving  pitch  hereafter  described  [see  §  576-77], 
thoroughly  mixed  with  as  much  hot  dry  sand  as  the  pitch  will  carry;  but  in  no 
case  shall  the  volume  of  the  sand  exceed  the  volume  of  the  pitch.  The  sand  shall 
be  fine  and  clean,  and  all  of  it  shall  pass  a  20-mesh  screen.  It  shall  be  heated  to  a 
temperature  of  not  less  than  300  nor  more  than  400°  F.;  and  shall  be  between 
these  limits  when  mixed  with  the  paving  pitch. 

"The  paving  pitch  shall  be  heated  in  kettles  properly  equipped  with  an 
approved  thermometer,  which  shall  register  the  temperature  of  the  pitch. 

"The  mixture  shall  be  flushed  on  the  surface  of  the  blocks  and  pushed  into 
the  joints  with  suitable  tools,  re-flushing  or  re-pouring,  if  necessary,  until  the 
joints  remain  permanently  filled  flush  with  the  surface  of  the  pavement.  As 
little  as  possible  of  the  mixture  shall  be  left  on  the  surface. 

*Specifications  for  Stone-block  Paving,  as  revised  in  1916. 


ART.   2] 


CONSTRUCTION 


585 


"  In  applying  the  filler  care  should  be  taken  that  the  pavers  are  closely  followed 
by  the  filler  gang,  and  in  no  case  shall  the  paving  be  left  over  night,  or  when  work 
is  stopped,  without  the  filling  of  the  joints  being  completed.  In  case  rain  stops 
the  filler  gang  before  its  work  is  finished,  the  joints  should  be  protected  by  the 
use  of  tarpaulins  or  other  means,  to  keep  out  water.  Under  no  circumstances 
shall  the  filler  be  poured  into  wet  joints." 

The  tar  pitch  should  comply  with  the  specifications  in  §  576-77, 
except  that  northern  cities,  or  rather  cities  that  are  subject  to  cool 


Modern  pavement  in  foreground;  old-style  pavement  in  background. 
FIG.  216. — FILLING  THE  JOINTS  WITH  TAB. 

weather  the  greater  part  of  the  year,  should  use  pitch  having  a  melting 
point  from  115  to  125°  F.,  and  cities  which  have  long-continued  hot 
weather  should  specify  a  melting  point  from  125  to  135°  F. 

1111.  Asphalt  and  Sand.  The  following  are  the  specifications 
for  this  form  of  filler  adopted  by  the  American  Society  of  Municipal 
Improvements.  * 

"The  joint  filler  used  shall  be  the  asphalt  cement  hereafter  described  [see 
§  544],  thoroughly  mixed  with  as  much  hot,  dry  sand  as  the  cement  will  carry; 
but  in  no  case  shall  the  volume  of  the  sand  exceed  the  volume  of  the  cement. 
The  sand  shall  be  fine  and  clean  and  all  of  it  shall  pass  a  20-mesh  screen.  The 

*  Specifications  for  Stone-block  Paving,  as  revised  in  1916, 


586 


STONE-BLOCK   PAVEMENTS 


[CHAP,  xvin 


sand  shall  be  heated  to  a  temperature  of  not  less  than  300  nor  more  than  400°  F.; 
and  shall  be  between  these  limits  when  mixed  with  the  asphalt  cement. 

"The  asphalt  cement  shall  be  heated  in  kettles  properly  equipped  with  an 
approved  thermometer,  which  shall  register  the  temperature  of  the  cement. 

"The  mixture  shall  be  flushed  on  the  surface  of  the  blocks  and  pushed  into 
the  joints  with  suitable  tools,  re-flushing  or  re-pouring,  if  necessary,  until  the 
joints  remain  permanently  filled  flush  with  the  surface  of  the  pavement.  As 
little  as  possible  of  the  mixture  shall  be  left  on  the  surface." 

The  specifications  for  the  asphalt  cement  referred  to  above  are 
given  in  §  544,  page  282. 

1112.  Cement  Grout.  To  secure  the  smoothest  and  most  durable 
stone-block  pavement,  the  joints  should  be  filled  with  portland- 
cement  grout,  which  should  be  mixed  and  applied  as  described  in 


FIG.  217. — GRANITE-BLOCK  PAVEMENT  EIGHT  YEARS  OLD. 


§  996-1005.  However,  since  larger  quantities  are  required  for  stone 
blocks  than  for  bricks,  it  is  usual  to  permit  the  grout  to  be  mixed  in  a 
batch  machine-mixer. 

Fig.  217  shows  a  grout-filled  granite-block  pavement  at  Lowell, 
Mass.,  eight  years  old. 

The  portland-cement  grout  makes  the  joint  impervious,  holds  the 


AET.   2]  CONSTRUCTION  587 

blocks  firmly  in  position,  prevents  the  edges  from  chipping  and  the 
top  face  from  wearing  round,  and  adds  materially  to  the  smoothness 
and  durability  of  the  pavement. 

1113.  EXPANSION  JOINTS.     If  the  joints  are  filled  with  port- 
land-cement  grout,  an  expansion  joint  of  J  to  1  inch  in  width  should 
be  provided  next  to  each  curb,  constructed  as  described  in  §  1017; 
and  expansion  joints  should  be  provided   around  manhole  covers, 
water  boxes,  etc.,  as  described  in  §  1021. 

If  the  joints  are  filled  with  pebbles  or  bituminous  cement,  no 
longitudinal  expansion  joints  are  necessary. 

Transverse  expansion  joints  should  not  be  provided,  as  they  are 
not  needed  and  are  a  decided  detriment  (see  §  1018-20). 

1114.  PAVING  ADJACENT  TO  TRACK.     Fig.  218  shows   the 
standard  method  employed  by  the  Paving  Commission  of  Baltimore 
in  laying  granite-block  pavement  next  to  street-car  rails.* 

^Bituminous  f/7/er 
9-Rafl     I'Morfarbet" '  «™*"L%!t>'  Z- 


FIG.  218. — GRANITE-BLOCK  PAVING  ADJACENT  TO  TRACK,  BALTIMORE. 

Granite  blocks  are  much  used  for  paving  the  railway  area,  because 
of  their  durability.  When  granite  blocks  are  laid  in  the  narrow  strip 
between  the  rail  and  some  other  form  of  paving,  they  should  be  laid 
as  stretchers  or  as  headers,  i.  e.,  without  toothing. 

In  Worcester,  Mass.,  an  expansion  joint  is  constructed  between 
the  pavement  on  the  track  area  and  that  on  the  remainder  of  the 
street.  The  joint  extends  through  the  wearing  coat  and  the  con- 
crete foundation.  The  joint  in  the  block  course  is  made  by  nailing 
a  pre-moulded  mastic  strip  (§  1017)  against  the  ends  of  the  ties.  The 
joint  is  to  prevent  the  rumbling  of  the  grout-filled  pavement  due  to 
the  passage  of  a  street  car;  and  is  effective.! 

1115.  MAXIMUM  GRADE.  Stone-block  pavements  are  freely 
employed  upon  grades  up  to  10  per  cent;  and  if  the  stone  5s  a  quality 
that  does  not  wear  smooth,  they  may  be  used  upon  grades  up  to 
15  per  cent. 

"Engineering  News,  Vol.  73  (1915),  p.  884. 
•\Ibid.,  News,  Vol.  74  (1915),  p.  398. 


588  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

It  has  been  recommended  that  on  steep  grades  to  afford  a  good 
foothold  for  the  horses,  (1)  the  edges  of  the  blocks  be  chamfered,  (2) 
that  the  joints  be  comparatively  wide,  and  (3)  that  the  joints  be  filled 
to  within  about  an  inch  of  the  top  with  cement  mortar.  It  is  not 
known  that  these  expedients  have  ever  been  employed;  but  the 
probabilities  are  that  wide  joints  would  be  equally  as  effective 
without  chamfering  the  blocks,  since  the  edges  spall  off  soon  when 
the  joints  are  wide  and  are  filled  with  either  gravel  or  tar.  Further, 
the  accumulation  of  dirt  in  the  wide  joints  would  probably  largely 
neutralize  their  effect.  Fig.  219  shows  another  method  that  has 
been  proposed,  but  it  is  not  known  that  it  has  ever  been  tried. 

1116.  MERITS  AND  DEFECTS.  The  only  merit  claimed  for 
stone-block,  particularly  granite-block,  pavement  is  durability. 
The  material  of  the  blocks  does  not  decay  or  wear  entirely  out. 
But  if  the  joints  are  filled  with  gravel  or  a  soft  filler,  the  face  of  the 
blocks  wear  round;  and  if  a  thick  sand  cushion  is  used,  some  blocks 
settle  more  than  others.  The  result  is  that  such  a  pavement  becomes 
excessively  rough  and  noisy;  and  if  the  granite  is  hard,  the  pave- 
ment is  slippery. 


FIG.  219. — STONE-BLOCK  PAVEMENT  ON  STEEP  GRADE. 

However,  if  the  blocks  are  carefully  dressed,  are  of  nearly  uniform 
size,  and  laid  with  thin  joints;  and  if  they  are  laid  in  a  mortar  bed- 
ding course  and  the  joints  are  filled  with  portland-cement  grout,  the 
pavement  is  very  durable  and  not  specially  noisy.  A  granite-block 
pavement  is  the  form  universally  chosen  where  there  are  many 
heavily  laden  steel-tired  wagons  and  trucks. 

1117.  DURAX  PAVEMENT.  This  is  a  pavement  made  of  cubes 
of  granite  placed  upon  a  concrete  foundation.  In  America  and 
England  it  is  known  as  durax  pavement,  and  in  Germany  as  klein- 
pflaster.  This  pavement  has  been  used  in  Europe  since  about  1885, 
but  the  first  in  this  country  was  laid  in  the  Brooklyn  Navy  Yard  in 
1913.  Since  then  it  has  been  laid  in  a  number  of  American  cities. 

The  blocks  are  approximately  cubes  having  faces  2J  to  3J  inches 
square.  They  are  usually  cut  to  lay  approximately  f-inch  joints, 


ART.    2]  CONSTRUCTION  589 

The  cubes  are  generally  machine  made,  and  can  be  turned  out  cheaper 
per  square  yard  than  large  hand-made  ones;  but  on  the  other  hand, 
they  are  not  usually  as  accurately  cut  as  the  best  large  blocks.  How- 
ever, since  the  durax  blocks  are  nominally  cubical,  they  may  be  laid 
on  any  one  of  three  sides,  which  gives  a  little  advantage  in  fitting 
them  into  place.  The  blocks  may  be  laid  with  any  form  of  bedding 
course  (§  1095-98),  or  with  any  joint  filler  (§  1107-12);  but  appa- 
rently they  are  usually  laid  on  a  1-inch  sand  cushion,  and  with 
asphalt  filler. 

The  blocks  are  not  usually  laid  in  straight  course,  but  in  concen- 
tric segments  of  circles,  in  what  is  sometimes  called  the  oyster-shell 


FIG.  220. — LAYING  A  DURAX  PAVEMENT. 

pattern.     Fig.  220  shows  the  process  of  laying  a  durax  pavement; 
and  Fig.  221,  page  590,  is  a  close  view  of  such  a  pavement.* 

The  advantages  claimed  for  the  segmental  form  of  courses  are: 
1.  There  are  only  a  few  joints  parallel  to  the  direction  of  travel,  and 
hence  the  stones  wear  better  than  in  the  ordinary  oblong  block  pave- 
ment. This  would  not  be  important,  if  a  grout  filler  is  used.  2. 
Since  there  are  no  continuous  transverse  joints,  opposite  wheels  of  a 
vehicle  can  not  drop  into  a  joint  at  the  same  time;  and  hence  there  is 
less  jar  and  less  wear  on  pavement  and  vehicle.  This  would  not  be 
an  important  advantage,  if  the  joints  are  filled  with  portland-cement 
grout.  3.  Since  the  courses  need  not  be  kept  straight,  the  blocks 
can  be  turned  so  as  to  give  the  narrowest  joints.  This  may  be  an 
advantage  in  placing  some  of  the  blocks;  but  it  is  a  disadvantage  in 

*  Engineering  News,  Vol.  72   (1914),  p.  529. 


590 


STONE-BLOCK   PAVEMENTS 


[CHAP,  xvin 


making  closures  between  different  segments.  On  the  whole,  the 
joints  can  not  be  as  narrow  as  with  large  blocks  equally  accurately 
cut. 

1118.  The  first  cost  of  the  blocks  for  durax  pavement  is  less  per 
square  yard  than  for  an  ordinary  granite-block  pavement;  but  the 
labor  of  laying  is  much  greater,  and  the  total  cost  of  the  small-cube 


FIG.  221. — OYSTER-SHELL  PATTERN  OF  DURAX  PAVEMENT. 

pavement  is  more  than  that  of  the  large-block  pavement.  The 
small  cubes  have  been  used  to  re-surface  old  macadam  or  other 
pavements.  One  advantage  of  the  small  cubes  is  that  they  may  be 
made  of  the  same  thickness  as  a  brick  or  asphalt  or  wood-block 
pavement,  and  hence  a  durax  surface  may  replace  the  old  one  with- 
out disturbing  the  old  foundation  or  changing  the  grade  of  the  pave- 
ment. It  is  said  that  durax  pavements  are  not  now  being  laid 
in  Europe  to  any  considerable  extent,  and  that  the  area  of  durax 
pavements  in  Europe  is  only  about  3  per  cent  of  that  of  oblong 
blocks. 


ART.    2]  CONSTRUCTION  591 

1119.  COST.     Price    of  Blocks.     The  following  is  the  market 
quotation  for  stone  paving-blocks  for  November  1,  1917.* 

New  York  City,  Manhattan,  standard  granite  .............  $2  .  50  sq.  yd. 

other  boroughs  standard  granite  .............  2  .  25 

other  boroughs  5-inch  granite  ...............  2  .  55  " 

Boston  standard  granite  .............  2  .  55  " 

Chicago,  ordinary  dressing,  standard  granite  .............   1  .  80  " 

best  dressing  standard  granite  .............  2.25  " 

St.  Paul  standard  sandstone  ..........   1  .  65  " 

Kansas  City  standard  limestone  ...........  2  .  15  " 

The  variation  in  price  is  partly  due  to  the  difference  in  freight 
and  in  price  of  labor,  but  chiefly  to  the  ease  with  which  the  available 
material  may  be  dressed.  For  example,  according  to  data  published 
by  the  U.  S.  Geological  Survey,  the  average  price  of  granite  paving 
blocks  per  thousand  in  1916  varied  from  $32  in  California  and  $33  in 
Georgia  to  $62  in  Minnesota  and  $66  in  Wisconsin,  the  first  two 
having  easily  worked  granites  and  the  last  two  granites  difficult  to 
work.  Ordinarily,  to  lay  a  square  yard  of  pavement  requires  28  to 
31  blocks. 

1120.  Granite-block  Pavement.    New  York.     In  New  York  City 
in  1917  the  cost  of  standard  granite-block  pavement  is  as  follows: 


COST 

per  Sq.  Yd. 

CONCRETE  BASE,  6  inches:  materials  and  labor  .......................  $1  .  10 

SAND  CUSHION,  1  inch:  material  and  labor  ............................  07 

WEARING  COAT: 
29  blocks  at  9  cents  on  street  ...........................     2.61 

labor  laying  ..........................................        .22 

Total  for  wearing  coat  .........................................  $2  .  83 

JOINT  FILLING:  2  gallons  of  bituminous  filler,  sand,  and  labor  of  applying.        .45 

Total  cost  to  contractor,  exclusive  of  administration,  tools,  etc.,  and 

grading  .............................  $4.45 

With  a  cement-grout  filler  in  place  of  the  bituminous  filler,  the 
cost  is  about  20  cents  less,  or  $4.25  per  square  yard.  With  a  cement- 
grout  filler  and  a  cement-mortar  cushion,  the  cost  is  5  to  10  cents 
per  square  yard  less  or  $4.35  to  $4.40  per  square  yard.  In  the 
Borough  of  Manhattan  with  the  improved  block  there  specified 
and  with  the  difference  in  working  conditions  in  that  Borough,  about 
40  cents  per  square  yard  should  be  added,  making  the  total  cost  about 
$4.85  per  square  yard. 

^Engineering  News-Record,  Vol.  79  (1917),  Construction  News,  p.  179. 


592  STONE-BLOCK    PAVEMENTS  [CHAP.    XVIII 

1121.  Chicago.  The  average  cost  to  the  contractor  of  laying 
specially  dressed  granite  blocks  (3|  to  4  inches  wide,  8  to  10  inches 
long,  5  inches  deep,  of  which  28  to  31  lay  a  square  yard)  at  Chicago 
in  1917,  was  about  as  follows:* 


ITEMS- 
CONCRETE  BASE  :  6  inches:  materials  and  labor  .......................  $0  .  92 

SAND  CUSHION: 
2  inches  of  sand  at  $2.50  per  cu.  yd  ....................  13 

labor  spreading  ...................................          -02f 

Total  cost  sand  cushion  .......................................   $  .  15| 

WEARING  COAT: 
granite  blocks  f  .o.b.  Chicago  .........................  $2  .  35 

hauling  to  street  ....................................  13 

carrying  to  paver  .................  ............  .......  06 

laying  and  ramming  ...............................          .  19 

Total  for  wearing  coat  ........................................  $2.73 

FILLING  JOINTS: 
paving  gravel  at  $2.00  per  cu.  yd  .....................  $  .13 

labor  spreading  ....................................        .04 

tar  at  9  cents  per  gallon  ..............................  10 

labor  applying  ....................................          .07 

Total  for  joint  filler  ..........................................  .  $   .34 

Total  cost  to  contractor,  exclusive  of  tools,  administration,  etc.,     . 

and  grading  ......................   $4  .  14f 

Ordinary  granite  blocks  cost  25  to  30  cents  per  square  yard 
less  than  the  special  dressed  blocks  above,  the  cost  of  laying  is 
8  cents  per  square  yard  less,  and  the  total  of  the  other  items  is  sub- 
stantially as  above,  thus  making  the  total  cost  of  the  ordinary  granite- 
block  pavement  on  concrete  foundation  about  $3.75  per  square  yard. 

1122.  Removing,  Re-cutting,  and  Re-laying.  /  kiladelphia.  The 
following  data  on  the  cost  of  removing,  re-cutting,  and  re-laying 
granite-block  pavement  are  from  experience  in  Philadelphia,  f 

Removing  old  blocks  ..................................  $0  .035  per  sq.  j  d. 

Clipping  the  old  blocks  to  Hnch  joints  ...................       ,50  "  " 

Piling  and  inspecting  new  blocks  .........................  07  "  " 

Sand  cushion  ..........................................  08  "  " 

Laying  and  grouting  .......  .  ............................  22  "  " 

Gravel  for  filling  joints  .................................        .04  "  " 

Cost  of  grout  ..................................                     .09  "  " 


Total $1.035     "     "     " 

*By  courtesy  of  W.  L.  Wccden,  Field  Secretary  of  Granite  Block  Producers  Association. 
tW.  H.  Connell,  Chief  of  Bureau  of  Highways  (Streets),  Philadelphia,  in  Engineering  and 
Contracting,  Vol.  40  (1913),  p.  290. 


ART.    2]  CONSTRUCTION  593 

1123.  Schenedady.   Table  65,  page  594,  shows  the  details  of  the  cost 
of  re-cutting  and  re-laying  2,578  square  yards  of  granite  blocks  in 
Schenectady,N.  Y.*   The  old  blocks  were  12  by  8  by  4  inches;  and  the 
new  ones  6  by  4  by  4  inches,  and  were  laid  on  a  new  4-inch  green 
concrete  base  with  a  thin  bedding  course  of  1  :  3  dry  cement  mortar. 
The  joints  were  filled  with  a  1  :  2  portland  cement  grout.     The  day 
was  8  hours.     The  work  was  done  without  interrupting  travel. 

1124.  Durax.     The  following  data  are  from  experience  in  Louis- 
ville, Ky.,  in  laying  a  small  area  of  cubical  granite  blocks,  f     The 
blocks  were  3|  to  4  inches  on  a  side,  were  made  in  North  Caro- 
lina, cost  $9.30  per  ton  f.o.b.  Louisville,  and  were  guaranteed  to  lay 
7  square  yards  per  ton,  and    consequently  the    guaranteed    price 
was  $1.33  per  square  yard,  which  was  67  cents  per  square  yard  less 
than  standard-size  blocks  from  the  same  quarry  would  have  cost. 
A  man  laid  2.2  square  yards  per  hour,  whereas  of  standard  blocks 
he  would  have  laid  3.3  to  4  square  yards  per  hour. 

1125.  Medina  Block.     Buffalo.     For  somewhat  obvious  reasons, 
the  prices  in  1917  were  quite  erratic,  and  hence  it  is  not  wise  to  cite 
them.   Table  66,  page  595,  shows  the  representative  cost  of  a  Medina- 
sandstone  block  pavement  in  Buffalo,  N.  Y.,  in  1916.J 

1126.  Cleveland.     Table  67,  page  596,  shows  the  representative 
cost  of  Medina-sandstone  pavements  in  Cleveland,  Ohio,  under  a 
5-year  guarantee,  in  1916.§ 

1127.  Rochester.     Table  68,  page  597,  shows  the  cost  of  Medina- 
sandstone  pavements  at  Rochester,  N.  Y.,  in  1917. 

1128.  Cost   of   Grouting. 1 1     Lawrence.     The   cost   at   Lawrence, 
Mass.,  of  grouting  standard  granite  blocks  was  as  follows:  Cement 
cost  $1.08  per  barrel,  pea  gravel  $2.30  per  cubic  yard,  sand  $1.00 
per  cubic  yard.     To  hold  the  blocks  in  place  while  being  rammed,  the 
joints  were  filled  to  a  depth  of  1  inch  with  pea  gravel.     The  grout 
was  mixed  1  :  1  in  iron  boxes  (§  997),  and  scooped  onto  the  pavement 
and  broomed.     With  wages  at  $2.25,  the  cost  of  applying  the  grout 
was  6.4  cents  per  square  yard.     The  total  cost  of  the  grout  in  place 
was  26f  cents  per  square  yard. 

1129.  Lowell.     At  Lowell,  Mass.,  grouting  granite  blocks  on  a 

*Chas.  A.  Mullen,  City  Engineer,  in  Municipal  Engineering,  Vol.  46  (1914),  p.  431. 

t  D.  R.  Lyman,  Chief  Engineer  of  Department  of  Engineering,  in  Engineering  News,  Vol.  72 
(1914),  p.  948. 

t  Frank  L.  Bapst,  President  German  Rock  Asphalt  Co.,  which  company  lays  much  stone- 
block  paving  in  Buffalo. 

§  Robert  Hoffman,  Commissioner  and  Chief  Engineer,  Department  of  Public  Works,  Cleve- 
land. O. 

||  Engineering  and  Contracting,  Vol.  44  (1915),  p.  350-51. 


594  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

TABLE  65 
COST  OF  RE-CUTTING  AND  RE-LAYING  GRANITE-BLOCK  PAVEMENT 

Schenectady,  N.  Y. 

Taking  up  old  pavement  and  preparing  subgrade:  Sq.  Yd. 

Labor  removing  asphalt  surface  and  concrete  base,  at  $2.25  per  day.  $0.0445 
Team  "  "  "       $5. 00  per  day.        .0156 

Labor  removing  old  granite  blocks  at  $2.25  per  day 0305 

"      regulating  and  preparing  subgrade  at  $2.25  per  day 0417 

Team  hauling  materials  from  subgrade  at  $5.00  per  day 0241 

Total $0. 1564 

Concrete  Foundation — 1  :  3  :  6.    4  inches  thick: 

Labor  mixing  and  placing,  at  $2.25  per  day $0 . 1192 

Cement,  delivered  ($1.24  per  bbl.  f.o.b.  cars) 1018 

Sand,  "       ($0.25  per  ton  f.o.b.  bank) 0275 

Stone,  "       f-inch  at  $1.30  per  ton  f.o.b.  track  > 

H-inch  at  $1.75  per  ton  on  job         f  - 

Total $0.3320 

Bedding  Course — 1  :  3  cement  mortar: 

Labor,  mixing  and  placing,  at  $2.25  per  day $0.0738 

Cement,  delivered  ($1.24  per  bbl.  f.o.b.  cars) 0868 

Sand,  "          (25  cents  per  ton  f.o.b.  bank) 0242 

Total $0.1848 

Re-cutting  and  re-laying  granite  blocks: 

Labor  breaking  and  dressing,  at  $5.00  per  day $0 . 7385 

"      sharpening  and  making  tools 0501 

Materials  for  sharpening  and  dressing  tools 0058 

Horse  and  wagon,  moving  blocks  at  $4.00  per  day 0059 

Labor  transporting  blocks  to  pavers  at  $2.25  per  day 0396 

"     setting  blocks,  at  $5.00  per  day 1796 

"     ramming  blocks,  at  $2.25  per  day 0065 

Total $1.0260 

Grout  Filler — 1  :  2  portland  cement- 
Labor  mixing  and  placing,  at  $2.25 $0.0466 

Cement,  delivered  ($1.24  per  bbl.  f.o.b.  cars) 0707 

Sand,  "       (25  cts.  per  ton  f.o.b.  bank) 0106 

Total $0. 1279 

Over-head  charges: 

Foreman  at  $4.00,  assistant  foreman  at  $3.50,  etc $0.0819 

Watchman  at  $2.25 0552 


Total $0.1371 

Extras: 

Repairs  to  curbs,  sidewalks,  sewers,  etc $0 . 0430 

Total  cost  of  removing,  re-cutting  and  re-laying $2 . 0072 


ART.    2]  CONSTRUCTION  595 

TABLE  66 
COST  OF  MEDINA-BLOCK  PAVEMENT  IN  BUFFALO  IN  1916. 


CONCRETE  FOUNDATION,  1:8: 
cement  at  $2.00  per  bbl.,  f.o.b.  Buffalo  .............................  $0.37 

sand  |  32 

crushed  stone  f  ' 

mixing  and  laying,  at  37£  cents  per  hour  ...........................       1.  15 

Total  for  concrete  base  ........................................  $0  .  84 

SAND  CUSHION:  labor  and  material  .................................  $0  .  15 

WEARING  COAT: 
blocks  f.o.b.  quarry  ..........  ....................................  $1  .  60 

freight  Medina  to  Buffalo  .........................................  18 

unloading  and  hauling,  at  75  cents  per  hour  for  team  and  driver.  ......       .20 

labor  laying  blocks,  at  60  cents  per  hour  ............................  35 

Total  for  wearing  coat  .........................................  $2  .  33 

FILLING  JOINTS: 

cement  at  $2.00  per  bbl.,  f.o.b.  Buffalo  .............................   $0  .  08 

labor  applying  grout  ..............................................  25 


Total  for  filling  joints $0 . 33 

MISCELLANEOUS: 

overhead  expenses $0 . 10 

indemnity  insurance  at  3.17%  of  pay  roll 025 

discount  on  City  tune  warrants,  3% 11 

maintenance  during  10-year  guarantee  period 10 

paid  city  for  water 02 

Total  miscellaneous $0 . 355 


Total  cost  of  pavement  exclusive  of  excavation $4 . 005 

2-inch  sand  cushion  with  1  :  1  grout  required  0.295  bag  of  cement  per 
square  yard ;  and  the  average  cost  was  24 \  cents  per  square  yard. 

1130.  Worcester.     At  Worcester,  Mass.,   a   1  :  1   grout  required 
0.36  cubic  foot  of  cement  per  square  yard;  and  the  total  cost  of 
grouting  was  24  cents  per  square  yard. 

1131.  Albany.     At  Albany,  N.  Y.,  the  cost  of  grouting  standard 
granite  blocks  having  side  joints  not  exceeding  \  inch,  using  1  :  1 
grout  mixed  by  machine,  cost  13.9  cents  per  square  yard.     The  cost 
of  mixing  by  machine  was   1.5  cents  per  square  yard,  and  by  hand 
5.25  cents  per  square  yard.     The  cement  required  was  0.4  bag  per 
square  yard. 

1132.  Philadelphia.     At  Philadelphia,   Pa.,  when  each  standard 
block  is  "  struck  in  "  at  the  base  to  secure  a  close  joint,  and  when 
2  inches  of  pea  gravel  are  deposited  in  the  joints  before  ramming,  a 


596  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

TABLE  67 

COST  OF  MEDINA-BLOCK  PAVEMENT  IN  CLEVELAND  IN  1916. 

COST 
ITEMS.  per  Sq.  Yd. 

CONCRETE  BASE,  6  inches  of  1  :  3  :  6: 
materials  and  labor* $0 . 98 

SLAG  CUSHION,  2  inches: 

material  and  labor  spreading 08 

WEARING  COAT,  6  to  6|  inches  thick: 

stone  blocks,  f  .o.b.  quarry 1 . 65 

freight  Medina  to  Cleveland 54 

transporting  from  car  to  street,  at  90  cents  per  hour  for  team 10 

laying .45 


Total  for  wearing  coat $2 . 74 

TAR  FILLER  :  material  and  labor 35 

FOREMAN:  supervision .02 


Net  cost  to  contractor $4.17 

OVERHEAD:  administration,  depreciation,  interest,  profits,  etc .78 

Price  bid,  exclusive  of  excavation $4 . 95 

Granite  blocks  cost  $2.50  per  square  yard,  f.o.b.  cars  Cleveland,  which  is 
31  cents  per  square  yard  more  than  Medina  blocks. 

1  :  1J  grout  required  0.27  bag  of  cement  per  square  yard.  The  total 
cost  of  grouting,  using  mixing  boxes  (§  997)  and  including  contractor's 
profits,  was  from  17  to  20  cents  per  square  yard. 

1133.  Cost  of  Tar-sand  Filler.  At  Englewood,  N.  J.,  the  total 
cost  of  applying  a  1  :  1  tar-sand  hand-mixed  filler  on  standard 
blocks  having  ^-inch  joints  in  which  had  previously  been  deposited 
"  a  small  amount  of  grit,"  was  as  follows:! 

Pitch, — 1.7  gallons  per  square  yard $0 . 143  per  sq.  yd. 

Labor  handling  the  pitch 02      "     "     " 

Grit Oil     "     "    " 

Sand *. 019    "    "    " 

Labor 038     "     "     " 

Total ; $0.231     "     "     " 

"  Investigation  showed  that  in  every  case  the  filler  penetrated 
to  the  bottom  of  the  block.  It  was  also  found  that  the  volume  of  tar 
required  to  fill  the  joints  was  less  than  that  smeared  over  the  surface 
of  the  pavement  in  the  two  or  three  successive  pourings." 

*Sand  $1.15  per  ton  f.o.b.  cars  Cleveland;  broken  stone  $1.35  per  ton  f.o.b.  cars.     Loading 
and  hauling  sand  and  stone,  30  cents  per  ton  per  mile.     Labor  from  30  to  43  cents  per  hour, 
t  Engineering  and  Contracting,  Vol.  47  (1917),  p.  134. 


ART.  3]  MAINTENANCE  597 

TABLE  68 
COST  OF  MEDINA-BLOCK  PAVEMENTS  IN  ROCHESTER  IN  1917  * 

COST 
ITEMS.  per  Sq.  Yd. 

CONCRETE,  6  inches: 

materials  and  labor $1 . 00 

SAND  CUSHION,  2  inches: 

sand  at  $1 . 50  per  cu.  yd.  in  place 09 

WEARING  COURSE,  6  inches  deep: 

blocks  f .o.b.  quarry 1 . 50 

freight  to  Rochester 07 

loading  and  unloading 03 

hauling  1  mile 05 

distributing  and  sorting 03 

laying 15 

FILLING  JOINTS: 

0.025  cu.  yd.  sand  at  $1.50  ...- 04 

1|  gallons  of  tar  at  10  cents 23 

labor  applying  filler 09 

SUPERINTENDENCE  : 

foreman  at  40  cents  per  hour  for  30  square  yards 015 

Total  cost  to  contractor  exclusive  of  administration,  tools,  etc $3.29 

1134.  Contract  Price.     Table  69,  page  598,  shows  the  contract 
price  of  stone-block  pavements  in  various  cities,  and  incidentally 
gives  considerable  detailed  information  as  to  the  practice  in  the 
several  cities. 

ART.  3.    MAINTENANCE 

1135.  There  are  almost  no  data  concerning  either  the  method  or 
the  cost  of  maintaining  granite-block  pavements.     Formerly,  when 
the  wide-joint  and  roughly  dressed  granite-block  pavement  was  the 
only  form,  little  or  no  attention  was  given  to  methods  or  cost  of 
pavement  maintenance,  and  this  was  particularly  true  of  granite- 
block  pavements.    Since  the  introduction  of  the  better  dressed  narrow- 
joint  granite-blocks,  there  has  not  been  time  in  which  to  develop  a 
system  of  maintenance  nor  to  determine  the  cost,  particularly  as  a 
modern  granite-block  pavement  is  very  durable  and  needs  no  repairs 
for  the  first  few  years. 

*  By  courtesy  of  Walter  L.  Weeden,  Secretary  Granite  Paving  Block  Manufacturers'  Asso- 
ciation. 


598 


STONE-BLOCK    PAVEMENTS 


CHAP.    XVIII 


TABLE  69 

CONTRACT  PRICE  OF  STONE-BLOCK  PAVEMENTS  IN  VARIOUS  CITIES  * 
Laid  in  1912 


LOCALITY. 

Amount 
Laid  in 
1912, 
sq.  yd. 

CONCRETE  BASE. 

Kind 
of 
Filler. 

Guar- 
antee, 
years. 

Total 
Thick- 
ness, 
inches. 

Aver- 
age 
Price  i 
sq.  yd. 

State. 

City. 

Thick- 
ness, 
inches. 

Propor- 
tions. 

California.  .  .  . 

Oakland  '. 

11  445 

6 

grout 

0 

S4.002 

San  Francisco 

21  565 

gravel 

7-9 

3.502 

Connecticut 

Ansonia. 

1  500 

6 

1     3:6 

2.  10 

Georgia  

LaGrange.  .  . 

5000 

tar 

0 

10 

1.75 

Louisiana  

New  Orleans  . 

4  119 

6 

1     3:6 

grout 

3 

14 

3.95 

Maine  

Portland.  .  .  . 

1  004 

grout 

2.30 

Massachusetts 

Lawrence.  .  .  . 

30704 

6 

1     3:6 

grout 

0 

7i 

2.72 

Leominster  .  . 

2  268 

4 

1     3:5 

pitch 

4 

2.47 

Lowell  

22418 

6 

1     4:11 

3 

'  'i2'  ' 

3.15 

New  Bedford. 

12310 

4 

Hassam 

grout 

10 

1.902 

Westfield.  .  .  . 

6  197 

4 

1     3.:  6 

"  l"  ' 

8£ 

3.30 

Minnesota.  .  .  . 

Duluth  

23  146^ 

5 

1     3:6 

grout 

5 

13 

2.  68  2 

Minneapolis.  . 

79004 

grout 

2.75* 

Missouri  

Kansas  City. 

13  248 

6 

1     3:6 

grout 

5 

13 

3.10 

New  Jersey  .  . 

Newark  

75735 

5 

3.03 

New  York  .... 

Rochester.  .  . 

42700 

6 

1     3:6 

grout 

5 

13 

3.18 

Troy  

4  000 

6 

1      5 

grout 

5 

3.35 

Oregon  

Portland*.  .  .  . 

3914 

6 

1     3:6 

grout 

12 

3.45 

Pennsylvania. 

Scranton.  .  .  . 
Wilkesbarre.  . 

1  688 
2  156 

"e 

i     2:  5 

sand    • 
grout 

5. 

9 
12 

2.10 
2.70 

Washington.  .  . 

Seattle  

17436 

6 

1     3:6 

grout 

12 

3.50 

Tacoma  

11  002 

6 

1     3:6 

grout 

in 

2.50 

Wisconsin  .... 

Superior  

10078 

5 

1     3:5 

5 

13 

2.52 

Canada  

Montreal.  .  .  . 

48000 

6 

1     3:6 

5 

12 

3.65 

Ottawa  

10041 

6 

1     3:6 

grout 

5 

13 

3.78 

Saskatoon  .  .  . 

16  662 

5 

1     1\ 

grout 

5 

11 

5.10 

Vancouver.  .  . 

55359 

6 

1     2i  :  P 

grout 

5 

13 

4.50 

1  Including  grading  and  concrete  base.     2  Not  including  grading. 
3  Part  grout  and  part  pitch  and  tar.  4  Kettle  River  Sandstone. 

1136.  REPAIRS    REQUIRED.     The    repairs    ordinarily    required 
are  re-laying  small  areas,  re-filling  the  joints,  repairing  spalling  joints, 
raising  low  blocks,  repairing  where  the  foundation  has  settled,  and  re- 
laying over  trenches  or  other  openings. 

1137.  Re-laying.     At  present   the  most  common  work  required 
in  connection  with  the  maintenance  of  stone-block  pavements  is 
taking  up  the  old  blocks,  re-cutting,  and  re-laying  them,  which  is  a  re- 
construction rather  than  repair  or  maintenance;    and  usually  the 
new  pavement  is  of  entirely  a  different  type  than  the  old.     This 
subject  has  already  been  considered  in  §  1101. 

*  Engineering  and  Contracting,  Vol.  39  (1913),  p.  378-79. 


ART.   3]  MAINTENANCE  599 

1138.  Re-filling  Joints.     If  a  bituminous  filler  is  used,  it  may 
run  out  of  the  top  of  the  joints,  particularly  near  the  crown,  or  be 
picked  out  by  the  traffic.     When  this  occurs,  the  joints  should  be 
poured  again.     With  dense  traffic  this  may  be  required  every  two  or 
three  years.     With  a  good  portland-cement  filler,  the  joints  are  not 
likely  to  need  re-filling;  but  if  it  develops  that  in  spots  the  filler  was 
poor,  the  joints  should  be  digged  out  and  again  filled. 

1139.  Spalling   Joints.     If  pea  gravel  or  sand  is  put  into  the 
joints,  there  is  danger  that  the  grout  filler  will  fill  only  the  top  of  the 
joint;  and  hence  that  the  pressure  due  to  expansion  of  the  pavement, 
being  concentrated  near  the  top  of  the  joint,  will  cause  the  edge  of  the 
block  to  spall.     Usually  there  will  be  no  spalling,  if  the  grout  pene- 
trates 3  inches;  but  if  the  penetration  is  1  inch  or  less,  the  blocks  are 
likely  to  spall  —  generally  the    first  summer  after  the  pavement 
is  completed,  but  sometimes  not  for  two  or  three  years,  depending 
upon  the  time  required  for  travel  to  fill  the  joints  opened  by  the 
contraction  of  the  pavement. 

The  spalling  at  joints  can  be  prevented  by  clearing  the  joints  of 
sand  or  gravel  to  a  depth  of  3  inches  before  applying  the  grout  filler. 
If  spalling  occurs  in  the  finished  pavement,  the  blocks  should  be 
taken  up,  the  joints  cleaned,  the  blocks  re-laid,  and  the  joints  re- 
filled. 

In  effect  this  is  substantially  the  same  defect  as  that  of  brick 
pavements  described  in  §  1059. 

1140.  Raising    Low    Blocks.     If    a    block    was    not    properly 
bedded  or  sufficiently  rammed,   it  may  become  depressed  under 
travel,  particularly  with  a  thick  sand  bedding-course;   and  if  so,  it 
should  be  taken  up,  and  re-laid.     If  a  grout  filler  is  used,  it  is  prac- 
tically impossible  to  remove  a  block  without  destroying  it;  and  this 
is  reason  for  special  care  in  bedding  and  tamping  blocks  that  are  to 
be  grouted. 

1141.  Settlement  of  Foundation.     If  there  is  a   depression  on 
the  surface,  it  may  be  due  to  a  settlement  of  the  foundation  and 
can  be  corrected  substantially  as  described  for  brick  pavements, 
see  §  1056. 

1142.  Settlement  of  Trench.     The   effect  and   the   remedy  of 
the  sinking  of  a   stone-block  pavement  is  the  same   as  that  of  a 
brick  pavement,  see  §  1057  and  1061. 

1143.  COST  OF  REPAIRS.     There  are  almost  no  data  on  the  cost 
of  repairs  for  stone-block  pavements,  probably  partly  because  of  their 
long  life  and  partly  because  only  few  repairs  are  ever  made,  the  pave- 


600  STONE-BLOCK   PAVEMENTS  [CHAP.    XVIII 

ments  usually  being  allowed  to  continue  in  a  bad  state  of  repairs 
The  only  data  on  record  seem  to  be  cost  of  repairs  in  Buffalo,  N.  Y 
(see  §  868),  where  most  of  the  stone-block   pavements  are  Medina 
sandstone  (§  1086).     In  1916  the  annual  cost  of  repairs  on  218,090 
square  yards  was  3.66  cents  per  square  yard.* 

*Report  of  Dept.  of  Pub.  Wks.— Bureau  of  Engineering,  1915-16,  p.  70. 


CHAPTER  XIX 
WOOD-BLOCK  PAVEMENTS 

1144.  KINDS  OF  PAVEMENTS.     There  are  two  forms  of  wood- 
block pavements,  viz. :  the  round-block  and  the  rectangular-block. 

1145.  Round-block  Pavement.     Fig.  222  shows  the  usual  form 


Fro.  222.  —  ROUND  WOOD-BLOCK  PAVEMENT. 


of  round  wood-block  pavement.  This  form  is  often  called  a  cedar- 
block  pavement,  since  the  blocks  are  usually  sections  of  cedar 
poles  or  trees.  These  blocks  are  generally  placed  on  a  foundation 
of  planks  nailed  to  scantling  which  are  placed  on,  or  rather  in, 
sand. 

Until  about  the  close  of  the  last  century,  untreated  round  wood- 
block pavements  were  laid  in  considerable  quantities  in  localities 
where  lumber  was  cheap;  but  now  they  are  seldom  laid,  owing  chiefly 
to  the  rapid  increase  in  price  of  lumber  and  partly  to  the  introduction 
of  other  cheap  forms  of  pavements.  Such  pavements  are  laid  now 
only  where  first-cost  is  the  controlling  factor,  as  for  example,  in  new 

601 


602 


WOOD-BLOCK    PAVEMENTS 


[CHAP,  xix 


city  additions  in  states  where  the  first  pavement  is  selected  and  paid 
for  by  the  owners  of  the  abutting  property,  and  the  cost  of  mainte- 
nance and  renewal  is  paid  from  the  general  property  tax.  In  view 
of  these  facts,  the  round  wood-block  pavement  will  not  be  further 
considered  in  this  volume. 

1146.  Rectangular-block  Pavement.     Fig.  223  shows  the  usuel 


FIG.  223.  —  RECTANGULAR  WOOD-BLOCK  PAVEMENTS. 


form  of  rectangular  wood-block  pavement.  The  rectangular  block 
is  usually  treated  with  a  preservative  when  used  for  paving  pur- 
poses. This  type  of  pavement  will  be  considered  in  detail  in  this 
chapter. 

1147.  HISTORICAL.  Wood  appears  to  have  been  employed  as  a 
paving  material  first  in  Russia,  where  though  rudely  fashioned  it 
has  been  used  for  some  hundreds  of  years.  Wood  pavements  were 
first  laid  in  New  York  City  in  1835-36,  and  in  London  in  1839. 

The  first  wood  used  for  pavements  was  untreated  (§1145).  The 
first  pavement  in  this  country  made  of  treated  blocks  was  laid  in 
Galveston,  Texas,  in  1874;  and  remained  in  service  for  29  years. 
A  treated  wood-block  pavement  was  laid  in  St.  Louis,  Missouri,  in 
1882-85;  and  remained  in  use  until  worn  out  by  the  traffic.  The 
first  extensive  use  of  treated  blocks  for  pavements  was  in  Indianap- 
olis in  1896;  and  after  21  years  these  blocks  are  still  in  use  and  are 
said  to  be  in  a  good  state  of  preservation.  Although  a  number  of 
methods  of  preserving  timber  piles,  railroad  ties,  etc.,  had  been  used 
in  this  country  for  several  years,  it  was  not  until  about  1900  that 


ART.    1]  MATERIALS   AND    TREATMENT  603 

there  was  any  considerable  use  here  of  a  preservative  for  wood 
paving-blocks. 

1148.  In  1909  3J  per  cent  of  the  pavements  in  the  United  States 
were  wood-block  (see  table  on  page  320);  but  in  1900  such  pave- 
ments constituted  10  per  cent  of  the  total  (see  a  table  similar  to  that 
on  page  320,  in  the  former  edition  of  this  volume).  This  seems  to 
prove  that  the  percentage  of  wood-block  pavements  is  rapidly 
decreasing.  However,  the  decrease  is  wholly  in  the  untreated-block. 
The  percentage  of  treated-block  pavements  is  rapidly  increasing, 
although  it  is  still  less  than  1  per  cent  of  the  pavements  included 
in  the  table  on  page  320. 

ART.  1.    MATERIALS  AND  TREATMENT 

1150.  THE  TIMBER.     Both  hard  and  soft  woods  have  been  em- 
ployed  for  making   paving   blocks.     The   hard   woods   were   used 
untreated,  and  the  soft  varieties  were  treated.     At  present  only 
treated  timber  is  used;   and  of  course  the  softer  and  cheaper  woods 
are  preferred,  since  they  only  can  be  impregnated  with  the  preserva- 
tive.    Exceptions  to  the  above  statement  are  two  Australian  hard 
woods,  jarrah  and  karri,  that  are  much  used  in  London  without 
treatment. 

1151.  Jarrah  and  Karri.     Jarrah  is  short  grained  and  free  split- 
ting, and  breaks  with  a  clean  fracture  and  burns  with  a  black  ash. 
In  color  it  looks  nearly  like  cherry.     When  seasoned,  it  has  a  specific 
gravity  of  1.01  and  absorbs  about  10  per  cent  of  water  when  im- 
mersed 48  hours.*     Its  transverse  and  crushing  strength  is  about 
the  same  as  that  of  English  oak  and  Indian  teak. 

Karri  is  interlocked  in  the  grain  and  is  difficult  to  split ;  it  splinters 
in  breaking  and  burns  with  a  white  ash.  It  is  a  little  lighter  colored 
than  cherry.  When  seasoned,  it  has  a  specific  gravity  of  1.12,  and 
absorbs  about  7  per  cent  of  water  when  immersed  48  hours.  Its 
transverse  strength  is  a  little  greater  than  that  of  English  oak  or 
Indian  teak,  and  its  crushing  strength  is  considerably  greater. 

For  street  paving,  there  is  little  difference  between  jarrah  and 
karri,  although  for  exceptionally  heavy  traffic  karri  shows  slightly 
less  wear.  Karri  shrinks  less  than  jarrah.  Both  timbers  are  very 
plentiful  in  Western  Australia,  the  trees  growing  with  large,  long 
straight  bodies  without  limbs.  Jarrah  and  karri  are  preferred  in 
some  vestries  of  London  to  any  other  form  of  wood  paving-blocks. 

*  Most  •aoft  -woods  will  absorb  20  to  25  per  cent. 


604  WOOD-BLOCK   PAVEMENTS  [CHAP.    XIX 

1152.  Wood  for  Treated  Blocks.     The  cost  of  lumber  in  the 
United  States  in  the  last  few  years,  even  before  the  Great  European 
War,  has  been  so  great  that  the  first  cost  of  the  blocks  is  an  important 
consideration.     Southern  long-leaf  yellow  pine  was  almost  exclusively 
used  in  early  treated  wood-block  pavements;    and  is  most  largely 
used  at  the  present  time.     It  makes  the  most  durable  blocks  of  any 
timber  yet  tried.     However,  it  is  not  entirely  satisfactory,  for  the 
hardness  which  gives  it  durability  against  wear  also  makes  it  slippery; 
and  further,  it  is  liable  to  split  when  the  blocks  are  taken  up  to 
repair  underground  work.     Nevertheless,  long-leaf  yellow  pine  is  the 
most  satisfactory  timber  for  treated  wood-block  pavements.     How- 
ever, as  this  timber  is  produced  in  only  one  section  of  the  country 
the  transportation  charges  are  likely  to  be  high,  and  besides  the 
supply  is  nearly  exhausted;  and  therefore  where  traffic  conditions  will 
permit,  it  is  desirable  to  use  a  cheaper  material. 

Some  years  ago  the  U.  S.  Forestry  Service,  to  obtain  data  as  to 
the  relative  value  of  different  species  of  wood  for  paving  purposes, 
laid  a  great  variety  of  woods  under  the  same  conditions  on  a  street  in 
Baltimore,  Maryland,  and  conducted  a  similar  experiment  in  Minne- 
apolis, Minnesota.  The  conclusion  reached  as  a  result  of  the  exami- 
nation of  the  Baltimore  experiment  after  four  years  of  service,  was 
that  the  different  varieties  could  be  grouped  into  classes  in  the  order 
of  the  value  for  paving  purposes  as  follows:  (1)  Southern  long-leaf 
yellow  pine;  (2)  Norway  pine,  white  birch,  tamarack,  eastern  hem- 
lock; (3)  Western  larch;  and  (4)  Douglas  fir.  The  conclusion  from 
the  Minneapolis  experiment  after  eight  years  of  service  was  sub- 
stantially the  same. 

The  1917  specifications  of  the  American  Wood  Preservers'  Asso- 
ciation and  also  those  of  the  American  Society  of  Municipal  Improve- 
ments call  for  Southern  pine,  which  permits  the  use  of  either  long- 
leaf  or  short-leaf  Southern  pine. 

1153.  Specifications  for  Blocks.     Dimensions.     The  width  was 
formerly  4  inches;  but  in  recent  practice  it  is  sometimes  3  inches. 
The  length  varies  widely  so  as  to  make  available  planks  of  different 
widths.     The  length  specified  is  usually  5  or  6  inches  to  10  inches; 
and  often  the  average  width  is  specified  to  prevent  the  use  of  too 
many  short  or  too  many  long  blocks.     The  average  length  is  usually 
6  or  8  inches.     The  depth  must  be  enough  to  give  the  block  stability, — 
say,  3  inches.     This  depth  is  used  where  the  traffic  is  light;    but 
under  heavy  traffic  the  depth  is  4  inches,  and  in  extreme  cases  4J 
inches,  as  for  pavements  in  the  Borough  of  Manhattan,  New  York 


ART.    1]  MATERIALS   AND   TREATMENT  605 

City.     If  a   3-inch   block   is   used   its    length   should    not   exceed 
8  inches. 

While  it  is  usual  to  specify  that  all  the  blocks  for  one  job,  or  at 
least  in  any  one  city  block,  shall  be  of  the  same  width,  it  is  customary 
to  permit  a  variation  of  f  inch  in  the  width.  A  variation  of  Y&  inch 
in  depth  is  generally  permitted.  Sometimes,  to  prevent  a  block  from 
being  laid  on  its  side,  it  is  specified  that  there  shall  be  a  difference 
between  the  width  and  depth  of  at  least  J  inch. 

1154.  Quality  of  Blocks.     The  blocks  should  be  sawed  square  and 
true.     They  should  be  free  from  large,  unsound,  loose,  or  hollow 
knots;  and  should  not  contain  any  shakes,  checks,  or  other  defects. 
The  blocks  should  be  free  from  any  blue  tinge,  which  is  a  sign  of 
incipient  decay. 

Specifications  usually  state  the  minimum  number  of  annual  rings 
permitted — some  permitting  5  or  6  per  linear  inch,  and  others  not 
less  than  8  or  9.  The  amount  of  sap  wood  allowed  varies  from  10  to 
40  per  cent.  In  the  early  use  of  treated  wood  blocks,  specifications 
were  more  rigid  in  limiting  the  amount  of  sap  wood ;  but  recent  expe- 
rience seems  to  indicate  that  there  is  no  noticeable  difference  in  wear 
between  heart  and  sap  wood. 

1155.  CAUSES   OF  DECAY.     The  decay  of  wood  is  due  to  a  low 
form  of  plant  life  called  fungi.     Air,  heat,  and  moisture  are  necessary 
for  the  existence  of  the  fungous  growth;  and  without  any  one  of  these 
the  fungi  can  not  live.     Since  air  and  heat  are  present  in  all  climates,  it 
is  necessary  to  eliminate  moisture  to  preserve  the  timber  from  decay. 
Seasoning,  both  air  drying  and  kiln  drying,  is  a  method  of  removing 
part  of  the  moisture,  and  hence  is  a  method  of  preserving  timber 
against  decay;  but  any  seasoned  timber  will  re-absorb  moisture,  and 
hence  seasoning  is  only  an  imperfect  method  of  preservation.     A 
more  effective  method  is  to  inject  into  the  timber  some  substance 
that  will  change  the  organic  matter  in  the  wood  so  it  will  no  longer 
serve  as  food  for  the  fungi. 

1156.  THE   PRESERVATIVE.     The    preservative    performs   two 
functions,  viz.:   (1)  acts  as  an  antiseptic  to  prevent  decay;   and  (2) 
acts  as  a  waterproofing  material  to  keep  out  moisture. 

It  is  desirable  that  the  preservative  should  be  stable  and  remain 
in  the  block  as  long  as  possible;  since  where  the  travel  is  light,  the 
life  of  the  pavement  depends  upon  the  resistance  of  the  blocks  to 
decay.  However,  if  an  antiseptic  has  thoroughly  penetrated  the 
block,  it  will  ordinarily  be  preserved  against  decay,  even  though  a 
larger  proportion  of  the  preservative  evaporates  or  is  washed  out. 


606  WOOD-BLOCK   PAVEMENTS  [CHAP.    XIX 

But  if  the  antiseptic  evaporates  or  is  washed  out,  the  timber  becomes 
again  susceptible  to  changes  in  volume  with  changes  of  moisture 
content,  which  is  objectionable  in  paving  blocks. 

Any  material  that  renders  a  block  waterproof  is  in  itself  a  fair 
preservative,  even  though  it  is  not  an  antiseptic.  For  example, 
petroleum  is  a  fair  preservative  of  timber,  although  it  is  not  anti- 
septic. If  water  could  be  kept  entirely  out  of  the  block,  there 
would  be  little  or  no  decay;  but  this  is  practically  impossible,  since 
no  amount  of  preservative  will  make  wood  absolutely  Waterproof. 

1157.  Creosote  is  a  distillate  of  coal  tar,  and  is  usually  called 
creosote  oil,  and  sometimes  dead  oil  of  tar.     Creosote  oil  was  the  first 
material  used  for  preserving  paving  blocks;  and  it  is  still  the  essen- 
tial constituent  in  all  such  preservatives. 

Creosote  oil  was  not  entirely  satisfactory  as  a  preservative  for 
paving  blocks.  It  preserved  the  wood  and  prevented  decay;  but 
it  gradually  evaporated  and  washed  out,  and  permitted  a  change  in 
the  moisture  content  of  the  blocks,  which  caused  expansion  and  con- 
traction. To  remove  this  objection,  coal  tar  was  added  to  the 
creosote  to  increase  the  waterproofing  qualities  of  the  preservative. 
The  addition  of  tar  also  cheapens  the  preservative. 

Coal  tar  may  contain  something  like  40  per  cent  creosote  oil,  and 
hence  is  in  itself  a  fair  antiseptic;  but  on  the  other  hand;  it  is  harder 
to  force  the  viscous  tar  into  the  wood  than  the  more  fluid  creo- 
sote oil. 

There  is  a  considerable  difference  of  opinion  as  to  the  relative 
merits  of  the  different  creosote  preservatives.  Some  argue  that  the 
preservative  should  be  pure  creosote  oil,  having  a  specific  gravity  of 
1.03  to  1.07,. and  a  few  claim  that  it  should  have  a  higher  specific 
gravity  but  still  be  free  from  tar;  while  others  prefer  a  mixture  of 
creosoted  oil  and  coal  tar  having  a  specific  gravity  of  1.08  to  1.12. 
Most  cities  now  use  an  oil  having  a  specific  gravity  of  1.10  to  1.14 
and  containing  a  large  proportion  of  coal  tar,  which  is  usually  re- 
quired to  be  nearly  free  from  carbon.  The  presence  of  any  consid- 
erable amount  of  carbon  is  likely  to  plug  the  pores  of  the  wood  and 
prevent  the  introduction  of  the  preservative. 

Some  engineers  claim  that  water-gas  tar  (§  564)  gives  satisfactory 
results,  even  though  it  has  no  antiseptic  properties;  but  others  claim 
that  experience  with  this  material  has  been  too  limited  in  both  extent 
and  time  to  warrant  its  use  on  a  large  scale.  It  is  usually  cheaper 
than  a  mixture  of  creosote  oil  and  coal  tar. 

1158.  Two  proprietary  methods  of  treating  wood  paving-blocks 


ART.    1]  MATERIALS   AND   TREATMENT  607 

were  introduced  comparatively  early.  One,  called  the  kreodone 
process,  consisted  in  impregnating  block  under  pressure  with  a 
secret  proprietary  preservative.  The  other,  called  the  creo-resinate 
process,  consists  in  mixing  melted  resin  and  formaldehyde  with  the 
creosote,  the  resin  being  to  waterproof  the  wood,  and  the  formalde- 
hyde is  to  increase  the  antiseptic  effect  of  the  preservative.  This 
process  was  very  efficient,  but  was  discontinued  on  account  of  the 
increase  in  the  cost  of  resin. 

1159.  Specifications    for   Preservatives.     The    specifications    in 
§  1160,  §  1161,  and  §  1162,  were  prepared  by  the  American  Wood- 
Preservers'  Association,*  and  have  virtually  been  approved  by  prac- 
tically all  of  the  national  engineering  societies  interested  in  wood 
preservation.! 

1160.  Creosote  Oil.     Creosote  oil  was  formerly  used  for  railway 
ties,  structural  timber  and  wood  pavmg-blocks ;  but  lately,  owing  to 
its  scarcity  and  cost,  has  not  been  used  much,  particularly  for  paving 
blocks.     Paving  blocks  do  not  require  as  perfect  a  preservative  as 
ties  and  structural  timber,  since  usually  their  life  is  limited  by  their 
resistance  to  wear  rather  than  to  decay.     Creosote  oil  when  tested 
in  accordance  with  the  standard   methods  of  the   American  Wood 
Preservers'  Association,  should  comply  with  the  following  require- 
ments : 

"1.  The  oil  shall  be  a  distillate  of  coal-gas  tar  or  coke-oven  tar. 

"2.  It  shall  not  contain  more  than  3  per  cent  of  water. 

"3.  It  shall  not  contain  more  than  0.5  per  cent  of  matter  insoluble  in  benzol. 

"4.  The  specific  gravity  of  the  oil  at  38°  C.  compared  with  water  at  15.5°  C. 
shall  be  not  less  than  1.03. 

"5  The  distillate,  based  on  water-free  oil,  shall  be  within  the  following  limits: 
up  to  210°  C.  not  more  than  5  per  cent;  and  up  to  235°  C.  not  more  than  25  per 
cent. 

"6.  The  specific  'gravity  of  the  fraction  between  235°  and  315°  C.  shall 
not  be  less  than  1.03  at  38°  C.,  compared  with  water  at  15.5°  C.  The  specific 
gravity  of  the  fraction  between  315°  and  355°  C.  shall  be  not  less  than  1.10  at 
38°  C.,  compared  with  water  at  15.5°  C. 

"7.  The  residue  above  355°  C.,  if  it  exceeds  5  per  cent,  shall  have  a  float-test 
of  not  more  than  50  seconds  at  70°  C. 

"8.  The  oil  shall  yield  not  more  than  2  per  cent  coke  residue." 

1161.  Coal-tar  Distillate  Oil.     Coal-tar  distillate  oil  for  paving 
blocks,  when  tested  in  accordance  with  the  standard  methods  of  the 

*Procccdings,  1917,  p.  307-9. 
•\Ibid.,  p.  41  and  325. 


WOOD-BLOCK   PAVEMENTS  [CHAP.    XIX 


American  Wood  Preservers'  Association,*  shall  comply  with  the  fol- 
lowing requirements : 

"1.  The  oil  shall  be  a  distillate  of  coal-gas  tar  or  coke-oven  tar  [§  564], 

"2.  It  shall  not  contain  more  than  3  per  cent  of  water. 

"3.  It  shall  not  contain  more  than  0.5  per  cent  of  matter  insoluble  in  benzol. 

"4.  The  specific  gravity  of  the  oil  at  38°  C.,  compared  with  water  at  15.5°  C., 
shall  be  not  less  than  1.06. 

"5.  The  distillate,  based  on  water-free  oil,  shall  be  within  the  following 
limits:  up  to  210°  C.  not  more  than  5  per  cent;  and  up  to  235°  C.  not  more  than 
15  per  cent. 

"6.  The  specific  gravity  of  the  fraction  between  235°  and  315°  C.  shall  be  not 
less  than  1.03  at  38°  C.,  compared  with  water  at  15.5°  C.  The  specific  gravity 
of  the  fraction  between  315°  and  355°  C.  shall  be  not  less  than  1.10  at  38°  C., 
compared  with  water  at  15.5°  C. 

"7.  The  residue  above  355°  C.,  if  it  exceeds  10  per  cent,  shall  have  a  float-test 
of  not  more  than  50  seconds  at  70°  C. 

"8.  The  oil  shall  yield  not  more  than  2  per  cent  coke  residue." 

1162.  Coal-tar  Paving  Oil.     Coal-tar  paving  oil  for  paving  blocks, 
when  tested  in  accordance  with  the  standard  methods  of  the  American 
Wood  Preservers'   Association,*  shall  comply  with   the   following 
requirements : 

"1.  It  shall  be  a  coal-tar  product  of  which  at  least  65  per  cent  shall  be  a  dis- 
tillate of  coal-gas  tar  or  coke-oven  tar,  and  the  remainder  shall  be  refined  or  fil- 
tered coal-gas  tar  or  coke-oven  tar  [§  564]. 

"2.  It  shall  not  contain  more  than  3  per  cent  of  water. 

"3.  It  shall  not  contain  more  than  3  per  cent  of  matter  insoluble  in  benzol. 

"4.  The  specific  gravity  of  the  oil  at  38°  C.,  compared  with  water  at  15.5°  C., 
shall  be  not  less  than  1.07  or  more  than  1.12. 

"5.  The  distillate,  based  on  water-free  oil,  shall  be  within  the  following  limits: 
up  to  210°  C.,  not  more  than  5  per  cent;  and  up  to  235°  C.  not  more  than 
25  per  cent. 

"6.  The  specific  gravity  of  the  fraction  between  235°  and  315°  C.  shall  be 
not  less  than  1.03  at  38°  C.,  compared  with  water  at  15.5°  C.  The  specific 
gravity  of  the  fraction  between  315°  and  355°  C.  shall  be  not  less  than  1.10 
at  38°  C.,  compared  with  water  at  15.5°  C. 

"7.  The  residue  above  355°  C.,  if  it  exceeds  35  per  cent,  shall  have  a  float-test 
of  not  more  than  80  seconds  at  70°  C. 

"8.  The  oil  shall  yield  not  more  than  10  per  cent  coke  residue." 

1163.  Water-Gas  Tar.     Refined  water-gas  tar  shall  conform  to 
the  following  requirements:! 

"1.  The  specific  gravity  shall  be  not  less  than  1.12  nor  more  than  1.14  at 
38°  C.,  referred  to  water  at  the  same  temperature. 

*  Proceedings,  1917,  p.  309-21. 

t  Specifications  for  Creosoted  Wood-block  Paving,  American  Society  of  Municipal  Improve-- 
ments,  1916,  p.  5-6. 


ART.    1]  MATERIALS   AND   TREATMENT  609 

"2.  Not  more  than  2.0  per  cent  shall  be  insoluble  by  hot  extraction  with 
benzol  and  chloroform. 

"3.  On  distillation  when  made  as  hereinafter  described,*  the  distillate, 
based  on  water-free  oil,  shall  be  within  the  following  limits:  up  to  210°  C., 
not  more  than  5.0  per  cent;  up  to  235°  C.,  not  more  than  15.0  per  cent;  up  to 
315°  C.,  not  more  than  40.0  per  cent;  and  up  to  355°  C.  not  less  than  25.0  per 
cent. 

"4.  The  specific  gravity  of  the  total  distillate  below  355°  C.  shall  not  be  less 
than  1.0  at  38°  C.,  referred  to  water  at  the  same  temperature. 

"5.  The  oil  shall  not  contain  more  than  2.0  per  cent  water;  and  due  allow- 
ance shall  be  made  for  all  water  and  insoluble  foreign  matter  it  may  contain  by 
injecting  a  corresponding  additional  quantity  into  the  blocks." 

1164.  TREATMENT    OF   BLOCKS.     There  are  two   methods    of 
treating  the  blocks  with  the  preservative,  viz.:    (1)  the  open-tank 
process,  and  (2)  the  pressure  process. 

1165.  Open-tank  Process.     In  this  process  the  blocks  are  im- 
mersed in  the  preservative  from  a  few  minutes  to  an  hour,  depending 
upon  the  kind  of  wood  and  the  degree  of  penetration  desired.     This 
method  is  largely  used  in  France.     In  this  country  early  in  the  his- 
tory of  treated  paving  blocks,  it  was  much  used;  but  it  is  not  now 
used. 

1166.  Pressure   Process.     The  standard   specifications  for  the 
treatment  of  the  timbers  mentioned  in  §  1152,  except  Douglas  fir, 
are  substantially  as  follows: 

"The  blocks  are  placed  in  a  closed  cylinder,  and  subjected  to  steam  at  a  tem- 
perature of  220°  to  240°  F.  for  not  less  than  2  hours  nor  more  than  4  hours. 
The  steam  and  moisture  are  blown  out  of  the  cylinder,  and  then  the  blocks  are 
subjected  to  a  vacuum  of  not  less  than  22  inches  of  mercury  for  at  least  1  hour. 
While  the  vacuum  is  still  on,  the  preservative,  heated  to  a  temperature  of  180° 
to  220°  F.,  is  run  in  until  the  cylinder  is  completely  filled,  care  being  taken  that 
no  air  is  admitted.  Pressure  is  then  gradually  applied  at  a  rate  not  to  exceed 
50  Ib.  per  square  inch  per  hour,  and  is  maintained  at  100  to  150  Ib.  per  square  inch 
until  the  wood  has  absorbed  the  required  amount  of  preservative.  Next  a  sup- 
plemental vacuum  of  at  least  20  inches  is  applied  for  at  least  30  minutes.  If 
desired  this  vacuum  may  be  followed  by  a  short  steaming  period. 

"The  timber  may  be  either  green  or  air  seasoned;  but  should  preferably  be 
treated  within  three  months  after  it  is  sawed.  Green  and  seasoned  timber  shall 
not  be  treated  together  in  the  same  charge. 

"After  treatment  the  blocks  shall  show  a  satisfactory  penetration  of  the  pre- 
servative; and  in  all  cases  the  preservative  must  be  diffused  throughout  the  sap 
wood.  The  surface  of  the  blocks  after  treatment  shall  be  free  from  deposit  of 
objectionable  substances;  and  all  blocks  that  have  been  materially  warped, 
checked,  or  otherwise  injured  in  the  process  of  treatment  shall  be  rejected." 

*  Accompanying  the  printed  specifications,  but  not  reproduced  in  this  volume. 


610  WOOD-BLOCK   PAVEMENTS  [CHAP.   XIX 

1167.  The  following  are  the  reasons  for  the  several  steps  in  the 
treatment. 

The  preliminary  steaming  softens  or  liquefies  the  sap,  so  it  may 
later  be  removed  from  the  pores  of  the  wood.  The  steaming  also 
equalizes  the  moisture  content  of  heart  and  sapwood,  which  equalizes, 
the  resistance  to  penetration,  and  thus  prepares  the  wood  to  receive 
the  preservative  more  uniformly.  The  preliminary  steaming  is . 
applied  whether  the  timber  is  green  or  seasoned.  In  green  timber 
the  sapwood  contains  more  water  than  the  heartwood;  and  unless 
the  excess  sap  is  removed,  the  blocks  will  contain  untreated,  or  at 
least  under-treated,  sapwood.  Therefore,  green  blocks  should  be 
subjected  to  steaming  and  the  vacuum  process  to  remove  the  excess 
water  from  the  sapwood.  On  the  other  hand,  seasoned  timber  is 
more  easily  treated,  i.  e.,  takes  the  preservative  more  easily,  than 
green  timber.  Sometimes  seasoned  timber  accepts  the  preservative 
so  easily  that  almost  no  pressure  is  required  to  produce  the  desired 
absorption ;  and  consequently  the  easily  treated  portion  receives  too 
much  preservative  and  the  portion  that  is  more  difficult  to  treat 
receives  too  little.  The  sapwood  is  likely  to  be  more  thoroughly 
seasoned  than  the  heartwood;  and  therefore  the  former  may  receive 
more  preservative  than  the  latter,  and  consequently  the  latter  may 
decay  because  of  incomplete  penetration  of  the  preservative.  The 
steaming  of  seasoned  blocks  expands  them,  so  that  when  treated  and 
laid  in  the  pavement  they  will  have  a  minimum  expansion  with  the 
absorption  of  moisture.  Therefore,  seasoned  timber  should  be  sub- 
jected to  steaming  to  prepare  it  to  receive  the  preservative  and  to 
decrease  expansion  when  laid  in  the  pavement. 

The  preliminary  vacuum  is  applied  to  remove  the  sap  and  mois- 
ture, which  equalizes  the  resistance  to  the  penetration  of  the  pre- 
servative. 

The  preservative  is  applied  under  pressure  and  for  a  considerable 
time  to  secure  complete  penetration;  but  the  pressure  is  applied 
slowly  so  as  not  to  injure  the  strength  of  the  wood. 

The  supplemental  or  final  vacuum  is  applied  to  equalize  the  distribu- 
tion of  the  preservative,  but  chiefly  to  remove  the  excess  preservative 
in  the  outer  portion  of  the  block  and  thus  decrease  bleeding  (§  1211). 

The  final  steaming  is  applied  to  soften  and  remove  the  excess 
preservative  from  the  surface  of  blocks. 

Finally,  green  and  seasoned  timber  should  not  be  treated  together 
in  the  same  charge,  since  they  have  unequal  resistance  to  the  pene- 
tration of  the  preservative. 


ART.    1]  MATERIALS  AND   TREATMENT  611 

1168.  Amount  of  Preservative.     The  blocks  laid  in  Indianapolis 
in  1896  (§  1147)  were  treated  by  the  open-tank  process  and  contained 
only  about  3  Ib.  of  creosote  oil  per  cubic  foot.     In  the  earlier  appli- 
cations of  the  pressure  process,  the  amount  was  usually  10  to  12  Ib. 
per  cubic  foot;  but  later  the  general  practice  was  to  inject  from  20  to 
24  Ib.  per  cubic  foot,  which  resulted  in  a  greatly  increased  cost  and 
an  excessive  bleeding  of  the  blocks  (§  1211).     Recently  the  tendency 
has  been  to  reduce  the  absorption;   and  at  present  the  average  is 
about  16  Ib.  of  water-free  preservative  per  cubic  foot.     The  standard 
specifications  of  the  American  Wood  Preservers'  Association  and  also 
those  of  the  American  Society  of  Municipal  Improvements  require 
16  Ib.  per  cubic  foot. 

The  amount  absorbed  is  determined  from  gages  on  the  treat- 
ing cylinder  and  a  knowledge  of  the  volume  of  the  charge;  and  may 
be  checked  by  weighing  several  blocks  before  and  after  treatment. 

1169.  Testing  the  Blocks  after  Treatment.     Usually  the  only 
test  made  is  to  determine  the  penetration  of  the  preservative.     "  To 
determine  this,  at  least  twenty-five  blocks  shall  be  selected  from 
various  parts  of  each  charge,  and  sawn  in  half  at  right  angles  to  the 
fibers,  through  the  center;  and  if  more  than  one  of  these  blocks  show 
untreated  sapwood,  the  charge  shall  be  retreated." 

1170.  Occasionally  a  test  is  made  to  determine  the  amount  of 
water  a  treated  block  will  absorb.     The  usual  specifications  for  this 
test  are:  "  The  treated  blocks  after  being  dried  in  an  oven  at  100°  F. 
for  24  hours,  and  then  immersed  in  clear  water  for  24  hours,  shall  not 
absorb  more  than  3J  per  cent  of  their  dry  weight  if  pine,  nor  more 
than  4J  per  cent  if  tamarack."     This  test  is  to  determine  the  pos- 
sibility of  the  block's  swelling  after  being  laid,  by  the  absorption  of 
moisture;    and  therefore  is  quite  important.     The  absorption  will 
increase  with  the  time  after  treatment;  and  therefore  the  blocks  for 
this  test  sho  ild  be  taken  from  those  about  to  be  laid  rather  than 
from  those  recently  treated. 

The  results  by  this  test  depend  more  upon  the  time  since  treat- 
ment and  upon  the  method  of  storage  than  upon  the  method  of 
treatment;  and  hence  this  is  not  an  accurate  test  of  absorbing  power. 
Further,  since  no  preservative  process  can  make  blocks  absolutely 
waterproof,  they  should  be  so  laid  as  to  reduce  to  a  minimum  the 
amount  of  moisture  absorbed  (see  §  1180). 

1171.  Care  of  Blocks  after  Treatment.     The  blocks  should  prefer- 
ably be  laid  in  the  street  as  soon  as  possible  after  being  treated.     If 
they  can  not  be  laid  within  two  days,  provision  should  be  made  to 


612  WOOD-BLOCK   PAVEMENTS  [CHAP.   XIX 

prevent  them  from  drying  out,  by  stacking  in  close  piles  and  covering 
them;  and  if  possible,  sprinkling  them  thoroughly  at  intervals.  To 
prevent  expansion  in  the  pavement  through  the  absorption  of  water, 
the  blocks  should  be  well  sprinkled  about  two  days  before  being  laid. 


ART.  2.     CONSTRUCTION 

1173.  FOUNDATION.     The    subgrade    should    be    prepared    as 
described  in  Art.  1  of  Chapter  XV, — Pavement  Foundations.     The 
usual  foundation  is  a  layer  of  hydraulic  concrete,  which  should  be 
constructed  as  described  in  Art.  2  of  Chapter  XV. 

The  specifications  of  the  American  Society  of  Municipal  Improve- 
ments state:  "  At  no  place  shall  the  surface  of  the  finished  concrete 
vary  more  than  a  half  inch  from  the  given  grade." 

1174.  BEDDING    COURSE.     A  bedding   course  is  necessary  to 
compensate  for  any  unevenness  in  the  top  surface  of  the  concrete 
foundation  and  to  afford  a  good  bearing  for  the  blocks.     Three 
forms  of  bedding  course  are  in  common  use,  which  for  brevity  may 
be  designated  as  sand,  hydraulic-cement  mortar,  and  bituminous 
cement. 

1175.  Sand  Cushion.     The  sand  bedding-course  varies  in  thick- 
ness from  1  to  2  inches.     The  disadvantages  of  a  thick  sand  cushion 
are  very  much  the  same  for  wood  blocks  as  for  brick— see  §  977. 
In  recent  years,  as  in  brick  pavements,  there  has  been  a  tendency  to 
reduce  the  thickness  of  the  sand  cushion.     The  1916  specifications 
of  the  American   Society   of  Municipal   Improvements   require  a 
cushion  1  inch  thick,  of  "  sand  that  will  pass  a  J-inch  screen  and 
contain  10  to  25  per  cent  of  loam  or  clay."     The  sand  cushion  should 
be  struck  with  a  template  to  a  surface  parallel  to  the  grade  and  con- 
tour of  the  finished  pavement;  and  should  then  be  rolled. 

Sometimes,  instead  of  striking  the  sand  cushion  with  a  template, 
screeds  are  laid  transversely  across  the  pavement  at  intervals  of  8  or 
10  feet,  being  placed  upon  a  ridge  of  sand  or  mortar  so  as  to  bring  the 
top  surface  of  the  screed  parallel  to,  and  at  the  right  distance  below, 
the  surface  of  the  pavement.  The  sand  is  spread  between  the  screeds, 
and  then  struck  off  to  the  right  depth  with  a  straight  edge  which 
rests  upon  the  screeds  and  is  kept  parallel  to  the  curb.  This  method 
requires  more  labor  and  does  not  give  as  accurate  a  surface  as  striking 
with  a  template.  The  only  disadvantage  of  using  a  template  is 
that  a  new  one  is  required  with  each  change  in  crown  or  width, 


ART.    2]  CONSTRUCTION  613 

although  this  objection  is  overcome  in  part  by  using  a  template 
which  is  slightly  adjustable. 

1176.  Substantially  all  of  the  comments  concerning  the  sand 
cushion  for  brick  pavements  (§  971-78)  apply  also  to  that  for  wood- 
block pavements.     In  addition,  a  sand  cushion  holds  moisture,  and 
hence  increases  the  absorption  of  the  blocks  and  adds  to  the  troubles 
due  to  their  expansion  in  the  pavement.     Formerly  the  sand  cushion 
was  the  most  common  form  of  bedding  course;  but  it  has  now  prac- 
tically been  abandoned. 

1177.  Dry-mortar  Bed.     The  method  of  laying  wood  blocks  on  a 
dry-mortar  bed  is  substantially  the  same  as  for  laying  bricks  on  a 
cement-sand  bedding  course  (see  §  979-81).     It  would  be  possible 
to  lay  wood  blocks  upon  a  wet-mortar  bed  by  either  of  the  processes 
employed  for  brick  pavements  (see  §  982);  but  it  is  not  known  that 
it  has  ever  been  done. 

The  1916  specifications  of  the  American  Society  of  Municipal 
Improvements  for  preparing  the  dry-mortar  bed  for  wood  blocks  are 
as  follows: 

"The  concrete  foundation  shall  be  cleaned  and  swept;  and  shall  be  thor- 
oughly dampened  immediately  in  advance  of  the  spreading  of  the  cushion  course. 
Upon  the  surface  of  the  foundation  thus  prepared  shall  be  spread  a  layer  of  mortar 
not  exceeding  \  inch  in  thickness,  made  of  one  part  portland  cement  and  three 
parts  of  sand.  Only  sufficient  water  shall  be  added  to  this  mixture  to  insure  a 
proper  setting  of  the  cement,  thelntention  being  to  produce  a  granular  mixture 
which  may  be  raked  or  struck  by  a  template  to  the  desired  grade.  The  mortar 
shall  be  [thoroughly  mixed,  and  shall  be  spread  in  place  upon  the  foundation  by 
means  of  a  template  immediately  in  advance  of  the  laying  of  the  blocks." 

1178.  It  is  doubtful  if  the  mixture  of  cement  and  sand  ever  gets 
enough  water  to  cause  it  to  set  fully,  since  the  joints  between  the 
blocks  are  quite  narrow.     The  only  reason  for  using  a  granular  mix- 
ture is  that  it  can  be  spread  and  struck  easily;    but  the  ordinary 
cement  mortar  containing  enough  water  to  insure  a  complete  set, 
can  be  spread  and  struck  without  serious  trouble.     Or,  better  still, 
if  the  concrete  foundation  is  finished  with  a  slight  excess  of  mortar  on 
the  surface,  the  wood  blocks  can  be  set  in  the  mortar  as  are  the 
brick  in  the  monolithic  brick  pavement  (§  892). 

An  objection  to  the  mortar  bedding  course  is  that  the  pavement 
can  not  be  used  until  the  mortar  has  set;  but  if  the  mortar  bedding 
course  is  laid  immediately  after  the  concrete  foundation  is  placed,  this 
objection  is  eliminated. 

A  serious  objection  to  the  dry-mortar  cushion  is  that  not  enough 


614  WOOD-BLOCK    PAVEMENTS  [CHAP.    XIX 

water  is  used  to  secure  a  good  quality  of  mortar.  When  the  dry- 
mortar  cushion  is  used  for  a  brick  pavement,  the  mortar  is  thor- 
oughly wet  by  sprinkling  the  brick  after  they  are  laid;  but  this 
should  not  be  done  with  wood  blocks,  since  they  absorb  more  water, 
and  since  with  wood  blocks  the  joints  are  usually  filled  with  bitumi- 
nous cement  (§  1187),  which  should  be  applied  only  when  the  blocks 
are  dry. 

1179.  Bituminous  Bed.  If  the  top  of  the  concrete  foundation 
has  not  been'  finished  to  an  accurate  surface  by  strkiing  with  a  tem- 
plate (§  461-62),  it  should  be  leveled  up  by  spreading  a  layer  of  1  :  2 


FIG.  224. — FINISHING  MORTAR  BEDDING-COURSE  WITH  A  STEEL  FLOAT. 

cement  mortar  on  it.  This  mortar  should  be  of  such  consistency 
that  it  may  be  easily  spread;  and  should  be  applied  to  the  surface 
of  the  concrete  before  initial  set  of  it  has  begun.  The  mortar  should 
be  then  worked  to  an  accurate  surface  by  means  of  a  long-handled 
wood  float  having  upturned  ends.  When  finished  the  surface  should 
not  show  any  depressions  greater  than  J  inch  under  a  5-foot  straight 
edge  laid  parallel  to  the  curb.  Fig.  224  shows  the  method  of 
finishing  the  mortar  bedding-course  with  steel  floats;  and  Fig.  225 
shows  the  method  of  finishing  the  bedding  course  with  a  wood- 
float. 

1180.  After  the  concrete  base  and  the  mortar  coat  have  set  and 
hardened,  and  after  the  surface  has  been  thoroughly  cleaned,  and 
while  it  is  perfectly  dry,  a  coat  of  coal-tar  pitch  is  spread  upon  the 
surface,  The  pitch  should  meet  the  specifications  of  §  576-77 


ART.   2] 


CONSTRUCTION 


615 


(page  295)  for  filler  for  wood  blocks;  and  should  be  applied  at  a 
temperature  between  250°  and  300°  F.  It  should  be  spread  to  a 
uniform  thickness  of  not  more  than  £  inch,  and  be  finished  to  a  smooth 
surface.  The  blocks  should  be  set  directly  upon  this  paint  coat 
within  30  minutes  after  it  has  been  applied.  If  the  work  is  properly 
done,  the  blocks  are  firmly  held  in  place;  and  in  tearing  up  such  a 
pavement  it  is  not  unusual  to  have  the  pitch  pull  up  a  film  of  the 
concrete  base.  If  the  surface  of  the  concrete  or  the  mortar  coat 
has  ridges  or  depressions  in  it,  the  blocks  are  likely  to  split  under 
travel.  If  the  pitch  coat  is  thicker  than  £  inch,  it  is  likely  to  flow 


FIG.  225. — FINISHING  MORTAR  BEDDING-COURSE  WITH  A  WOOD  FLOAT. 


and  split  the  blocks.  The  slipping  of  the  blocks  on  the  pitch  coat  is 
sometimes  called  "  floating." 

The  chief  advantage  of  the  bituminous  bedding  course  is  that 
the  bituminous  cement  completely  seals  the  pores  of  the  block,  and 
prevents  the  absorption  of  any  water  that  may  reach  the  top  of  the 
concrete  base  through  cracks  in  the  wearing  coat.  This  method 
represents  the  best  modern  practice,  and  has  recently  been  adopted 
quite  generally. 

1181.  LAYING  THE  BLOCKS.  Upon  one  of  the  three  bedding 
courses  described  above,  the  blocks  are  set  with  the  fiber  vertical, 
in  straight  parallel  courses,  leaving  a  space  at  the  curb  1  inch  in 
width  for  the  expansion  joint.  The  blocks  are  laid  "hand  tight"; 
but  each  eight  or  ten  courses  are  driven  together  by  laying  a  4-  by 


616 


WOOD-BLOCK   PAVEMENTS 


[CHAP,  xix 


4-inch  scantling  against  the  last  course  and  striking  it  with  an  axe  or  a 
sledge.  No  joint  should  be  more  than  £  of  an  inch  in  width,  although 
some  good  authorities  permit  a  width  of  Y&  of  an  inch.  The  blocks 
should  lap  at  least  2  or  2J  inches.  Only  whole  blocks  should  be  used, 
except  in  starting  and  closing  a  course.  The  block  used  in  starting 
or  closing  should  have  its  cut  face  perpendicular  to  the  top.  In 
placing  the  blocks  the  workman  should  stand  upon  the  blocks  already 
placed,  and  never  upon  the  bedding  course. 

There  is  no  agreement  as  to  whether  the  courses  shall  be  per- 
pendicular to  the  curb,  or  at  an  angle  of  45°  or  67J°.  It  is  claimed 
that  if  the  courses  are  oblique  to  the  curb,  the  wear  at  the  joints,  par- 
ticularly the  transverse  ones,  will  be  less  than  if  the  courses  are  per- 
pendicular to  the  curb.  But  this  claim  has  not  been  established; 
and  as  it  is  most  convenient  and  costs  about  2  cents  per  square  yard 
less  to  lay  the  courses  at  right  angles  to  the  curb,  this  method  has 
generally  been  adopted. 


FIG.  226.— LAYING  WOOD-BLOCK  PAVEMENT  BETWEEN  STREET-CAR  RAILS. 

1182.  Fig.  226  shows  the  method  of  laying  wood  blocks  between 
the  rails  of  a  street-car  track  on  a  mop  coat  of  tar  over  a  smooth 


ART.    2] 


CONSTRUCTION 


617 


concrete  base.  Fig.  227  shows  the  manner  of  laying  wood  blocks 
between  the  railway  area  and  the  curb.  Note  the  plank  next  to 
the  curb  to  provide  space  for  the  longitudinal  expansion  joint. 
The  blocks  are  being  laid  on  a  mop  coat  of  tar  on  a  concrete  base. 


FIG.  227. — LAYING  WOOD-BLOCKS  ON  A  MOP  COAT  OF  TAR. 


1183.  Rolling.  After  being  placed,  the  blocks  are  inspected,  and 
the  rejected  blocks  are  removed  and  replaced  with  acceptable  ones. 
The  surface  should  then  be  swept  clean,  and  be  rolled  with  a  tandem 
roller  weighing  from  3  to  6  tons.  The  roller  should  begin  at  the  side 
of  the  pavement,  run  slowly  parallel  to  the  center  line,  and  work 
inwardly  until  the  center  of  the  road  is  reached.  It  should  then  move 
to  the  opposite  side  of  the  pavement  and  proceed  as  before.  As  the 
roller  passes  back  and  forth,  it  should  overlap  its  course  each  time. 
After  one  rolling  of  the  entire  surface,  the  speed  of  the  roller  may  be 
increased  and  the  rolling  continued  until  the  blocks  are  thoroughly 
and  evenly  bedded. 

Portions  of  the  pavement  inaccessible  to  the  roller  should  be 
thoroughly  rammed  with  a  paver's  rammer  (§  992)  weighing  not  less 
than  50  lb.,  striking  upon  a  plank  not  less  than  6  feet  long,  10  to  12 
inches  wide,  and  2  inches  thick.  The  plank  should  be  laid  parallel 
to  the  curb,  and  moved  so  that  the  surface  will  be  equally  rammed 
and  brought  to  the  proper  elev.stion. 


618  WOOD-BLOCK    PAVEMENTS  [CHAP.    XIX 

If  the  bedding  course  is  green  mortar,  the  rolling  should  be  com- 
pleted before  the  mortar  has  set. 

When  the  rolling  and  ramming  are  completed,  the  surface  of  the 
pavement  should  conform  so  nearly  to  that  indicated  on  the  plans, 
that  it  will  nowhere  depart  more  than  one  fourth  of  an  inch  from 
properly  formed  templates  or  from  a  10-foot  straight  edge  applied 
parallel  to  the  center  line. 

1184.  JOINT  FILLER.  The  joints  between  the  blocks  are  filled 
with  grout,  sand,  tar  pitch,  or  asphalt. 

,1185.  Grout.  Occasionally  the  attempt  is  made  to  fill  the  joints 
of  a  wood-block  pavement  with  hydraulic-cement  grout;  but  if  the 
blocks  are  set  as  closely  together  as  they  should  be,  the  joints  will  be 
so  narrow  that  it  is  impossible  to  get  a  good  grout  to  fill  them.  If 
the  grout  is  thin  enough  to  flow  into  narrow  joints,  it  will  not  con- 
tain enough  cement  to  make  it  of  much,  if  any,  value;  and  besides 
it  will  not  adhere  well  to  the  sides  of  treated  wood-blocks,  and  even 
the  preservative  seems  to  kill  the  cement. 

1186.  Sand.     Fine  clean  dry  sand  is  swept  into  the  joints;   and 
the  surface  is  covered  with  sand  to  the  depth  of  |  an  inch.     The 
sand  is  placed  upon  the  surface  to  insure  that  every  joint  is  filled 
and  to  permit  travel  to  grind  the  sand  into  the  surface  of  the  blocks. 
After  the  sand  has  remained  on  the  pavement  for  a  time,  depending 
upon  the  density  of  the  travel,  it  is  swept  up  and  hauled  away. 

Sand  was  formerly  the  most  common  joint  filler,  but  it  has 
practically  been  abandoned.  It  is  cheap  and  makes  a  fairly  water- 
tight pavement;  but  it  has  little  or  no  effect  in  binding  the  blocks 
together  to  prevent  settlement  due  to  shrinkage  of  the  sand  cushion 
(§  1055)  or  other  causes  (§  1056-57),  or  to  prevent  upheavals 
(§  1060). 

1187.  Tar  Pitch.     The  tar-pitch  filler  should  conform  to  the  spe- 
cifications stated  in  §  576-77,  page  295.     It  is  heated  in  kettles  or 
tanks  on  the  street.     The  temperature  should  never  exceed  325°  F.; 
and  it  should  be  applied  to  the  pavement  at  a  temperature  between 
250  and  300°  F.     The  hot  pitch  may  be  poured  into  the  joints  as 
described  for  brick  pavements  (see  the  second  paragraph  of  §  1011). 
Fig.  228  shows  a  somewhat  antiquated  method  of  applying  the  filler 
to  a  wood-block  pavement.     This  method  is  more  suitable  for   a 
brick  pavement  than  a  wood  one,  since  with  the  former  the  joint  is 
wider,  and  hence  it  is  easier  to  follow  a  joint  with  the  point  of  the 
can;    and  also  since  more  tar  is  required,  and  hence  the  tar  flows 
better  through  the  tip  of  the  can. 


ART.    2] 


CONSTRUCTION 


619 


FIG.  228. — FILLING  THE  JOINTS  WITH  A  CONICAL  CAN. 


Fig.  229  shows  a  better  method  of  applying  the  tar  joint-filler. 


Fio.  229. — APPLYING  TAR  JOINT-FILLER  WITH  BUCKET  AND  SQUEEGEE. 


620 


WOOD-BLOCK   PAVEMENTS 


[CHAP,  xix 


Fig.  230  shows  a  buggy  for  transporting  and  applying  pitch  joint- 
filler. 


Fia.  230. — BUGGY  FOR  APPLYING  PITCH  JOINT-FILLER. 

Fig.  231  shows  the  finished  wood-block  pavement. 


FIG.  231. — FINISHED  WOOD-BLOCK  PAVEMENT. 

11)88.  In  filling  the  joints  care  should  be  taken  to  leave  as  little 
bituminous  cement  on  the  surface  as  possible,  since  one  of  the  ob- 
jections to  a  treated  wood-block  pavement  is  that  the  preservative 
exudes,  i.  e.,  "  the  pavement  bleeds,"  and  covers  the  surface  with  a 
sticky  mass  (§  211).  If  any  joint  filler  is  left  upon  the  surface,  it 
increases^this  objection.  Further,  some  object  to  a  bituminous  filler, 
because  it  is  liable  to  be  forced  out  of  the  joints  onto  the  surface  of  the 


ART.    2] 


CONSTRUCTION 


621 


pavement.  An  advantage  of  a  bituminous  filler  is  that  it  virtually 
surrounds  each  block  with  an  expansion  joint,  and  hence  decreases 
the  expansion  due  to  the  absorption  of  moisture  by  the  blocks. 

Under  no  circumstances  should  the  pitch  be  applied  when  there 
is  any  moisture  in  the  joints;  and  therefore  a  pitch  filler  should  not 
be  used  with  a  cement-sand  bedding  course  (§  1177). 

1189.  The  specifications  of  the  Ohio  Department  require  that 
the  lower  half  of  the  joints  shall  be  filled  with  tar  pitch  as  in  §  1187, 
and  the  upper  half  with  sand  as  in  §  1186. 

1190.  OPEN- JOINT  CONSTRUCTION.     Since  a.wood-block  pave- 
ment made  as  described  above  is  quite  smooth,  it  is  customary  to 
modify  the  construction  on  steep  grades  so  as  to  give  a  good  foothold 
to  horses.     Such  construction  is  not  employed  unless  the  grade  is 
more  than  3  or  4  per  cent — see  Table  15,  page  57. 

Formerly  a  corner  was  cut  out  of  the  upper  edge  of  a  block 
as  shown  in  Fig.  232  so  as  to  give  an  open  joint  J  inch  wide  and 


FIG.  232. — RECTANGULAR  WOOD-BLOCK  PAVEMENT  WITH  OPEN  JOINTS. 

1^  inches  deep;  but  at  present  it  is  more  common  and  cheaper 
to  place  between  adjacent  courses  a  creosoted  wood  lath  3^  of  an  inch 
thick  and  2  inches  wide.  The  joint  is  poured  about  half  full  of 
bituminous  filler;  and  then  the  space  above  the  lath  is  filled  with 
hot  crushed  stone,  and  the  interstices  between  the  stones  are  filled 
with  bituminous  cement. 

Sometimes  the  upper  edge  of  the  face  of  the  blocks  is  chamfered 
at  an  angle  of  about  45°  to  a  depth  of  about  f  of  an  inch;  and  this 
space  is  filled  with  the  ordinary  joint  filler.  This  method  is  not  as 


622  WOOD-BLOCK  PAVEMENTS  [cHAP.   XIX 

satisfactory  nor  as  cheap  as  the  second  one  described  in  the  preceding 
paragraph. 

1191.  EXPANSION  JOINTS.     The    expansion    of  wood    paving- 
blocks  due  to  changes  of  temperature  is  not  great  enough  to  require 
any  consideration;  but  the  expansion  and  contraction  due  to  change 
in  the  moisture  content  requires  attention.     The  amount  of  expan- 
sion and  contraction  due  to  moisture  depends  somewhat  upon  the 
method  of  treatment.     If  the  blocks  are  seasoned  before  being  treated, 
and  are  not  steamed  before  they  are  impregnated  with  the  preserva- 
tive, they  are  likely  to  absorb  moisture  and  swell  after  the  preserva- 
tive has  evaporated.     Again,  with  green  timber  the  steaming  and  the 
vacuum  liquefies  and  removes  the  sap,  and  reduces  the  volume  of  the 
wood  so  it  will  be  less  likely  to  shrink  when  laid  in  the  pavement. 
Finally,  the  amount  of  preservative  should  be  such  as  to  fill  the  cells 
of  the  wood  and  cover  the  fibers,  thus  making  the  blocks  partially 
waterproof.     An  additional  means  of  reducing  the  expansion  and 
contraction  due  to  moisture  is  to  lay  the  blocks  in  a  paint  or  mop 
coat  of  bituminous  cement  (see  §  1179-80). 

1192.  To  allow  for  expansion  due  to  temperature  and  moisture, 
it  is  customary  to  construct  a  longitudinal  expansion  joint  next  to 
each  curb.     There  are  two  forms  of  expansion  joints,  the  poured  and 
the  pre-moulded. 

The  poured  joint  is  made  by  placing  a  f  or  a  1-inch  board  next 
to  the  curb,  and  setting  the  blocks  against  it  (see  Fig.  227,  page  617); 
and  after  the  blocks  are  set  and  the  joints  are  filled,  the  board  is 
removed  and  the  space  is  filled  with  bituminous  joint-filler  poured  hot 
(see  Fig.  233).  For  an  illustration  of  a  brick  pavement  showing  a 
thin  board  in  place  with  wedges  behind  it  to  facilitate  its  removal, 
see  Fig.  171,  page  483. 

The  pre-moulded  expansion  joint  is  a  sheet  of  felt  or  its  equivalent 
saturated  with  tar  pitch  or  asphalt.  There  are  several  somewhat 
similar  forms  on  the  market.  For  a  few  additional  items  concern- 
ing pre-moulded  expansion  joints,  see  the  second  paragraph  of 
§  1017. 

1193.  Formerly   transverse   expansion    joints   were   inserted   in 
wood-block  pavements;    but  they  have  been  discontinued,  because 
they  are  not  needed,  and  are  a  positive  detriment.     The  objections 
to  the  transverse  expansion  joint  for  wood-block  pavements  are  sub- 
stantially the  same  as  for  brick  pavements — see  §  1018. 

Care  should  be  taken  to  fill  the  joints  around  manholes,  water- 
boxes,  etc.,  to  prevent  water's  reaching  the  foundation,  where  it  will 


ART.    2J 


CONSTRUCTION 


623 


freeze  and  lift  the  pavement,  or  be  absorbed  by  the  blocks  and  cause 
them  to  expand. 

1194.  CROWN.     The  surface  of  a  wood-block  pavement  should  be 
quite  smooth;    and  therefore  the  crown  should  be  comparatively 
small.     A  special  committee  of  the  American  Society  of  Civil  Engi- 
neers recommended  that  the  transverse  slope  be  between  J  and  \  of 
an  inch  per  foot — see  Table  16,  page  65. 

1195.  MAXIMUM   PERMISSIBLE    GRADE.     The   maximum  per- 
missible grade  for  a  close-joint  wood-block  pavement  is  3  or  4  per 
cent  (see  Table  15,  page  57);   and  with  the  open-joint  construction 
(§  1190),  the  maximum  grade  may  be  6  or  7  per  cent. 


FIG.  233. — POURING  THE  LONGITUDINAL  EXPANSION  JOINT. 

1196.  PAVING  ADJACENT  TO  TRACK.    Wood-blocks  are  laid 
adjacent  to  the  rails  of  street  railway  tracks  in  substantially  the  same 
manner  as  bricks — see  Fig.  196  and  197,  page  540. 

1197.  COST  OF  CONSTRUCTION.     Price   of  Blocks.     Table  70, 
page  624,  shows  the  market  quotation  for  treated  wood  paving- 
blocks  for  November  1,  1917.     For  more  recent  quotations,  see  con- 
struction news  in  current  technical  journals. 

1198.  Cost  of  Pavement.     The  following  estimate  was  prepared 
for  this  volume  by  a  specialist  in  wood  preservation  and  wood  paving 
who  has  no  financial  interest  in  contracting.! 

1199.  Cost  of  Blocks.     Table  71,  page  624,  shows  the  amount  of 
timber  and  preservative  required.      The  method  of  using  Table  71  in 

t  Mr.  Walter  Buehler,  Mem.  Amer,  Soc,.gf  Civil  Engra.,  Chicago. 


624 


WOOD-BLOCK   PAVEMENTS 


[CHAP,  xix 


TABLE  70 

MARKET-PRICE  FOR  TREATED  WOOD  PAVING-BLOCKS* 


LOCALITY. 

ABSORPTION, 
Ib.  per  cu.  ft. 

DEPTH, 
inches. 

PRICE, 
per  sq.  yd. 

New  York  City    

16 

3£ 

$2.00 

((                 U                 (I 

16 

4 

2  25 

Chicago                                              

16 

4 

1  85 

St  Louis                                          

16 

31 

1  80 

u       n 

16 

4 

2  03 

Kansas  City                         

16 

4 

2.50 

St  Paul 

16 

31 

2  00 

San  Francisco 

16 

3 

2  15 

<;               « 

16 

31 

2  26 

(.               ti 

16 

4 

2  57 

Seattle                        

16 

4 

2  35 

TABLE  71 
AMOUNT  OF  LUMBER  AND  PRESERVATIVE  REQUIRED  FOR  PAVING  BLOCKS 


LUMBER  REQUIRED, 

PRESERVATIVE  REQUIRED, 

Feet,  B.M.,perSq.  Yd. 

Gallons  per  Sq.  Yd. 

Depth  of 
Block, 
Inches. 

Absorption,  Lb.  per  Cu.  Ft. 

Net. 

Waste. 

Total. 

10 

12 

16 

3 

2.7 

2.7 

30 

2.443 

2.945 

3.928 

3| 

31.5 

3.15 

35 

2.862 

3.546 

4.583 

4 

36 

3.6 

40 

3.274 

3.928 

5.238 

determining  the  cost  of  blocks  is  as  follows:  Assume  the  cost  of 
lumber  at  the  treating  plant  at  $30  per  thousand  feet,  B.  M.,  which 
under  normal  conditions  prevailing  a  few  years  ago  would  not  be  over 
$25.  Assume  that  the  preservative  meets  the  specifications  of 
§  1161;  and  that  it  costs  9  cents  per  gallon.  Assume  that  the  blocks 
are  to  be  3|  inches  deep,  and  are  to  be  treated  with  16  Ib.  per  cubic 
foot.  The  cost  of  the  blocks  are: 

Lumber  35  feet  B.M.,  at  $30 $1 .05    per  sq.  yd. 

Preservative  4.583  gallons  at  9  cents 414     "     "     " 

Labor,  depreciation,  interest,  etc 240     "     "     " 

Factory  cost $1 . 704     "     "     " 

Profits  at  15  per  cent  gross 30       "     "     " 

Factory  selling  price $2 . 004     "     "     " 

The  approximate  weight  of  wood  blocks  treated  with  16  Ib.  per 
cubic  foot  is  as  follows:  3-inch,  130  Ib.  per  square  yard;  3j-inch 
150  Ib.  per  square  yard;  and  4-inch,  170  Ib.  per  square  yard. 

*  Engineering  News-Record,  Vol,  79  (1917),  Construction  News,  p.  180, 


AfcT.  2] 


CONSTRUCTION 


625 


1200.  Cost  of  Wearing  Coat.  Table  72  shows  in  detail  the  esti- 
mated cost  of  a  3i-inch  wood-block  pavement  with  two  forms  of  bed- 
ding course. 

TABLE  72 
ESTIMATED  COST  OF  3|-iNCH  WOOD-BLOCK  PAVEMENT 


BEDDING 

COURSE. 

ITEMS. 

Tar  Paint 
Coat. 

Dry  Mortar. 

Sub-grade  see  Table  56  page  546 

$0  217 

$0  217 

Concrete  Foundation  see  Table  56,  page  546. 

568 

568 

extra  finish  to  surface. 

01 

Bedding  Course  1  '.  4  dry  cement  and  sand  .  .        

18 

0  5  gallon  of  pitch,  and  labor  

.06 

Wood  Blocks,  see  §  1199  

2  004 

2  004 

freight,  say,  200  miles  at  6  cents  per  100  blocks  
hauling  at  60  cents  per  hour. 

.09 
05 

.09 
05 

laying  at  25  cents  per  hour. 

08* 

08* 

rolling 

01 

02 

Joint  Filler,  including  longitudinal  expansion  joint  
Top  Dressing,  purchasing  and  spreading  sand    .  .      .    . 

.12 
02 

.12 
02 

Total  cost,  exclusive  of  interest,  insurance,  deprecia- 
tion profit  etc 

$3  229 

$3  349 

1201.  Example  of  Cost.    Table  73,  page  626,  shows  the  cost  of 
laying   150,000  square  yards  of  creosoted   wood-block  paving  in 
Minneapolis  by  city  force,  and  are  given  in  unusual  completeness,  and 
hence  are  specially  valuable  in  making  estimates. 

1202.  The  following  are  the  details  of  the  cost  of  laying  a  wood- 
block pavement  in  Cambridge,   Mass.,  in  1913. f     Common  labor 
was  31  cents  per  hour.     The  foundation  consisted  of  5  inches  of 
1  :  2 J  :  5  concrete;  and  the  bedding  course  was  1  inch  of  cement  and 
sand.     The  blocks  were  southern  long-leaf  yellow  pine  treated  with 
20  Ib.  of  preservative.     The  joints  were  filled  with  1  :  1  cement 
grout.     Longitudinal  expansion  joints  were  provided  at  each  curb, 
and  transverse  contraction  joints  at  each  30  feet.     4-inch  blocks  cost 
$2.59  per  square  yard  delivered  on  the  street,  and  $4.11  per  square 
yard  complete  in 'the  pavement;  and  3|-iLch  blocks  cost  $2.29  and 
$3.81,  respectively. 

1203.  Contract  Price.     Table  74,  page  627,  shows  the  contract 
price  of  wood-block  pavements  in  various  cities,  and  incidentally 
also  gives  considerable  information  as  to  the  details  of  practice  of 
these  cities. 


*If  there  is  a  street-railway  track,  add  2  cents. 
^Engineering  News,  Vol.  71  (1914),  p.  1131. 


626  WOOD-BLOCK   PAVEMENTS  [CHAP.   XIX 

TABLE  73 
COST  OF  WOOD-BLOCK  PAVEMENT  IN  MINNEAPOLIS* 

ITEMS.  PER  SQ.  YD. 

SUBGRADE:  grading  and  shaping $0.2  87 

CONCRETE  BASE,  1:3:6,   6  inches  thick: 

cement,  $1.12  per  bbl.,  f.o.b.  cars 1222 

sand  $0 . 60  per  cu.  yd.,  delivered 0395 

stone,  $1.00  per  cu.  yd.,  f.o.b.  quarry,  $1.70,  delivered ; 2386 

labor  mixing  and  placing  by  hand  at  28  cents  per  hour 1392 

hauling  cement,  plank,  etc.,  at  59  cents  per  hour 0238 

concreting  strip  H  ft.  wide  between  railway  ties 0189 

Total  for  concrete  base $0. 5822 

SAND  CUSHION,  1  inch:  sand  at  60  cents  per  cubic  yard  on  job $0 . 200 

WOOD  BLOCKS,  Norway  pine  and  tamarack: 

i    4-inch,  treated  with  12  Ib.  of  creodone  creosote 1 .3275 

I    hauling  blocks  at  59  cents  per  hour .0495 

laying  blocks  at  22£°  with  curb,  at  40  cents  per  hour 0716 

Total  for  blocks $1 .4486 

JOINT  FILLER,  sand  and  pitch: 

sand  at  60  cents  per  cu.  yd.,  on  job 0055 

pitch  at  5.7  cents  per  gallon 0490 

labor  at  28  cents  per  hour 0175 

Total  for  filler $  .0720 

HEADERS: 

4  X  10-inch  plank  at  cross  streets  and  alleys 0030 

MISCELLANEOUS: 

materials .  0077 

labor 0002 

cleaning  up 0071 

tools..  .0200 


Total  miscellaneous. .  .  $   .  0350 


Total  average  cost $2 . 3795 

1204.  MERITS  AND  DEFECTS.  The  merits  of  a  treated  wood- 
block pavement  are:  1.  It  has  a  smooth  surface,  and  therefore  is  a 
quiet  pavement.  It  is  less  noisy  than  sheet  asphalt,  brick  or  stone- 
block;  and  from  the  standpoint  of  tenants,,  this  is  an  important 
advantage.  2.  It  is  a  reasonably  durable  pavement,  even  under 
heavy  and  dense  travel.  This  conclusion  has  been  established  in 
many  cities  in  this  country  and  in  Europe.  3.  The  pavement  is 
easy  to  clean;  and  its  surface  does  not  grind  up  and  make  dirt.  4. 
It  has  a  low  tractive  resistance. 

*B.  H.  Durham,  Street  Engineer,  in  Engineering  and  Contracting,  Vol.  35,  p.  451. 


ART.   2] 


CONSTRUCTION 


627 


TABLE  74 

CONTRACT  PRICE  OF  WOOD-BLOCK  PAVEMENTS  IN  VARIOUS  CITIES* 
Laid  in  1912 


LOCALITY. 

Amount. 
Laid  in 
1912, 
Sq.  Yd. 

CONCRETE  BASE. 

Kind 
of 
Filler. 

Guar- 
antee, 
Years. 

Total 
Thick- 
ness, 
Inches. 

AVER- 
AGE 
PRICE. 
Per 
Sq.  Yd.» 

State. 

i 
City. 

Thick- 
ness, 
Inches. 

Propor- 
tions. 

Connecticut.  .  . 

Georgia  
Illinois  

Bridgeport.  .  . 
S.  Norwalk.  .  . 

Albany  

Granite  City  . 
Quincy  

Burlingtor..  . 
Louisville.  .  .  . 

New  Orleans. 
Shreveport.  .  . 

Bangor  
Springfield  .  .  . 

Hibbing  
Minneapolis.  . 
Owatonna.  .  . 
Virginia  

10000 
3400 

10000 

10000 
9  171 

22000 
1  578 

18401 
54000 

1  700 
12949 

44608 
130000 
29  1843 
22854 

6 
5 

5 

6 
5 

6 

6 
5 

6 

1      3     6 
1     3     5 

1     3     6 

1     3     5 
1     3     6 

1     3     6 

1     3     6 
135 

1     3     6 

sand 
sand 

sand 

pitch 
asphalt 

asphalt 
sand 

pitch 
sand 

pitch 

5 
5 

9* 
9 

10* 
9* 

10i 

$3.10 
3.19 

2.18 

2.542 
2.66 

2.77 
2.85* 

3.002 
2  24 

3.88 
3.13 

2.77 
2.43 
2.292 
2.692 

"5" 
5 
5 
3 

5 
0 

5 
.  „.  . 

5 

Iowa         

Kentucky.  .  .  . 

10 

8 

9i 

Maine  

Mass 

Minnesota.  .  .  . 

5 

"5" 
6 

1     3     6 
136 
1     8 
1     2     4 

asphalt 
pitch 
pitch 
pitch 

9J 

"9J" 
10» 

Mississippi.  .  . 
Missouri  

Greenwood.  .  . 
Kansas  City.  . 
Miles  City.  .  . 

25000 
1  021 
4500' 

6 
6 
5 

1     3     5 
1     3     6 
1     6 

bitu. 
sand 
asphalt 

5 
5 
3 

11 
11 
9 

2.422 
2.95 
3.202 

Montana  

New  Jersey  .  .  . 

Jersey  City.  .  . 

11  891 

5 

1     3     5 

sand 

9J 

3.00 

New  York.... 

Plattsburg  .  .  . 
Rochester.  .  .  . 

1  979 
8392 

6 
6 

1     3     6 
1     3     6 

sand 
sand 

3 
5 

10 
10* 

3.08" 
3.32 

S.  Carolina.  .  . 
Texas  
Canada  

Charleston.  .  . 
Brownsville.  . 

Hamilton.  .  .  . 
Vancouver.  .  . 

9000 
29000 

12000* 
189  875 

4 

1     3     5 

sand 

5 

8 

2.72» 
2.70 

2.85 
3.202 

6 
6 

1     3     6 
1     2J  :5 

10 
10* 

pitch 

5 

1  Including  grading  and  concrete  base    2  Exclusive  of  grading.     3  3-inch  block     *  20  Ib.  per  cu.  ft. 

The  defects  of  a  treated  wood-block  pavement  are:  1.  It  is  some- 
what slippery,  particularly  when  its  surface  is  moist.  In  this  respect 
it  is  about  on  a  par  with  sheet  asphalt.  2.  Its  surface  is  liable  in 
hot  weather  to  become  covered  with  a  sticky  mass  which  adheres  to 
wheels  of  vehicles  and  tracks  into  houses  (§  1211).  3.  It  is  rather 
high  in  first  cost. 

1205.  SPECIFICATIONS.  The  American  Society  of  Municipal 
Improvements  in  1916  adopted  complete  specifications  for  Creosoted 
Wood-block  Paving;  and  substantially  the  same  specifications  have 
been  adopted  by  the  American  Wood  Preservers'  Association  and 

*  Engineering  and  Contracting,  Vol.  39  (1913),  p.  380-81. 


628 


WOOD-BLOCK   PAVEMENTS 


[CHAP,  xix 


other  associations  interested  in  wood  paving.  Copies  of  these  speci- 
fications may  be  had  for  a  nominal  sum  of  the  secretary  of  the  first- 
mentioned  society. 

ART.  3.    MAINTENANCE 

1206.  The    experience  with  treated  wood-block  paving  has  been 
comparatively  short,  and  hence  there  has  not  been  developed  any 
general  method  of  maintenance  or  repairs. 

1207.  DEFECTS    TO    BE    REMOVED.     The    principal    matters 
requiring  attention  are:    removing  poor  blocks,  raising  low  spots, 
re-laying  over  trenches,  lowering  bulges,  removing  exudation. 

1208.  Removing  Poor  Blocks.     Blocks  fail  owing  to  defects  in 
the  timber  or  to  imperfect  treatment.     The  latter  do  not  usually 
appear  until  after  the  pavement  is  several  years  old.     Generally, 
only  a  portion  of  a  single  block  fails,  and  usually  only  a  few  blocks 
in  each  city  block.     Fig.  234  shows  the  failure  of  a  single  block; 


FIG.  234. — FAILURE  OF  A  BLOCK  ON  WESTMINSTER  PLACE,  ST.  Louis. 

and  Fig.  235  shows  the  failure  of  several  blocks.  Not  infrequently 
the  failing  blocks  are  in  a  bunch,  indicating  that  there  was  prob- 
ably something  wrong  with  a  single  charge.  The  failure  or  decay 
is  ordinarily  due  to  insufficient  preservative  in  either  the  sap- 


ART.   3] 


MAINTENANCE 


629 


wood  or  the  heartwood  (see  §  1167).  At  first  the  hole  is  small  and 
shallow,  and  does  no  great  harm,  although  it  gradually  enlarges,  par- 
ticularly if  there  is  much  heavy  steel-tired  traffic.  The  defect  can  be 
temporarily  cured  by  filling  the  hole  with  bituminous  joint-filler 


Treated  in  1903.     Photographed  in  1915. 
FIG.  235. — FAILURE  OF  SEVERAL  BLOCKS  ON  WESTMINSTER  PLACE,  ST.  Louis. 

or  better  with  mortar  or  fine  concrete  made  with  bituminous  cement. 
The  only  permanent  remedy  is  to  cut  out  the  defective  block  and 
replace  it  with  a  good  one,  which  can  be  done  easily  and  quickly. 
With  a  little  care  and  attention  a  new  block  can  be  inserted  so  that 
the  patch  is  hardly  visible. 

1209.  Raising  Low  Spots.  Frequently  shallow  depressions  appear 
in  the  pavement.  These  holes  may  be  due  to  the  settlement  of  the 
foundation  (§  1056),  to  the  settlement  of  the  soil  in  a  trench  (§  1057), 
or  to  the  shrinkage  or  shifting  of  the  sand  cushion  (§  1055).  Such 
holes  are  objectionable  because  they  are  unsightly,  particularly  when 
filled  with  water;  and  they  hold  water  which  dissolves  the  preserva- 
tive, and  also  causes  the  blocks  to  swell  and  perhaps  buckle.  The 
sinking  of  the  blocks  break  the  bond  of  the  joint  filler,  particularly 
if  it  is  not  bituminous;  and  may  permit  water  to  reach  the  founda- 
tion, which  if  it  freezes  may  lift  the  pavement.  The  remedy  is  to 
take  up  the  spot,  remove  the  cause  and  re-lay  the  blocks.  For  a 


630  WOOD-BLOCK    PAVEMENTS  [CHAP.    XIX 

discussion  of  precautions  to  be  taken  in  re-laying  a  brick  pavement 
under  similar  conditions,  many  of  which  are  equally  applicable  in  re- 
laying wood-block  pavements,  see  §  1061. 

1210.  Re-laying  over  Trenches.     It  is  frequently  necessary  to  re- 
lay a  wood-block  pavement  over  a  trench  on  account  of  the  settle- 
ment of  the  soil  in  the   trench  or   because  a   trench  is  opened  to 
repair  or  lay  a  pipe  or  sewer.     For  a  discussion  of  the  method  of  re- 
laying a  brick  pavement  over  a  trench,  see  §  1061. 

1211.  Lowering  Bulges.     A  bulge  or  ridge  is  sometimes  formed 
in  a  wood-block  pavement  by  the  expansion  due  to  the  absorption  of 
moisture.     If  the  pavem  entis  not  provided  with  adequate  longitudi- 
nal expansion  joints,  the  bulge  may  be  longitudinally  along  the  crown 
of  the  pavement;    or  a  bulge  may  take  place  at  a  raised  footway 
crossing  or  at  the  crown  of  an  intersection  pavement  (see  §  1060). 
Usually  a  bulge  can  be  replaced  by  removing  a  few  blocks  along  the 
crest  of  the  bulge,  pressing  the  adjoining  pavement  back  to  place,  and 
re-laying  the  blocks  that  were  removed. 

1212.  Bleeding.     In  some  cases  the  preservative  fluid  oozes  out 
of  the  blocks  and  forms  a  thick  sticky  mass  on  the  surface  of  the 
pavement,  which  is  picked  up  by  the  wheels  of  passing  vehicles  and  is 
tracked  into  houses.     When  this  occurs  the  pavement  is  said  to 
bleed.     The  bleeding  may  be  due  to  one  or  more  of  the  following 
causes,  viz.:   1.  The  expansion  by  heat  of  the  air  in  the  pores  of  the 
block  may  force  out  the  preservative.     2.  The  absorption  of  moisture 
by  one  part  of  a  block  or  one  portion  of  the  pavement  may  cause  an 
expansion  which  forces  the  preservative  out  at  some  other  point. 
3.  Too  much  preservative  may  have  been  injected. 

Steaming  and  the  vacuum  treatment  of  green  blocks  decreases  the 
bleeding  by  removing  the  air  from  the  cells  and  by -reducing  the 
absorption  of  preservative  in  the  sapwood;  and  the  steaming  of 
seasoned  blocks  reduces  bleeding  by  expanding  the  blocks  to  their 
maximum  size  so  that  when  laid  they  will  be  less  likely  to  expand 
by  the  absorption  of  moisture.  The  bleeding  occurs  only  in  hot 
weather.  The  character  of  the  preservative  makes  little  or  no  dif- 
ference in  the  amount  of  bleeding,  many  claims  to  the  contrary  not- 
withstanding. Apparently  a  pavement  bleeds  less  under  heavy  than 
under  light  travel,  partly  because  the  traffic  seals  the  pores  of  the 
wood  and  prevents  the  oil  from  escaping,  and  partly  because  passing 
wheels  carry  away  the  sticky  materials  as  rapidly  as  it  oozes  out. 

1213.  The  remedy  for  a  bleeding  pavement  is  to  sprinkle  it  with 
fine  dry  sand,  and  remove  the  sand  after  it  has  absorbed  the  bitumi- 


ART.   3]  MAINTENANCE  631 

nous  material.  In  extreme  cases  it  may  be  necessary  to  apply  a 
second  coat  of  sand.  Usually  the  worst  cases  do  not  bleed  after 
the  first  year  or  two. 

1214.  COST  OF  MAINTENANCE.  There  is  an  unfortunate 
dearth  of  data  concerning  the  cost  of  repairs  or  of  maintenance  of 
any  pavement;  but  the  lack  is  greater  for  treated  wood-block  pave- 
ments than  for  any  other  kind,  since  the  experience  with  such  pave- 
ments is  comparatively  limited  (§  1147),  and  since  most  such 
pavements  have  been  laid  under  a  5-year  guarantee. 

The  following  examples  are  from  a  report  by  George  W.  Tillson, 
Engineer  of  the  Borough  of  Brooklyn,  New  York  City,  presented  at 
the  Third  International  Road  Congress  in  London  in  1913.* 

Wood-blocks  treated  by  the  creo-resinate  process  (§  1158)  were 
laid  on  Tremont  Street,  Boston,  in  1900;  and  after  the  pavement 
"  had  been  in  use  12  years  it  had  cost  absolutely  nothing  for  repairs, 
and  was  said  to  be  in  such  a  condition  that  it  would  probably  remain 
intact  for  10  years  longer.  It  is  stated  by  the  engineer  in  charge  of 
the  Boston  pavements  that  the  same  is  true  of  14,000  square  yards 
laid  at  about  the  same  time  and  in  the  same  way." 

"  In  the  Borough  of  Brooklyn,  the  first  creosoted  wood  pave- 
ment was  laid  in  1902,  without  any  guarantee,  and  has  cost  abso- 
lutely nothing  for  repairs.  Pavements  that  were  laid  later  and  have 
been  out  of  guarantee  from  3  to  4  years,  have  been  kept  in  repair  by 
the  Borough;  and  an  accurate  record  kept  of  their  cost.  Some  of 
these  pavements  have  cost  absolutely  nothing,  and  the  average  cost 
for  the  entire  area  out  of  guarantee  has  been  1.05  cents  per  square 
yard  per  year.  Many  of  these  pavements,  however,  have  been 
opened  for  sub-surface  work;  and  the  engineer  in  charge  of  pave- 
ments states  that  in  his  opinion  practically  all  of  the  repairs  are  due 
to  settlements  over  trenches  and  damage  caused  by  fires,  and  not  to 
actual  wear  and  tear  of  traffic." 

"  The  Borough  of  Manhattan  has  three  streets  which  have  been 
out  of  guarantee  three  years,  one  of  heavy  traffic,  one  of  medium 
traffic,  and  one  of  light  traffic.  The  heavy  traffic  street  has  cost  7 
cents  per  square  yard  per  year,  while  the  average  of  all  has  been 
6  cents  per  square  yard  per  year.  But  the  repairs  have  been  due  to 
wear  and  tear  only  on  the  heavy  traffic  street,  which  is  a  wholesale 
street  in  the  business  section.  Repairs  on  the  other  streets  are  due  to 
settlements  over  trenches,  and  damage  caused  by  fire;  and  prac- 
tically nothing  to  wear  and  tear  of  traffic." 

*  Engineering  and  Contracting,  Vol.  40  (1913),  p.  7-9. 


632  WOOD-BLOCK   PAVEMENTS  [CHAP.  XIX 

"  The  City  of  Minneapolis,  Minn.,  has  1,000,000  square  yards  of 
wood-block  pavements,  the  first  of  which  was  laid  in  1902.  The 
City  Engineer  states  that  these  pavements  have  required  practically 
no  repairs,  the  cost  in  1911  being  less  than  -&  cent  per  square  yard. 
He  also  states  (in  1913)  that  the  street  paved  in  1902  is  in  good  con- 
dition, and  looks  as  if  it  might  last  for  10  years  longer." 

"  In  St,  Louis,  Mo.,  in  1909,  the  repairs  to  50,000  square  yards 
of  wood  pavement  laid  in  1903  cost  $2.10;  and  in  1911  these  same 
50,000  square  yards  cost  less  than  A  cent  per  square  yard,  so  that 
the  total  cost  of  repairing  the  50,000  square  yards  of  wood  pavement 
the  first  nine  years  they  were  laid  was  3%  cent 'per  square  yard. 
These  pavements  are  all  on  light  traffic  streets." 


CHAPTER  XX 
SELECTING  THE  BEST  PAVEMENT 

1217.  KINDS    OF    PAVEMENTS.      Pavements    have    been    con- 
structed of  a  variety  of  materials;   but  the  forms  discussed  in  the 
preceding  chapters — hydraulic  concrete,  bituminous  concrete,  asphalt, 
brick,  stone  block,  and  wood  block — are  the  only  ones  of  importance 
now  constructed;  and  it  is  improbable  that  any  other  paving  material 
of  value  will  be  introduced.     From  time  to  time  notices  appear  in 
the  general  newspapers  of  the  introduction  of  some  new  pavement. 
Among  the  new  paving  materials  of  which  somewhat  laudatory 
notices  have  appeared  are  compressed  hay,  devitrified  glass,  cork,  and 
rubber.     All  such  novelties  are  either  an  attempt  of  an  eccentric 
inventor  to  sell  his  goods,  or  a  construction  to  meet  limited  and 
peculiar  conditions.     For  example,  it  has  been  stated  that  rubber 
has  been  tried  as  a  paving  material  in  London;  but  the  facts  are  that 
it  has  been  used  only  to  the  extent  of  300  or  400  square  feet  in  a  hotel 
porte  cochere. 

1218.  Table  75,  page  634,  shows  the  number  of  miles  and  the 
percentages  of  the  different  kinds  of  pavements  in  the  158  cities 
having  a  population  of  over  30,000  in  1909.    These  data  are  the  same 
as  those  in  the  table  on  page  320. 

It  is  interesting  to  note  that  (1)  practically  one  half  of  all  the 
pavements  in  Table  75  are  in  the  16  cities  having  a  population  of 
300,000  or  over;  (2)  two  thirds  of  the  asphalt  pavements  are  in  cities 
having  a  population  of  300,000  or  over,  and  that  one  third  of  this 
amount  is  in  New  York  City;  (3)  New  York  City,  Indianapolis  and 
Minneapolis  have  more  than  one  half  of  the  creosoted  wood-block 
pavements;  and  (4)  nearly  one  half  of  the  water-bound  macadam  is 
in  cities  having  a  population  of  over  300,000. 

Table  76,  page  635,  shows  the  percentages  of  the  different  kinds 
of  pavements  for  three  different  dates  in  the  larger  cities  of  the 
United  States.  These  data  are  interesting  as  showing  the  progressive 

633 


634 


SELECTING  THE  BEST  PAVEMENT 


[CHAP,  xx 


TABLE  75 

PERCENTAGES  OP  DIFFERENT  KINDS  OF  PAVEMENTS* 
In  1909  in  cities  having  a  population  of  30,000  or  over 


Ref. 
No. 

Kinds  of  Pavement. 

Length,  Miles. 

Per  Cent. 

1 

Asphalt-sheet               

4293 

20.4 

0 

block                              

261 

1  2 

3 

Bitulithic  .            

192 

0.9 

4 

Brick                  

2807 

13.4 

5 

Cobble  stone                        

5361 

2  6 

« 

Concrete  —  Portland  cement 

25 

0  1 

7 

G  ravel  —  water-bound 

2  556 

12  2 

Q 

bituminous-bound 

274 

1  3 

q 

Ivlacadam  —  water-bound 

6325 

30  1 

10 

tar-bound.                

142 

00  7 

11 
12 

portland-cement  grouted  
Stone  block                       

16 
2596 

0.08 
12  4 

13 

Wood  block  —  creosoted  

156 

0  7 

14 

untreated 

6142 

2  9 

it; 

Other  kinds 

211 

1  0 

Total 

21  004 

100  0 

Nearly  half  in  Baltimore. 


2  More  than  half  in  Chicago. 


changes  in  the  percentages  of  the  different  forms  of  pavements. 
For  example,  note  the  increase  in  the  percentage  of  asphalt  pave- 
ments, and  the  decrease  in  cobble-stone.  Notice  that  no  brick  pave- 
ments were  reported  separately  in  1890.  It  is  interesting  to  note 
that  the  percentages  of  water-bound  macadam  and  stone-block 
pavements,  the  extremes  as  to  durability,  remained  nearly  stationary. 
The  increase  in  the  total  number  of  miles  of  pavements,  is  shown 
below. 


YEAR. 

NUMBER  OF  CITIES. 

CITIES  HAVING  POPU- 
LATION OF  OVER. 

TOTAL  MILES  OF 
PAVEMENT. 

1890 
1901 
1909 

262 
135 
158 

10000 
30000 
30000 

12453 
15099 
21004 

Doubtless  if  Table  76  were  brought  up  to  date,  there  would 
be  some  material  changes.  For  example,  cobble-stone  pavements 
would  practically  disappear,  portland-cement  concrete  would  greatly 
increase,  bituminous  concrete  (other  than  bitulithic)  would  appear 
in  the  list,  a  considerable  proportion  of  the  water-bound  gravel  and 
macadam  pavements  would  change  to  bituminous  bound,  and  un- 
treated wood  block  would  nearly  disappear. 

*  Compiled  from  "General  Statistics  of  Cities  for  1909,"  Bureau  of  Census,  Washington, 
D.  C.r  1913,  p.  154~59. 


ART.  1] 


THE  DATA  FOR  THE  PROBLEM 


635 


TABLE  76 
PERCENTAGES  OF  DIFFERENT  KINDS  OF  PAVEMENTS  AT  DIFFERENT  DATES 


Ref 

p 

ERCENTAGE8    I 

f 

No. 

18901 

1901  2 

1909  » 

1 

Asphalt-sheet   { 

3.2 

13     6 

21  6 

2 

3 

block  J  ' 
Bitulithic                 

0  9 

Brick 

7  9 

13  4 

K 

Cobble-stone 

15  1 

6  8 

2  6 

6 

Concrete  —  portland  cement. 

0  1 

7 

Gravel  —  water-bound 

31  0 

14  7 

12  2 

g 

bituminous-bound 

1  3 

9 

Macadam  —  water-bound 

27  0 

30  6 

30  1 

10 

tar-bound. 

0  7 

11 

portland-cement  grouted.  . 

0  08 

12 

Stone  block.          

12.1 

13.4 

12  4 

13 

Wood-block  —  creosoted  

0.7 

14 

untreated. 

7  8 

8  7 

2  9 

15 

Other  kinds 

2  9 

4  3 

1  0 

Total                     

100  0 

100  0 

100  0 

1  Compiled  from  "Social  Statistics  of  Cities,  Eleventh  U.  S.  Census,  1890,"  p.  15-16. 

2  Compiled  from  "Statistics  of  Cities,  Bull.  No.  36,  U.  S.  Dept.  of  Labor,  Sept.,  1910," 
p.  876-79. 

3  Compiled  from  "General  Statistics  of  Citiea  for  1909,"  U.  S.  Bureau  of  Census,  Washing- 
ton, D.  C.,  1913,  p.  154-55. 

ART.  1.    THE  DATA  FOR  THE  PROBLEM 

1219.  DURABILITY  OF  PAVEMENTS.     The  durability  or  life  of  a 
pavement  is  the  most  important  factor  in  determining  which  is  the 
best  pavement.     The  durability  of  a  perishable  paving  material,  as 
untreated  wood  and  to  some  extent  macadam  and  asphalt,  depends 
upon  both  the  climate  and  the  traffic;  but  in  general  the  durability 
of  paving  materials  depends  chiefly  upon  the  amount  of  the  travel, 
and  consequently  the  durability  of  different  pavements  can  be  accu- 
rately compared  only  when  the  nature  and  the  amount  of  the  travel 
over  each  is  known.     Unfortunately  there  are  very  little  definite 
data  as  to  the  amount  of  travel  upon  American  pavements.     Not 
infrequently  the  travel  on  a  particular  pavement  is  referred  to  as 
being  "  heavy  "  or  "  light,"  but  such  general  terms  are  practically 
worthless  in  comparing  the  durability  of  different  kinds  of  pavements. 

1220.  Travel  Census.     Although  data  on  the  use  made  of  pave- 
ments are  of  vital  importance  in  attempting  to  compare  the  relative 
durability  of  different  paving  materials,  comparatively  few  obser- 
vations have  been  made  concerning  the  travel  upon  American  pave- 


636  SELECTING   THE   BEST  PAVEMENT  [CHAP.    XX 

ments.  For  a  discussion  of  the  causes  that  have  led  to  this  surprising 
result,  see  §640-42  (page  321-24).  For  a  statement  of  the  im- 
portance of  a  travel  census  in  considering  the  cost  of  construction 
and  maintaining  a  road  or  pavement,  see  §  29  (page  25).  For  a  brief 
account  of  some  observations  made  concerning  the  nature  and 
amount  of  the  travel  on  rural  roads,  see  §  30-33  (page  26-28);  and 
for  a  brief  reference  to  the  few  censuses  that  have  been  taken  of  travel 
on  American  streets,  see  §  34  (page  28). 

In  some  respects  the  most  elaborate  census  of  street  travel  taken 
in  this  country  was  that  made  by  the  Barber  Asphalt  Paving  Com- 
pany in  1885.  Table  77,  page  637,  shows  the  results.  It  is  not  worth 
while  to  describe  the  methods  employed  in  making  the  observations 
or  in  computing  the  results,  since  the  data  are  very  greatly  out  of 
date  owing  to  radical  changes  in  both  the  character  and  the  amount 
of  the  travel.  For  example,  in  St.  Louis  from  1914  to  1915,  the  total 
travel  on  certain  business  streets  increased  20  per  cent,  the  motor- 
driven  traffic  increasing  53  per  cent  and  the  horse-drawn  decreasing 
15  per  cent.*  However,  apparently  the  data  in  Table  77  are  the 
most  elaborate  that  have  yet  been  published.  Table  77  is  instruc- 
tive as  showing  the  great  variation  in  the  travel  on  different  streets 
of  any  particular  city  and  also  of  different  cities. 

1221.  Table  78,  page  638,  gives  the  travel  record  of  certain  streets 
in  London  and  Liverpool.     The  marked  difference  in  the  travel  on 
the  pavements  of  London  and  on  those  of  New  York  is  due  chiefly 
to  the  use  of  omnibuses  in  London  and  street  cars  in  New  York  City. 
This  example  illustrates  the  importance  of  having  definite  data  as 
to  the  amount  of  travel;   and  also  shows  the  importance  of  taking 
account  of  local  conditions  in  attempting  to  compare  the  results  in 
one  city  with  those  in  another. 

1222.  It  is  desirable  that  engineers  in  charge  of  streets  should 
ascertain  by  direct  observation  the  amount  of  tonnage  passing  over 
each  pavement,  in  order  that  the  service  per  unit  of  cost  of  different 
pavements  may  be  accurately  compared.     The  only  measure  of  the 
durability  of  a  pavement  is  the  amount  of  travel  tonnage  it  will  bear 
before  it  becomes  so  worn  that  the  cost  of  replacing  it  is  less  than  the 
expense  incurred  by  its  use.     It  is  also  desirable  that  all  such  observa- 
tions should  be  made  in  accordance  with  a  standard  plan,  so  the 
results  from  different  cities  will  be  comparable  (see  §  35-38). 

1223.  Elements  Modifying  Durability.    Although  the  effect  of 

*  Engineering  News,  Vol.  76,  (1916),  p.  832-34. 


ART.   1] 


THE  DATA  FOR  THE  PROBLEM 


637 


TABLE  77 
TRAVEL  ON  CERTAIN  STREETS  IN  VARIOUS  AMERICAN  CITIES  IN  1885* 


Ret. 
No. 

LOCALITY. 

«! 

"o'l 

JS  C 

-S  g 

s 

NUMBER  OF  TONS. 

City. 

Street. 

Total 
per  Day. 

Per 
Vehicle. 

II  * 

fc&° 

£* 

1 
2 

3 

4 
5 
6 

7 
8 
9 
10 

11 
12 

13 
14 
15 
16 

17 
18 
19 
20 
21 

22 
23 

24 
25 
26 
27 

28 
29 
30 
31 

32 
33 
34 

35 
36 

New  York  .  . 

u       it 

<t       a 

Philadelphia 

« 

Chicago  .... 

It 

U 

l( 
Boston 

Broadway,  near  Pine  
Fifth  Ave.,  opp.  Worth  Monum't. 
Wall,  corner  of  Broad. 

40 
40 

27 

65 
65 
26 

50 
45 
36 
38 

27 
32 
26 
39 
26 
24 

36 
50 
36 
36 
36 

40 
30 

70 
50 
50 
60 

56 
42 
38 
50 

61 
36 
35 

63 
60 

10  905 

3744 
2357 

9237 
6302 
1928 

7561 
6398 
2756 
2604 

5301 
5028 
3265 
2938 
1  130 
744 

3691 
3618 
2554 
942 
259 

6204 
1065 

4622 
1  688 
1455 
1289 

2613 
1505 
825 
714 

4176 
2402 
977 

2967 
1  449 

1.39 
.68 
1.00 

1.52 
1.24 
1.06 

2.08 
1.46 
.90 
1.11 

.99 
1.02 
.93 
.80 
.79 
.67 

1.13 
1.23 
1.16 
.90 

.84 

1.81 
.94 

1.02 

.87 
.88 
1.01 

.83 
1.88 
1.47 
1.24 

1.25 
1.05 
1.05 

.62 
.59 

273 
94 

87 

142 
97 

74 

151 
142 

77 
69 

196 
157 
126 
75 
43 
32 

103 
72 
70 
27 

7 

155 
35 

66 
34 
29 
21 

47 
36 
22 
14 

69 
67 

28 

47 
24 

Broad,  in  front  of  P.R.R.  Station 
Filbert,  in  front  of  City  Hall.  .  .  . 
Chestnut,  corner  of  Fourth  

Wabash,  near  Lake 

Clark,  near  Madison.  . 

La  Salle,  near  Locust  
Dearborn,  opp.  Washington  P'k  . 

Devonshire,  opp.  Post  Office  .... 
Devonshire,  near  Milk 

(t 

Kilby,  near  State  

u 
ii 

St.  Louis..  .  . 

«     « 

«     K 
(t     a 

u       « 

New  Orleans 

u            u 

Washington. 
(i 

Buffalo.  .  . 

Washington  
Arch,  near  Summer  

Court  Square  

Locust,  near  Beaumont  

Broadway,  near  Olive  
Pine,  near  Garrison. 

Chestnut,  near  Beaumont 

Olive,  near  Beaumont  

Tchoupitoulas,  near  Poydras.  .  .  . 
St.  Charles,  near  Washington  .  .  . 

15th,  opposite  Treasury  

9th,  between  D  and  E  

7th,  between  D  and  E  
6th,  between  Pa.  Ave.  and  B.  .  .  . 

Main,  near  Swan 

« 

Main,  near  Bouck  Ave 

« 
it 

Louisville.  .  . 

u 
(I 

Omaha  

« 

Lin  wood,  near  Ferry. 

Main,  near  Glenwood  
Main,  near  3d 

8th,  near  Walnut  

7th,  near  Jefferson 

Douglass,  near  15th. 

Farnham   near  14th 

*  Trans.  Ainer.  Soc.  of  Civil  Engra.,  Vol.  15  (1886),  p.  123-38. 


638 


SELECTING   THE    BEST   PAVEMENT 


[CHAP.    XX 


TABLE  78 
TRAVEL  ON  CERTAIN  STREETS  IN  LONDON  AND  LIVERPOOL  IN  1879* 


Ref. 
No. 

LOCALITY. 

PAVEMENT. 

NUMBER  OF  TONS. 

City. 

Street. 

Kind. 

a 

51 
3fe 
£ 

Per  Day. 

Per 

Vehicle. 

ii* 

*|a 

r* 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 

18 
19 
20 
21 

London.  .  . 
(i 

u 
n 
it 

(i 

a 
K 
ti 
n 
n 
(( 
u 

u 

Liverpool.  . 

« 

Gracechurch  
King  William  
Poultry  

Asphalt 
Wood 
Asphalt 
Wood 
Macadam 
Wood 
Asphalt 

Macadam 
Granite 
Wood 
Granite 

u 

Macadam 
Wood 
Macadam 
Wood 

Granite 
ti 

Wood 

a 

32 
40 
22 
37 
45 
57 
32 
30 
37 
44 

'32 
43 
52 

13507 

16484 
8330 
13596 
14380 
17076 
9260 
7588 
9358 
10658 

6  506 
8376 
9668 

1.11 

1.06 
1.02 

.84 
1.01 
1.01 
.98 
1.08 
.87 
.88 

i  '  02 

1.02 
.90 

422 
412 
378 
367 
322 
300 
290 
253 
253 
242 
216 
203 
195 
186 
156 
145 
93 

382 
232 
231 
100 

Strand  and  Fleet.  .  . 
Parliament 

Oxford 

Cheapside 

Leadenhall  
Piccadilly  
Euston  
Brompton  

King  William  

Edgeware 

Regent. 

King's  

Victoria  
Sloane  

40 

5780 

.96 

(Not  named)  

U                  U 

Great  Howard.  .  .  . 
Bold  

travel  is  dependent  chiefly  upon  the  number  of  tons  per  foot  of  width, 
its  influence  is  modified  somewhat  by  (1)  the  character  of  the  pave- 
ment, (2)  the  state  of  repairs,  (3)  the  degree  of  cleanliness,  (4)  the 
presence  or  absence  of  car  tracks,  (5)  the  width  of  pavement,  (6) 
the  character  of  the  travel,  and  (7)  the  climate. 

1.  The  durability  of  a  particular  kind  of  pavement  will  vary 
with  the  details  of  the  method  of  construction.     The  foundation 
may  be  more  qr  less  rigid,  the  materials  may  differ  greatly  in  dura- 
bility, with  any  form  of  block  pavement  the  joints  may  be  more  or 
less  open,  and  the  surface  may  also  vary  more  or  less  in  roughness. 

2.  The  durability  will  depend  upon  the  care  employed  in  repairing 
the  pavement.     If  holes,  depressions,  or  ruts  are  allowed  to  remain 
for  any  length  of  time,  whatever  the  material  the  pavement  will 
wear  abnormally  fast. 

3.  The  degree  of  cleanness  will  materially  modify  the  durability 


*  Trans,  Amer.  Soc.  of  Civil  Eng'rs,  V<?1.  15,  (1886),  p.  131, 


ART.  1]  THE  DATA  FOR  THE  PROBLEM  639 

of  a  pavement.  An  imperishable  material  is  benefited  by  a  cov- 
ering of  detritus,  since  it  serves  as  a  carpet  to  protect  the  pave- 
ment; and  if  the  covering  is  heavy  enough,  the  pavement  virtually 
becomes  a  foundation  and  is  entirely  protected  from  wear.  On  the 
other  hand,  the  decay  of  a  perishable  material,  as  wood  and  asphalt, 
is  hastened  by  a  covering  of  street  dirt  which  collects  moisture  and 
hastens  the  decay  and  disintegration  of  the  pavement. 

4.  The  presence  of  a  street-car  track  on  a  street  concentrates 
traffic  at  the  two  sides,  thus  virtually  narrowing  the  street,  and  also 
causes  the  travel  to  go  substantially  in  one  track,  a  result  which 
is  particularly  destructive  of  gravel  and  macadam  roads. 

5.  The  wider  a  pavement  the  more  evenly  will  it  wear,  and  conse- 
quently the  longer  it  will  last.     If  several  irregular  lines  of  travel  can 
be  maintained,  the  wear  will  be  much  more  even  and  the  durability 
greater  than  if  the  vehicles  are  restricted  to  practically  a  single  line. 

6.  The  durability  of  the  pavement  will  vary  with  the  weight  per 
unit  width  of  tire,  the  method  of  shoeing  the  horses,  and  the  rapidity 
of  the  travel.     In  Europe,  the  weight  per  unit  of  width  of  tire  is 
generally  regulated  by  law,  and  calks  on  the  horses'  shoes  are  pro- 
hibited ;  but  in  America  there  are  no  such  restrictions.     Rapid  travel 
is  more  destructive  to  a  block  pavement  than  slow  travel. 

7.  The  climate  affects  the  durability  of  several  kinds  of  pave- 
ment.    The  durability  of  an  untreated  wood  pavement  is  affected  by 
heat  and  moisture  conditions,  that  of  macadam  and  gravel  by  mois- 
ture and  winds,  and  that  of  asphalt  by  moisture,  particularly  by 
street  sprinkling. 

1224.  There  are  two  facts  of  a  somewhat  different  character 
that  should  not  be  overlooked  in  attempting  to  determine  the  life  of 
a  pavement. 

1.  The  average  wear  does  not  determine  the  life  of  a  pavement, 
since  even  the  most  carefully  constructed  pavements  wear  so  unevenly 
as  to  require  re-laying  before  the  wearing  coat  is  entirely  worn  out. 
This  is  true  of  sheet  asphalt  and  gravel  and  macadam  (both  water- 
bound  and  bituminous),  which  have  a  comparatively  thin  wearing 
coat;  and  is  particularly  true  of  pavements  made  of  blocks,  as  brick, 
stone  and  wood,  since  the  edges  of  the  blocks  wear  off  and  leave 
the  top  face  rounded,  and  when  the  pavement  reaches  this  stage  the 
wear  is  much  more  rapid  than  previously. 

2.  In  a  block  pavement  the  blocks  must  have  a  certain  depth  to 
enable  them  to  keep  their  place;  and  consequently  bricks  and  shallow 
wood  blocks  can  not  be  worn  more  than  about  half-way  through. 


640 


SELECTING   THE   BEST   PAVEMENT 


[CHAP.   XX 


If  the  blocks  are  made  deeper,  the  durability  of  the  pavement  is 
not  increased  much,  if  any,  since  owing  to  unequal  wear  the  pave- 
ment must  be  re-laid  before  any  considerable  depth  is  worn  off. 

Asphalt  and  macadam  have  some  decided  economic  advantages 
over  other  forms  of  pavements,  since  the  wearing  coat  is  compara- 
tively thin  and  can  be  replaced  when  it  is  worn  out  or  wears  rough, 
without  proportionally  as  much  loss  as  when  a  block  pavement  is  re- 
surfaced. A  further  economic  advantage  of  these  pavements  is 
that  when  holes  begin  to  form,  a  patch  may  be  applied  and  thus  the 
uniformity  of  the  surface  may  be  preserved  and  the  life  of  the  pave- 
ment be  extended. 

1225.  Data  on  Life  of  Pavements.     Until  more  complete  data 
concerning  the  volume  of  travel  on  pavements  and  the  amount  of 
wear  are  obtained,  it  will  be  impossible  to  make  any  reliable  estimates 
as  to  the  durability  of  different  paving  materials.     At  present  the 
best  that  can  be  done  is  to  accept  the  estimate  of  those  most  com- 
petent to  give  an  intelligent  opinion. 

1226.  Asphalt,  Brick,  and  Wood.    Table  79  gives  the  estimated 
life  of  sheet  asphalt,  brick,  and  wood-block  pavements  as  reported 
to  the  U.  S.  Census  Bureau.    An  extensive  list  was  given  for  asphalt 

TABLE  79 
ESTIMATED  LIFE  OF  PAVEMENTS* 


Ref. 

No. 

City. 

SHEET  ASPHALT. 

BRICK. 

WOOD-BLOCK. 

fl* 

c  2 

fS 

W 

SS 

c'E 

o  -^ 

fi 

K 

i| 

si  2 

|S 

PQ 

11 

dj  -*i 

;§• 

'S5  Q 
tf 

i! 

o  2 

fa 

PQ 

§i 

c'C 
v  += 

Is 

PH 

1 
2 
3 
4 
5 
6 
•    7 
8 
9 
10 
11 
12 
13 
14 
15 
16 

New  York  City,  N.  Y  
Boston,  Mass  

12 
10 

15 
15 
14 
12 
10 
14 
20 
14 
25 

11 
5 

"l6" 

11 

10 
25 
25 

15 
11 

9 
13 

"l7" 
15 
10 
10 

8 

"30" 
25 
15 

20 
20 

30 
18 

15 
30 

20 

Cleveland  O 

Indianapolis,  Ind  

10 
5 
11 
10 
11 
15 
12 
12 
10 
10 
18 
12 
7 

Portland,  Ore 

Columbus,  Ohio. 

18 
10 
12 

18 
25 
25 

Toledo,  Ohio. 

Atlanta  Ga 

Oakland,  Calif  

Cambridge,  Mass  
Springfield,  Mass  
Holyoke,  Mass. 

12 
15 
15 
20 
12 

ii 

12 

18 

"25" 
20 

10 
15 
18 
20 
10 

South  Bend,  Ind  
Saginaw,  Mich  . 

Sacramento,  Calif  
Galveston,  Tex  

11 

>      *  General  Statistics  of  Cities  for  1909,  Bureau  of  the  Census,  1913,  Washington,  D.  C. 
p.  69-70. 


ART.  1]  THE  DATA  FOR  THE  PROBLEM  641 

and  brick,  but  only  a  brief  list  for  wood-block;  and  only  those  in 
the  former  list  are  presented  in  Table  79,  for  which  data  were 
given  also  for  wood-block. 

Of  course,  the  life  of  a  pavement  will  depend  upon  the  specifica- 
tions and  the  details  of  the  construction,  which  can  not  be  pre- 
sented in  a  tabular  statement.  Further,  the  life  of  a  pavement 
depends  upon  the  extent  and  character  of  the  annual  repairs.  It  is 
possible  to  keep  some  types  of  pavements,  as  for  example  sheet 
asphalt,  going  almost  perpetually  by  patching;  while  with  other 
forms,  as  for  example  brick,  it  is  not  possible  to  greatly  prolong  the 
life  of  the  pavement  by  patching. 

1227.  Granite.     The  census  report  from  which  the  data  in  Table 
79  were  taken,  contained  no  statistics  for  the  life  of  stone-block  pave- 
ments.    The  following  data  to  some  degree  supply  that  lack. 

Mr.  Samuel  Whinery,  a  competent  authority,  estimates  the  life 
of  granite-block  pavements  in  Boston  as  follows :  * 

A,  central  business  streets  having  a  very  large  volume  of  the  heaviest 

travel 13  years 

B,  secondary  business  streets  with  travel  of  moderate  volume  and  weight .   17  years 

C,  main  city  or  suburban  thoroughfares  having  a  large  volume  of  com- 

paratively light  travel .16     " 

D,  suburban  or  residence  streets  where  the  travel  is  mainly  of  a  local 

character  and  light 20     " 

Mr.  George  W.  Tillson,  Consulting  Engineer,  Borough  of  Brook- 
lyn, estimates  the  life  of  a  granite-block  pavement  at  25  years,  f 

The  Bureau  of  Municipal  Research  of  Cincinnati,  Ohio,  estimates 
the  life  of  granite  blocks  at  25  years.f 

1228.  Water-bound  Macadam.     Macadam  varies  in  quality  more 
widely  than  any  of  the  preceding  forms  of  pavements,  and  hence 
there  is  a  wider  range  in  its  estimated  life. 

Mr.  Whinery  estimates  the  life  of  trap-rock  water-bound  mac- 
adam for  the  four  classes  of  streets  mentioned  in  the  second  para- 
graph of  §  1227,  as  follows: 

A,  5  years;  B,  6  years;  C,  7  years;  and  D,  10  years. 

The  Bureau  of  Municipal  Research  of  Cincinnati  estimates  the 
life  of  limestone  water-bound  macadam  at  8  years,  t 

*  Report  to  the  Finance  Committee  of  Boston,  Mass.,  Engineering  and  Contracting,  Vol.  73 
(1910),  p.  30-31. 

t  Paper  presented  to  International  Municipal  Congress,  Chicago,  Engineering  and  Con- 
racting,  Vol.  36  (1911),  p.  405. 

t  Municipal  Engineering,  Vol.  42  (1912),  p.  459. 


642  SELECTING   THE    BEST   PAVEMENT  [CHAP.    XX 

1229.  REQUIREMENTS  OF  AN  IDEAL  PAVEMENT.    The  perfect 
pavement  is  an  ideal  which  will  never  be  attained,  since  some  of 
the  qualities  required  in  a  perfect  pavement  are  antagonistic  to 
each  other.     For  instance,  perfect  durability  would  require  a  pave- 
ment without  friction,  for  friction  causes  wear  and  ultimately  destruc- 
tion of  the  pavement;  but  without  friction  there  could  be  no  adequate 
foothold  for  horses  drawing  loads.     Again,  to  be  the  least  injurious 
to  horses,  a  pavement  should  be  soft  and  yielding;   but  a  soft  and 
yielding  pavement  is  opposed  to  ease  of  traction.     The  conditions  to 
be  fulfilled  by  the  ideal  pavement  will  first  be  considered;  and  sub- 
sequently an  attempt  will  be  made  to  estimate  the  degree  to  which 
each  kind  of  pavement  approximates  the  perfect  ideal. 

A  perfect  pavement  should  satisfy  the  following  conditions: 

1.  It  should  be  low  in  first  cost. 

2.  It  should  be  durable,  i.  e.,  the  cost  of  perpetually  maintaining 
its  surface  in  good  condition  should  be  small. 

3.  It  should  have  a  smooth,  hard  surface,  so  as  to  have  a  low 
tractive  resistance. 

4.  It  should  afford  a  good  foothold  to  enable  horses  to  draw 
heavy  loads,  and  to  prevent  them  from  slipping  and  falling  and 
possibly  injuring  themselves  and  blocking  traffic. 

5.  It  should  be  smooth,  so  as  to  be  easily  cleaned. 

6.  It  should  be  comparatively  noiseless. 

7.  It  should  be  impervious,  so  as  to  keep  in  good  sanitary  condition. 

8.  It  should  yield  neither  dust  nor  mud. 

9.  It  should  be  comfortable  to  those  who  ride  over  it. 

10.  It  should  not  absorb  heat  excessively. 

Each  of  the  ordinary  forms  of  pavements  will  be  considered  under 
each  of  the  above  requirements. 

1230.  Cost  of  Construction.    The  cost  of  construction  of  a  pave- 
ment varies  with  the  specifications,  the  character  of  the  work,  and 
the  locality.     For  detailed  data  on  this  subject,  see  the  several  kinds 
of  pavements  in  the  preceding  chapters. 

Since  the  subject  under  consideration  in  this  chapter  is  a  com- 
parison of  the  different  forms  of  wearing  coats,  the  cost  to  be  con- 
sidered here  is  that  of  the  pavement  exclusive  of  the  expense  for 
curbs,  gutters,  catch-basins,  etc.  To  have  data  for  use  in  sample 
computations  later  in  this  chapter,  the  cost  of  the  best  grade  of  each 
of  the  several  kinds  of  pavements  is  assumed  to  be  as  follows: 

1.  Asphalt,  sheet $1 . 75  per  sq.  yd. 

2.  Brick 1 .80     "     "     " 


ART.  1]  THE  DATA  FOR  THE  PROBLEM  643 

3.  Granite  block 3 . 00  per  sq.  yd. 

4.  Macadam,  water-bound 1 . 25 

5.  Wood  block,  creosoted 2.75 

1231.  Cost  of  Maintenance.     By  the  cost  of  maintenance  is 
meant  the  expenditure  necessary  to  keep  the  pavement  in  practically 
as  good  a  condition  as  when  it  was  new.     Unfortunately  there  is  a 
greater  dearth  of  exact  information  under  this  head  than  for  almost 
any  other  phase  of  pavement  construction.     For  a  general  state- 
ment of  the  causes  of  this  lack,  see  §  640^3  (page  321-26) ;  and  for  a 
more  detailed  explanation  why  there  is  little  accurate  information 
on  the  cost  of  repairs  of  asphalt  pavements,  which  reasons  substan- 
tially apply  also  to  all  kinds  of  pavements,  see  §  881-82  (page  455-57). 

The  cost  of  maintenance  consists  of  two  distinct  elements,  viz.: 
(1)  the  cost  of  repairing  or  patching  of  small  areas;  and  (2)  the  cost 
of  renewing  or  re-surfacing  large  areas.  Each  will  be  considered 
separately. 

1232.  Cost  of  Repairs.    The  cost  of  repairs  is  here  used  as  mean- 
ing the  cost  of  correcting  any  defects  that  may  have  developed 
through  use.     Not  infrequently  this  term  is  used  to  include  also  the 
expense  of  re-laying  or  restoring  the  surface  of  the  pavement  after 
it  has  been  opened  to  lay  or  repair  sewers,  water  or  gas  pipes,  etc. 
In  one  sense  the  cost  of  restoring  the  surface  after  the  pavement  has 
been  opened  is  part  of  the  cost  of  maintenance;    but  the  cost  of 
restoring  the  surface  is  practically  independent  of  the  quality  or  life 
of  the  pavement,  and  hence  should  not  be  included  in  comparing  the 
economic  value  of  the  different  pavements. 

Unfortunately,  not  many  municipalities  keep  accurate  accounts 
of  the  cost  of  repairing  and  restoring  pavements,  and  very  few 
separate  the  costs  of  patching  and  restoring;  and  hence  there  are 
almost  no  reliable  data  on  the  cost  of  repairs. 

Further,  many  of  the  accounts  purporting  to  show  the  cost  of 
repairs  contain  one  or  both  of  the  two  serious  errors  discussed  in  the 
two  succeeding  sections. 

1233.  REPAIRS  vs.  NEW  CONSTRUCTION.     In  computing  the  cost 
of  repairs  of  roads  and  pavements,  it  is  common  to  fail  to  discrim- 
inate between  the  renewal  of  an  old  surface  and  the  substitution 
of  a  new  and  better  surface;    or,  in  other  words,  it  is  common  to 
charge  to  maintenance  an  item  which  should  be  charged  to  new 
construction    (see   paragraph  6,  §  881,  page  455).     For  example,  if 
some  form  of  bituminous  wearing  coat  is  applied  to  an  old  water- 
bound  macadam  road,  the  new  surface  is  not  a  renewal  of  the  old  one, 


644  SELECTING   THE   BEST   PAVEMENT  [CHAP.    XX 

but  is  the  construction  of  a  new  pavement  surface  upon  the  old  one 
as  a  foundation;  and  consequently  the  cost  of  the  new  construction 
should  not  be  regarded  as  the  cost  of  repairs  to  the  old  pavement. 
This  mistake  in  almost  the  exact  form  of  the  above  example  has 
often  been  made  in  discussions  of  highway  economics.  For  example, 
it  is  often  stated  that  the  cost  of  road  maintenance  has  recently 
taken  an  enormous  leap  upward  due  to  the  advent  of  the  automo- 
bile; while  the  truth  is  that  the  advent  of  the  automobile  has  caused 
the  substitution  of  a  more  expensive  form  of  road  construction,  and 
little  or  nothing  is  known  as  to  the  cost  of  maintenance  of  the  new 
form  of  construction,  partly  because  the  methods  of  both  construc- 
tion and  maintenance  have  been  experimental,  and  partly  because 
not  enough  time  has  elapsed  to  secure  trustworthy  data. 

Because  of  the  above  error,  many  estimates  of  the  cost  of 
annual  repairs  are  radically  wrong,  as  also  the  resulting  conclusions. 
Errors  of  this  character  have  for  somewhat  obvious  reasons  been 
more  common  in  connection  with  surfaces  suitable  for  light  travel 
than  for  heavy  travel. 

1234.  AGE  OF  PAVEMENT.  Sometimes  an  error  is  made  by  not 
duly  considering  the  age  of  the  pavement.  During  the  first  part  of 
the  life  of  a  pavement,  there  should  be  but  few,  if  any,  repairs; 
and  hence  to  secure  a  trustworthy  value  for  the  annual  cost  of  repairs, 
an  average  should  be  taken  for  a  number  of  years,  the  exact  number 
being  proportional  to  the  life  of  each  particular  pavement. 

Obviously,  the  greater  the  area  of  the  pavements  included  and 
the  greater  the  number  of  years  the  better;  but  it  is  not  correct  to 
simply  take  an  average  of  the  annual  cost  of  repairs  for  each  of  sev- 
eral pavements  of  different  ages  and  areas.  Assume  that  the  areas 
and  ages  of  the  several  pavements  are  as  below;  that  the  total  cost 
of  repairs  for  each  pavement  is  known ;  and  that  the  correct  average 
annual  cost  of  repairs  is  desired.* 

AREA,  Sq.  Yd.  AGE,  Years.  YARDS  XYEARS. 

10  000  16  160  000 

8  000  12  96  000 

7  000  10  70  000 

8  000  8  64  000 


33  000  390  000 

Multiplying  the  area  by  the  corresponding  age  gives  the  number 
of  yard-years  that  have  been  repaired,  i.  e.,  gives  the  equivalent  of 

*  Engineering  and  Contracting,  Vol.  37  (1912),  p.  311, 


ART.  1]  THE  DATA  FOR  THE  PROBLEM  645 

the  number  of  yards  that  have  been  maintained  for  1  year.  The 
total  cost  of  repairs  divided  by  the  total  yard-years  (390,000)  will 
give  the  correct  average  annual  cost  of  repairs.  The  weighted  average 
age  of  the  pavement  in  the  above  example  is:  390,000 -j- 33,000  = 
11.8  years. 

1235.  DATA  ON  COST  OF  REPAIRS.  For  a  few  data  on  the  cost 
of  the  different  kinds  of  pavements,  see  the  preceding  pages,  as 
follows:  Sheet  asphalt,  Table  47,  page  457,  and  Fig.  162,  page  459; 
brick,  §  1069,  page  564;  wood  block,  §  1214,  page  631. 

Table  80  shows  the  best  general  data  available,  although  it  is 
not  certain  that  these  values,  or  the  ones  referred  to  in  the  paragraph 
above,  are  free  from  the  two  errors  mentioned  in  the  two  pre- 
ceding sections.  Table  80  is  given  primarily  to  have  data  for  use  in 
sample  computations  to  be  presented  later. 

TABLE  80 
ASSUMED  AVERAGE  ANNUAL  COST  OF  REPAIRS 


Kind  of  Pavement. 

CENTS  PER  SQ.  YD.  PER  ANNUM. 

Heavy  Travel. 

Light  Travel. 

Asphalt  sheet,  —  see  §  886  

5.0 
3.0 
10.0 
2.0 
4.0 

3.5 
2.0 
2.0* 
2.0 
2.0 

Brick,—  see  §  1069  

Macadam,  water-bound  

Stone  block,  —  see  §  1142  

Wood  block,  creosoted,  —  see  §  1214  

1236.  Cost  of  Renewal  The  second  element  in  the  cost  of  main- 
tenance of  a  pavement  is  the  cost  of  renewal.  Since  the  foundation 
or  base  of  a  modern  pavement  is  not  subjected  to  wear  or  disinte- 
gration, the  cost  of  renewal  is  only  the  cost  of  adding  a  new  wearing 
coat;  although,  not  infrequently,  the  cost  of  a  new  concrete  base  is 
erroneously  included  in  the  cost  of  renewal  (see  §  1233). 

The  annual  cost  of  renewal  is  that  sum  which  each  year  must  be 
placed  at  compound  interest  to  accumulate  a  sum  equal  to  the  esti- 
mated cost  of  renewal  at  the  end  of  the  life  of  the  wearing  coat  of  the 
pavement.  As  an  example,  it  will  be  assumed  that  the  cost  of 
renewal  is  desired  for  a  sheet  asphalt  pavement.  It  will  be  assumed 
that  the  pavement  costs  $1.75  per  square  yard  (§  1230),  and  that  the 
grading  and  concrete  base  cost  $1.00  and  the  wearing  coat  $0.75. 
The  latter  is  a  little  less  than  the  pro  rata  share  according  to  Table  45, 

*  Report  Dept.  pf  Public  Works,  Bureau  of  Engineering,  Buffalo,  N.  Y.,  1915-16,  p.  70. 


646  SELECTING   THE   BEST   PAVEMENT  [CHAP.    XX 

page  453;  but  round  numbers  are  sufficient  for  an  example.  The 
life  of  the  wearing  coat  will  be  taken  as  15  years,  which  is  probably 
too  small,  notwithstanding  the  data  in  Table  79,  page  640.  It  will 
be  assumed  that  the  life  of  the  concrete  base  is  30  years,  which  is 
probably  too  small;  and  that  its  removal  will  cost  20  cents  per  square 
yard.  Interest  will  be  assumed  at  3J  per  cent.  It  will  be  further 
assumed  that  the  removal  of  the  old  wearing  coat  will  cost  10  cents 
per  square  yard.  Finally,  to  find  the  annual  sum  to  be  placed  at 
interest,  a  sinking  fund  table  is  desirable. 

From  a  sinking-fund  table  it  is  found  that  $0.05182  deposited 
annually  at  3J  per  cent  interest  will  amount  to  $1.00  in  15  years; 
and  that  at  the  same  interest  $0.01937  will  amount  to  $1.00  in  30 
years.  Then  the  annual  cost  of  renewal  is  as  follows : 

TTFM  ANNUAL  COST 

Cts.  per  Sq.  Yd. 

Concrete  base:  renewal,  $0.75  X  0.01937 1 . 452 

removal,  20  cents  ^30 0 . 667 

Wearing  coat:  renewal,  $1.00  X  0.05182 5. 182 

removal,  10  cents  -*-  15 0 . 667 


Total 7.968 

In  some  cases  there  is  a  salvage  value  to  the  old  wearing  coat,  in 
which  case  its  value  after  removal  divided  by  the  life  of  the  pave- 
ment should  be  subtracted  from  the  annual  cost  found  as  above. 

1237.  Economic  Life  of  a  Pavement.     When  does  the  cost  of 
repairs  become  great  enough  to  justify  renewal,  that  is,  what  is  the 
economic  life  of  a  pavement?     This  subject  has  frequently  been  dis- 
cussed by  writers  on  road  and  pavement  economics,  and  generally 
the  method  employed  has  been  radically  wrong.     For  an  elaborate, 
correct,   and  instructive  discussion  of  this  subject,   see  Gillette's 
Hand-book  of  Cost  Data,  Second  Edition,  page  27-34. 

1238.  Tractive  Resistance.     Table  8,  page  21,  gives  the  tractive 
resistance  of  different  pavements,  from  which  it  is  seen  that  the 
rank  of  the  various  pavements  according  to  tractive  resistance,  in 
order  beginning  with  the  one  offering  the  least  resistance,  is  about  as 
follows:   portland-concrete,  sheet  asphalt  during  cold  weather,  brick, 
best  water-bound  macadam,  asphalt  during  warm  weather,  rectan- 
gular  wood   block,   good   gravel,    accurately   dressed   stone   block, 
ordinary  water-bound  macadam,  gravel,  roughly  dressed  stone  block. 
The  tractive  resistance  will  vary  greatly  with  the  state  of  repair  of 
the  surface. 

Many  attempts  have  been  made  to  compute  the  financial  advan- 


ART.  1] 


THE  DATA  FOR  THE  PROBLEM 


647 


tage  of  a  decreased  tractive  resistance;  but  it  is  impossible  to  deter- 
mine its  value  with  any  degree  of  accuracy,  although  it  is  certain 
that  the  tractive  resistance  of  the  pavements  of  a  city  are  impor- 
tant factors  in  determining  the  cost  of  conducting  transportation. 
Ease  of  traction  is,  however,  not  relatively  as  important  for  city 
pavements  as  for  country  roads,  since  in  the  latter  ease  of  traction 
is  a  matter  of  first  importance  (see  §  4-7),  while  in  the  former  it  is 
comparatively  unimportant  (see  §  634).  On  the  other  hand,  the 
cost  of  transportation  per  ton-mile  is  considerably  more  in  the  cities 
than  in  the  country. 

1239.  Slipperiness.  The  method  of  comparing  pavements  in 
this  respect  is  to  determine  the  distance  a  horse  travels  on  the  dif- 
ferent pavements  before  he  falls.  The  most  complete  observations 
made  in  the  United  States  to  ascertain  the  prevalence  of  accidents  on 
the  different  pavements  were  made  under  the  direction  of  Capt. 
F.  V.  Greene.*  The  observations  were  made  from  7  a.m.  to  7  p.m. 
on  six  consecutive  days  in  October  and  November,  1885,  in  ten  of  the 
leading  American  cities  on  thirty-three  streets  having  the  heaviest 
travel  for  each  kind  of  pavement  in  the  particular  city.  The  number 
of  horses  observed  on  sheet  asphalt  pavement  were  360,254,  on  old- 
style  granite  block  376,384,  and  on  wood  70,914;  and  the  number 
of  miles  traveled  by  the  horses  while  under  observation  was  41,427 
on  the  asphalt  pavements,  34,723  on  the  granite,  and  4,901  on  the 
wood.  A  summary  of  the  results  is  shown  in  Table  81. 

TABLE  81 
MILES  TRAVELED  BY  A  HORSE  ON  AMERICAN  PAVEMENTS  BEFORE  AN  ACCIDENT 

OCCURS 
Observations  made  in  1885 


Ref. 
No. 

Kind  of  Pavement. 

Fall  on 
Knees. 

Fall  on 
Haunches. 

Complete 
Fall. 

Accident 
of  Any 
Kind. 

1 

Asphalt,  sheet  

1  534 

2  180 

1  647 

KQQ 

2 

(  Imnite  block  —  old  style  

510 

5  954 

3  472 

41  Q 

3 

Wood1. 

408 

983 

4  QOI 

979 

1  The  kind  of  wood-block  is  not  stated,  and  apparently  it  can  not  now  be  determined. 

These  data  are  very  much  out  of  date,  and  are  not  of  much  value, 
since  the  character  of  the  prevailing  forms  of  construction  has 
materially  changed;  but  nevertheless  Table  81  contains  the  only 


*  Trans.  Amer.  Soc.  of  Civil  Engineers,  Vol.  15  (1886),  p.  123-28. 


648  SELECTING    THE    BEST   PAVEMENT  [CHAP.    XX 

definite  data  on  record  for  American  pavements.  No  observations 
similar  to  the  preceding  have  been  made  for  brick  pavements;  but 
it  is  probable  that  they  are  less  slippery  than  asphalt,  wood-block,  or 
stone-block.  It  is  certain  that  modern  stone-block  pavements,  i.  e., 
those  with  comparatively  small  blocks  and  narrow  joints  filled  with 
bituminous  or  hydraulic  cement,  are  less  slippery  than  the  old-style 
stone-block  pavement  upon  which  the  observation  in  Table  81  were 
made. 

1240.  The  above  observations  relate  to  the  slipperiness  for  a 
horse;   but  slipperiness  is  nearly  as  important  for  an  automobile  as 
for  a  horse.     However,  no  systematic  observations  have  been  made 
as   to   the   effect   of  slipperiness   of   pavements   upon    automobile 
traffic. 

1241.  Conclusion.     It   is   generalty    conceded   that    wood-block 
pavements  are  the  most  slippery,  sheet  asphalt  next,  brick  next,  and 
then  granite. 

1242.  Investigation   in   Progress.     In    1916   the  State   Highway 
Commission  of  Massachusetts  built  ten  sections  of  pavements  pri- 
marily to  determine  their  relative  slipperiness.     Incidentally  it  is 
expected   that   the  experiment   will   demonstrate   other   things   of 
interest.* 

The  test  sections  were  built  on  Washington  Street,  Boston,  on  a 
4  per  cent  grade,  and  each  is  500  feet  long.  The  surfaces  of  the 
several  sections  are  those  ordinarily  employed  for  rural  and  suburban 
roads  rather  than  those  used  on  city  streets;  and  consist  of  two  of  tar 
macadam  (§  604),  two  of  asphalt  macadam  (§  604),  two  of  tar  con- 
crete (§  604),  one  of  asphalt  concrete  (§  604,  891,  and  901),  one  of 
hydraulic  concrete  (Chap.  VII),  one  of  tar  and  sand  (§  614),  and  one 
of  Topeka  asphalt  mixture  (§  897-98).  Each  of  the  first  three  forms 
of  pavements  was  laid  with  a  "  rough  "  and  a  "  smooth  "  surface. 

The  different  materials  are  subject  to  the  same  conditions,  and 
hence  are  strictly  comparable;  and  the  same  materials  were  used  in 
the  several  sections,  and  hence  the  methods  of  construction  are 
strictly  comparable.  The  only  noteworthy  conclusions  that  have 
been  announced  relate  to  the  durability  of  the  several  forms  of  con- 
struction; and  no  decision  has  been  reached  as  to  the  relative  slip- 
periness of  the  different  surfaces. 

1243.  Ease  of  Cleaning.     The  facility  with  which  a  pavement 
may  be  cleaned   is  an  important  matter  both  economically  and 

*  Engineering  News,  Vol,  76  (1916),  p.  1162, 


ART.    1]  THE   DAtA    *Cfc   THE    PROBLEM  649 

esthetically.  Col.  Geo.  E.  Waring,  noted  for  his  service  as  Street 
Cleaning  Commissioner  of  New  York  City,  in  1896  estimated  that 
if  all  the  streets  of  New  York  City  were  paved  with  asphalt  where  the 
grades  would  permit,  the  cost  of  street  cleaning  would  be  reduced 
from  $1,200,000  to  $700,000  per  year.  At  that  time  New  York  had 
431  miles  of  pavement  of  which  94  were  asphalt,  and  the  above  annual 
saving  is  equal  to  3  per  cent  of  the  cost  of  laying  asphalt  pavements 
upon  all  of  the  streets  not  already  asphalted. 

Sheet  asphalt  and  hydraulic  concrete  pavements  are  most  easily 
cleaned,  and  next  in  order  are :  wood  blocks  with  close  joints,  asphalt 
blocks,  brick  with  joints  filled  with  hydraulic  cement,  accurately- 
dressed  stone  blocks  with  cement  joints,  and  old-style  stone  blocks. 

Macadam  and  gravel  are  smooth  and  for  this  reason  are  easily 
cleaned;  but  their  surfaces,  particularly  if  water-bound,  grind  up 
into  powder  under  dense  or  heavy  travel,  and  for  this  reason  there  is 
considerable  detritus  to  be  removed,  a  fact  which  adds  to  the  expense 
of  cleaning. 

1244.  Noiselessness.  The  noise  made  by  travel  upon  a  pave- 
ment has  an  important  effect  upon  the  comfort  and  health  of  the 
people  using  the  pavement  or  living  adjacent  to  it.  A  quiet  pavement 
is  particularly  desirable  adjacent  to  office  buildings,  schools,  churches, 
hospitals,  etc.;  and  the  noise  of  travel  upon  a  rough  pavement 
aggravates,  if  it  does  not  cause  nervous  disorders. 

On  sheet  asphalt  and  hydraulic  concrete,  and  well-grouted  brick, 
the  only  noise  is  the  sharp  click  of  the  horses'  shoes;  and  on  asphalt 
block  and  re-pressed  brick  filled  with  tar  or  grout,  there  is  the  click 
of  horse's  shoes  and  a  slight  rumbling  of  the  wheels  passing  over  the 
joints.  On  well-dressed  granite  blocks  filled  with  hydraulic  cement, 
there  is  a  considerable  rumbling  due  to  the  passage  of  heavy  steel- 
tired  vehicles;  and  on  the  old-style  granite  block  with  sand-filled 
joints,  there  is  a  deafening  roar  due  both  to  the  rumbling  of  the 
wheels  and  to  the  blows  of  the  horses'  shoes.  Upon  wood  pavements 
the  horses'  feet  produce  no  noticeable  noise;  while  the  wheels  make  a 
dull  rumbling  noise,  but  not  loud  enough  to  be  seriously  objection- 
able. Macadam  and  gravel  are  more  quiet  than  wood. 

In  order  of  their  freedom  from  noise,  pavements  rank  about  as 
follows:  wood-block  having  the  joints  filled  with  tar  or  grout, 
sheet  asphalt,  asphalt  block,  asphalt  concrete,  hydraulic  concrete, 
square-edged  brick  having  grouted  joints,  re-pressed  brick  having 
joints  filled  with  tar  or  grout,  accurately  dressed  stone  blocks  having 
joints  filled  with  grout,  and  old-style  stone  block.  Bituminous  gravel 


650  SELECTING   THE   BEST   PAVEMENT  [CHAP.   XX 

and  macadam  are  nearly  as  quiet  as  sheet  asphalt,  and  water-bound 
gravel  and  macadam  are  not  seriously  noisy.  However,  the  freedom 
from  noise  on  any  pavement  depends  greatly  upon  the  care  used  in 
construction  and  maintenance. 

1245.  Healthfulness.     The  effect  of  a  pavement  upon  the  health 
of  the  residents  in  its  locality  will  depend  upon  the  tendency  of  the 
materials  composing  it  to  decay,  and  also  upon  its  permeability. 
The  healthfulness  of  a  pavement  was  much  more  important  formerly 
than  at  present.     The  form  of  pavements  that  were  most  unhealth- 
ful  have  gone  out  of  use.     These  are:  cobble-stone  and  stone-block 
pavements  having  joints  filled  with  pebbles,  cylindrical  wood-blocks 
with  sand-filled  joints,   and  stone-block  having  wide  joints  filled 
with  sand  or  pebbles.     The  difference  in  healthfulness  of  the  best 
modern  pavements  is  negligible. 

1246.  Freedom  from  Dust  and  Mud.    The  materials   of  an 
ideal  pavement  should  not  grind  up  and  make  dust  in  dry  weather 
or  mud  in  wet  weather.     The  dust  and  mud  not  only  add  to  the 
expense  of  cleaning  the  pavement,  but  are  a  discomfort  to  those  who 
use  the  pavement  and  to  those  who  live  or  do  business  adjacent  to  it. 

1247.  Comfort  in  Use.     If  the  pavement  is  to  be  used  for  pleasure 
driving,  the  comfort  of  the  users  must  be  considered;  and  therefore 
the  pavement  should  have  a  smooth  surface  which  is  free  from  dust 
when  it  is  dry  and  free  from  mud  when  it  is  wet. 

1248.  Temperature  of  Pavements.     During  hot  weather,  there 
is  frequently  complaint  that  one  pavement  reflects  or  radiates  more 
heat  than  another.     Observations  made  in  Washington,  D.  C.,  when 
the  temperature  of  the  air  2  feet  above  the  pavement  was  104°  F., 
showed  the  temperature  of  three  pavements  to  be  as  follows:  sheet 
asphalt  140°,  asphalt  block  122°,  and  macadam   118°.*     Observa- 
tions in  Boston,  when  the  temperature  of  the  air  in  the  shade  was 
98°  F.,  gave  the  temperature  of  four  pavements  as  follows:   wood 
block  124|°,  granite  block  115°,  sheet  asphalt  113°,  and  macadam 
102J°.     The  observations  are  not  conclusive  as  to  the  relative  tem- 
peratures of  different  pavements,  but  show  that  there  is  no  very 
great  difference  between  the  several  kinds.     The  temperature  of  the 
pavement  depends  upon  its  color,  which  varies  with  the  material. 

*  Proc.  Amer.  Soc.  Municipal  Improvements,  Vol.  5,  p.  161. 


ART.    2]  THE    SOLUTION    OF   THE    PROBLEM  651 


Art.  2.  THE  SOLUTION  OF  THE  PROBLEM 

1250.  The  problem  of  selecting  the  best  pavement  for  any  partic- 
ular case  is  a  local  one,  not  only  for  each  city  but  also  for  each  of  the 
various  parts  into  which  the  city  is  imperceptibly  divided;    and 
involves  so  many  elements  that  the  nicest  balancing  of  the  relative 
values  for  each  kind  of  pavement  is  required  to  arrive  at  a  correct 
conclusion. 

There  are  two  methods  that  may  be  employed  in  deciding  which 
is  the  best  of  several  pavements.  One  method  assumes  that  the 
selection  should  be  made  upon  economic  grounds  alone,  in  which  case 
the  best  pavement  is  that  for  which  the  total  annual  expense  is  a 
minimum.  The  other  method  assumes  that  the  decision  should  be 
based  upon  other  factors  beside  the  economic  features.  Each  of 
these  two  methods  will  be  considered;  and  for  convenience  the  first 
will  be  called  a  Problem  in  Economics,  and  the  second  a  Non-economic 
Problem. 

1251.  PROBLEM  IN  ECONOMICS.     As  a  problem  in  economics, 
the  selection  of  the  best  pavement  consists  in  finding  that  form  of 
pavement,  or  wearing  course,  for  which  the  total  annual  cost  is  least. 
The  annual  cost  consists  of  (1)  interest  on  the  cost  of  construction, 
(2)  the  annual  cost  of  repairs,  (3)  the  annual  cost  of  renewal,  (4) 
the  annual  cost  of  cleaning,  (5)  the  annual  cost  of  conducting  the 
transportation  the  pavement  carries,  and   (6)  the  annual  cost  of 
sprinkling, — when  that  is  necessary. 

The  first  three  items  of  cost  have  already  been  considered  in 
Art.  1.  The  last  three  items  of  expense  have  not  been  considered; 
but  will  now  be  briefly  discussed. 

1252.  Cost   of    Cleaning.     Some   forms   of   pavements   require 
sprinkling  for  economic  maintenance,  as  for  example  water-bound 
macadam;  and  some  types  require  sprinkling  for  the  comfort  of  the 
users  and  of  those  living  adjacent  to  it. 

The  cost  of  cleaning  depends  upon  the  amount  and  character  of 
the  travel  and  also  upon  the  smoothness  of  the  pavement,  and  more 
upon  the  former  than  the  latter.  The  difference  in  cost  of  cleaning 
the  different  forms  of  modern  pavements  is  not  great.  For  example, 
in  1895-97  in  New  York  City  it  was  determined  that  the  relative 
ease  of  cleaning  different  pavements  was  as  follows:  asphalt,  brick 
and  rectangular  wood-block,  each  100;  granite  block,  150;  Belgian 
block,  160;  and  cobble-stone,  400,  But  since  the  date  of  that  inves- 


652  SELECTING   THE   BEST   PAVEMENT  [CHAP.    XX 

tigation,  cobble-stone  pavements  have  been  practically  eliminated, 
and  Belgian  blocks  nearly  so;  while  the  prevailing  type  of  con- 
struction of  granite-block  pavements  has  changed  so  that  now 
they  are  practically  as  smooth  as  brick  pavements.  Hence,  if  the 
cost  of  cleaning  is  omitted,  it  will  not  materially  affect  the  con- 
clusion as  to  the  relative  economic  merits  of  the  different  pave- 
ments. 

1253.  Cost  of  Transportation.    At  first  thought,  it  does  not  ap- 
pear that  the  cost  of  conducting  the  transportation  over  a  pavement 
is  part  of  its  annual  cost;  but  really  the  cost  of  the  transportation  is  a 
part  of  the  cost  of  operating  the  pavement,  and  hence  is  a  part  of  its 
annual  cost.     The  cost  of  construction  is  usually  paid  by  the  abutting 
property  holder,  and  ordinarily  he  pays  also  the  cost  of  repairs  and 
renewals;  the  city  usually  pays  the  cost  of  cleaning;  and  the  owners 
of  the  horses  and  wagons  and  of  the  motor  cars  pay  the  cost  of  trans- 
portation. 

Modern  pavements  are  so  nearly  alike  in  the  smoothness  and 
hardness  of  their  surfaces  that  there  is  no  material  difference  in  the 
cost  of  conducting  transportation  on  them.  Further,  the  transpor- 
tation is  conducted  by  so  many  different  parties  under  so  many 
different  conditions,  that  it  is  practically  impossible  to  determine 
its  cost  with  any  degree  of  accuracy. 

Therefore,  for  lack  of  the  requisite  data,  it  is  necessary  to  omit 
the  cost  of  transportation  from  the  summary  of  the  annual  cost  of 
the  pavement;  but  this  omission  does  not  materially  affect  the  con- 
clusion as  to  the  most  economical  pavement. 

1254.  Cost    of    Sprinkling.    Water-bound    macadam    is    about 
the  only  pavement  that  requires  sprinkling,  either  for  maintenance 
or  for  the  comfort  of  users  of  the  pavement  or  of  those  living  adja- 
cent to  it.     The  cost  of  sprinkling  will  vary  widely  with  the  local 
conditions — the  character  and  amount  of   the   travel,  the   climate, 
the  material  of  the  road  surface,  the  quality  of  the  construction, 
etc. 

Since  this  form  of  pavement  surface  is  rapidly  going  out  of  use, 
there  are  no  recent  data  on  this  subject.  Further,  such  pavements 
are  not  likely  to  be  constructed  to  any  appreciable  extent  in  the 
future.  Therefore,  this  item  will  be  disregarded. 

1255.  Total  Annual  Cost.     The  cost  of  construction  is  stated  in 
§  1230  (page  642);  Table  80  (page  645)  shows  the  cost  of  repairs; 
and  §  1236  (page  645)  shows  the  method  of  computing  the  cost  of 
renewals, 


ART.    2] 


THE   SOLUTION   OF   THE   PROBLEM 


653 


The  method  of  computing  the  total  annual  cost  of  a  sheet  asphalt 
pavement  under  heavy  travel  is  as  follows: 

TTFMS  ANNUAL  COST 

Cts.  per  Sq.  Yd. 

Interest:   concrete  base,  $1.00  at  3£  per  cent 3 . 500 

wearing  coat,  $0.75  at  3£  per  cent 2.625 

Repairs  (Table  80,  page  645) 5.000 

Renewals  (see  §  1236,  page  645) 7.968 


Total  interest,  repairs,  and  depreciation 19 . 093 

1256.  A  Common  Error.     Not  infrequently,  in  computing  the 
annual  cost  of  a  pavement,  there  is  added  to  the  above  items  an 
annual  contribution  to  a  sinking  fund  sufficient  to  redeem  the  bonds 
issued  to  pay  for  the  pavement.     This  is  incorrect,  since  a  bond  issue 
is  only  a  means  of  deferring  the  payment  of  the  first  cost;  and  it  is 
clearly  wrong  to  include  both  interest  on  first  cost  and  an  annuity 
to  pay  the  first  cost.     If  the  bond  matures  at  the  end  of  the  life 
of  the  pavement,   an  annuity  to  redeem  the  bond  is  precisely  the 
same  thing  as  a  depreciation  fund  to  renew  the  pavement;  and 
hence  it  is  clearly  an  error  to  include  both  in  the  annual  cost. 

1257.  Comparison  of  Total  Annual  Costs.     To  compare  the  eco- 
nomic value  of  different  pavements,  a  computation  similar  to  that  in 
§  1255  should  be  made  for  each  pavement;  and  the  one  showing  the 
least  annual  cost  is  the  most  economical.     Substantially  the  above 
method  was  applied  to  three  forms  of  pavements  and  four  classes 
of  streets  in  a  certain  large  city,  with  results  in  Table  82.  * 

TABLE  82 
ANNUAL  COST  OF  PAVEMENTS 


CLASSES  o 

p  STREETS. 

A. 

B. 

C. 

D. 

Granite  block  

$0.511 

$0.372 

$0.365 

$0  294 

Sheet  asphalt  

.521 

.324 

.283 

222 

Water-bound  macadam  

.669 

.430 

.312 

151 

Although  the  error  mentioned  in  §  1233,  i.  e.,  not  discriminating 
between  renewal  and  new  construction,  was  made  in  computing  the 
cost  of  repairs  of  the  macadam,  the  results  are  sufficiently  correct  to 
show  the  method  of  utilizing  such  an  investigation.  The  compu- 
tations included  a  charge  for  cleaning  for  each  form  of  pavement,. and 

*  Engineering  and  Contracting,  Vol.  33  (1910),  p.  31. 


654  SELECTING   THE    BEST   PAVEMENT  [CHAP.    XX 

also  an  item  for  sprinkling  the  macadam.  Of  the  four  classes  of 
streets,  A  had  the  heaviest  travel  and  D  the  least.  The  results 
show  that  for  Class  A  streets,  a  granite  block  pavement  is  most 
economical;  for  Class  B,  sheet  asphalt;  for  Class  C,  sheet  asphalt; 
and  for  Class  D,  macadam. 

Of  course,  the  value  of  an  investigation  similar  to  that  above 
depends  upon  the  values  assumed  for  the  life  of  each  of  th'e  pave- 
ments, and  also  upon  the  value  used  for  the  cost  of  repairs;  but  unfor- 
tunately there  are  practically  no  reliable  data  for  either  of  these,  and 
hence  this  method  is  not  as  exact  as  its  form  implies.  For  example, 
for  any  of  the  four  classes  of  streets  in  Table  82,  the  differences  in 
the  annual  costs  of  the  three  pavements  are  so  small  that  the  con- 
clusions might  be  changed  by  a  change  in  the  assumed  life  of  one 
or  the  other  of  the  pavements. 

1258.  NON-ECONOMIC  PROBLEM.     Sometimes  the  selection  of  a 
pavement  is  determined  by  a  single  factor,  as  for  example,   the 
proximity  of  one  or  more  paving  materials,   and  sometimes  the 
selection  can  be  made  from  a   consideration  of  only  the   economic 
features  (see  §  1251-58) ;  but  usually  the  selection  requires  a  careful 
consideration  of  all  the  factors  involved.     The  benefits  to  be  derived 
from  good  pavements  are  stated  in  §  634  (page  318-19),  from  which  it 
appears  that  only  two  of  the  eight  advantages  relate  directly  to 
economics.     The  requirements  for  an  ideal  pavement  are  enumer- 
ated in  §  1229,  from  which  it  appears  that  only  five  of  the  ten  items 
concern  economics. 

The  decision  as  to  which  is  the  best  pavement  will  often  be  largely 
a  matter  of  judgment;  and  when  this  is  the  case,  the  engineer  should 
reach  his  conclusion  by  a  series  of  carefully  considered  steps,  and  not 
by  a  single  haphazard  leap.  He  should  weigh  all  the  evidence,  and 
not  base  a  decision  upon  a  single  item,  as  is  too  often  the  case;  nor 
should  he  adopt  the  practice  of  some  other  locality  without  a  careful 
consideration  of  the  local  resources  and  of  the  needs  of  the  place  in 
which  the  pavement  is  to  be  laid,  as  is  frequently  done. 

Local  conditions  should  always  be  considered,  and  hence  it  is 
not  possible  to  lay  down  any  fixed  rule  as  to  what  material  makes  the 
best  pavement;  but  a  careful  study  of  the  requirements  of  the  ideal 
pavement  and  of  the  qualities  of  the  different  kinds  of  pavements 
will  promote  an  intelligent  selection  in  any  particular  case. 

1259.  Relative  Merits  of  Pavements.     It  is  proposed  to  compare 
different  kinds  of  pavements  by  assigning  percentages  to  the  different 
qualities  of  an  ideal  pavement,  and  then  with  this  as  a  guide  to  assign 


ART.    2] 


THE    SOLUTION    OF   THE    PROBLEM 


655 


numerical  values  to  the  various   qualities  of  the  several  kinds  of 
pavements. 

The  various  qualities  of  a  perfect  pavement  have  been  discussed 
in  §  1229  to  §  1248,  and  these  qualities  have  been  grouped  in  Table 
83  under  the  three  heads:  (1)  economic  qualities,  (2)  sanitary 
qualities,  and  (3)  acceptability.  Opposite  each  of  these  qualities 
in  the  first  column  of  Table  83  is  placed  a  number  which  is  believed 
to  represent  the  average  relative  importance  of  that  particular 
quality  on  a  scale  of  100. 

TABLE  83 
RELATIVE  VALUES  FOR  THE  DIFFERENT  QUALITIES  OF  VARIOUS  PAVEMENTS 


±-E! 

ICENTAG! 

.    ASSIGN 

ED    TO    T 

HE    yUAI 

,ITY. 

Ref. 
No. 

Qualities. 

Ideal 
Pave- 
ment. 

Sheet 
Asphalt. 

Brick. 

Granite 
Block. 

Water- 
bound 
Mac- 
adam. 

Wood 
Block. 

I 

Economic  qualities: 
Low  first  cost 

20 

11 

10 

3 

18 

5 

2 
5 

Low  cost  of  repairs  
Ease  of  traction       

20 
10 

14 
9 

18 
8 

20 

7 

8 

6 

16 
9 

4 

Good  foothold        

5 

2 

4 

4 

5 

2 

5 

Ease  of  cleaning  

10 

10 

9 

6 

3 

9 

Total      

65 

46 

49 

40 

40 

41 

6 

Sanitary  qualities: 
Noiselessness 

15 

10 

7 

5 

15 

14 

7 

Healthfulness 

5 

5 

4 

3 

2 

4 

Total             

20 

15 

11 

8 

17 

18 

8 
9 
10 

Acceptability: 
Free  from  dust  and  mud  .  .  . 
Comfortable  to  use  
Non-absorbent  of  heat  

10 
3 

2 

10 
3 

1 

9 

1 
1 

8 
1 
1 

2 
3 
2 

9 
2 
1 

Total 

15 

14 

11 

10 

7 

12 

Grand  total  

100 

75 

71 

58 

64 

71 

The  most  important  matter  in  preparing  Table  83  is  the  assign- 
ment of  the  numbers  for  the  Ideal  Pavement,  for  the  number  assigned 
to  any  one  quality  limits  the  range  of  the  corresponding  assignments 
to  the  different  pavements.  The  assignment  of  the  numbers  is 
wholly  a  matter  of  judgment,  and  different  individuals  will  differ 
greatly  as  to  the  relative  values  to  be  given  to  each  quality;  but  the 
table  is  only  to  show  a  method  whereby  the  good  and  the  bad  qualities 


656  SELECTING   THE  3EST   PAVEMENT  [CHAP.    XX 

of  one  kind  of  pavement  may  be  balanced  against  those  of  another 
kind,  and  a  conclusion  may  be  reached,  step  by  step,  which  repre- 
sents the  algebraic  sum  of  the  judgment  on  each  item. 

Different  values  should  be  assigned  to  the  same  quality  according 
to  the  attendant  conditions.  If  the  street  is  in  a  manufacturing 
district  and  subject  to  heavy  traffic,  ease  of  traction  should  be 
assigned  a  comparatively  high  value,  and  noise  a  very  low  value. 
For  an  office  district,  quietness  is  the  controlling  factor,  and  should 
therefore  have  a  relatively  high  value.  Similarly,  for  a  residence 
district  with  its  light  driving,  healthfulness  and  freedom  from  dirt 
and  dust  may  be  the  most  important  element;  for  a  residence  dis- 
trict where  the  property  owners  can  not  afford  an  expensive  pavement, 
the  first  cost  may  determine  the  kind  of  pavement;  and  on  a  steep 
grade  slipperiness  may  out- weigh  all  other  conditions  in  determining 
the  kind  of  pavement  to  be  employed.  The  application  of  the 
principles  is  likely  to  be  complicated  by  the  personal  interests  of  the 
residents  or  property-holders,  since  opinions  are  likely  to  differ 
according  to  whether  the  point  of  view  is  that  of  a  tenant,  a  resident 
property-holder,  or  a  non-resident  property-holder. 

1260.  Each  quality  of  a  pavement  will  now  be  considered,  and 
the  degree  of  perfection  of  this  quality  possessed  by  each  kind  of 
pavement  will  be  indicated  by  a  numerical  value. 

1261.  First  Cost.     In  §  1230  (page  642)  are  given  assumed  values 
for  the  average  cost  of  construction  for  the  best  of  each  kind  of  pave- 
ment. These  values  are  repeated  below  in  the  order  of  their  cheapness : 

KIND  OF  PAVEMENT.  COST  PER  SQ.  YD.  RELATIVE  WEIGHT, 

1.  Macadam,  water-bound $1 . 25 18 

2.  Asphalt,  sheet 1 . 75 11 

3.  Brick 1.80 10 

4.  Wood  block,  creosoted 2.75 5 

5.  Granite  block 3.00 3 

The  last  column  of  the  above  exhibit  shows  the  relative  weights 
assigned  to  the  quality  of  cheapness.  Since  macadam  is  the  lowest 
in  first  cost,  it  possesses  the  quality  of  cheapness  in  the  highest 
degree;  and  consequently  it  is  given  a  weight  of  18 — nearly  the  value 
assigned  to  the  ideal  pavement  in  Table  83.  The  weights  assigned 
to  this  quality  decrease  from  gravel,  the  cheapest,  to  granite  block, 
the  most  expensive.  The  .several  weights  assigned  above  to  low 
first  cost  are  entered  opposite  this  quality  in  the  table. 

1262.  The  first  cost  of  a  pavement  not  infrequently  has  undue 
weight  in  comparing  the  relative  merits  of  different  kinds  of  pave- 


ART.   2]  THE   SOLUTION   OF  THE   PROBLEM  657 

ments.  The  pavement  which  costs  the  most  to  construct  is  not 
always  the  most  expensive,  nor  is  the  one  lowest  in  the  first  cost 
always  the  cheapest  in  the  end  (see  §  1251-57). 

A  pavement  is  sometimes  selected  because  of  its  low  first  cost, 
for  other  than  economic  reasons.  Often  the  cost  of  construction  is 
charged  against  the  abutting  property,  while  maintenance  is  paid 
for  by  the  whole  city;  and  the  result  is  that  many  property  owners 
perfer  a  cheap  pavement  because  they  must  pay  for  it,  notwithstand- 
ing the  fact  that  the  cheaper  pavement  may  cost  more  for  main- 
tenance and  be  dearer  in  the  long  run.  Again,  the  property  holders 
are  sometimes  really  unable  to  pay  for  the  most  economical  pave- 
ment, and  hence  a  pavement  low  in  first  cost  is  selected  as  a  tem- 
porary expedient. 

1263.  Cost  of  Repairs.   Table  80,  page  645,  contains  the  best  avail- 
able data  on  the  cost  of  repairs,  although  they  are  not  very  reliable. 
The  data  for  heavy  travel  are  re-arranged  and  transcribed  below : 

KIND  OF  PAVEMENT.  COST  PER  SQ.  YD.  RELATIVE  WEIGHT. 

1.  Stone  block 2.0 20 

2.  Brick 3.0 18 

3.  Wood  block 4.0 16 

4.  Asphalt 5.0 14 

5.  Macadam,  water-bound 10.0 8 

1264.  Ease  of  Traction.     Under  this  head  may  be  included  not 
only  the  power  required  to  move  loads,  but  also  the  consequential 
damages  to  vehicles,  since  they  both  vary  with  the  roughness  of  the 
pavement.     From  a  study  of  the  results  in  Table  8,  page  21,  remem- 
bering that  the  tractive  resistance  of  the  best  type  of  several  of  the 
pavements  has  decreased  since  the  observation  in  Table  8  were 
made,  the  weights  are  assigned  to  this  quality  for  the  different 
kinds  of  pavements,  as  shown  in  Table  83. 

1265.  Foothold.     From  a  study  of  §  1239,  the  relative  degree  of 
slipperiness  is  stated  in  numbers  and  entered  in  Table  83.     If  the 
pavement  is  to  be  upon  a  steep  grade,  this  quality  may  be  a  con- 
trolling factor. 

1266.  Ease  of  Cleaning.     The  relative  ease  with  which  certain 
types  of  pavements  may  be  swept,  as  determined  by  the  cost  of  doing 
the  work  in  New  York  City,  is  as  follows:  asphalt,  100;  brick,  100; 
rectangular  hard-wood  blocks,   100;    granite  blocks,   150;    Belgian 
blocks,  160;   cobble  stones,  400.*     For  sanitary  reasons,  New  York 

*  Street  Cleaning  in  New  York  City  in  1895-97,  p.  157 — Supplement  to  Vol.  II.  of  Municipal 
Affairs.     New  York,  1898. 


. 


658  SELECTING   THE   BEST   PAVEMENT  [CHAP.    XX 

City  has  spent  a  million  dollars  a  year  for  the  past  few  years  in  sub- 
stituting sheet  asphalt  pavements  for  stone-block  in  the  congested 
tenement  districts,  chiefly  on  account  of  the  greater  ease  with  which 
the  asphalt  is  kept  clean. 

The  cost  of  sweeping  ordinary  stone-block,  round  wood-block,  and 
brick  with  sand  filler  usually  ranges  between  40  and  48  cents  per  1,000 
square  yards  for  each  sweeping,  and  sheet  asphalt  from  30  to  38 
cents,  depending  upon  the  thoroughness  of  doing  the  work,  the  fre- 
quency of  sweepings,  the  kind  of  business  in  the  property  adjoining, 
and  the  amount  of  the  traffic.  The  relative  weight  to  be  assigned 
to  this  item  will  vary  with  the  frequency  of  cleaning. 

The  estimated  weight  to  be  assigned  to  the  several  pavements 
on  account  of  their  ease  of  cleaning  is  entered  in  Table  83. 

1267.  Value  for  Other  Qualities.     From  a  consideration  of  the 
discussion  in  §  1244-48,  the  percentages  for  the  other  qualities  are 
inserted  in  Table  80. 

1268.  Conclusion.     The  totals  at  the  foot  of  Table  83  represent 
the  summation  of  the  individual  decisions  on  the  several  qualities, 
and  the  larger  the  total  the  more  desirable  the  pavement.     The 
particular  results  in  this  example  may  not  be  applicable  to  any 
locality,  and  each  person  will  have  his  own  opinion  as  to  the  merits 
and  defects  of  any  particular  pavement;  but  the  method  of  analysis 
is  applicable  to  any  particular  case,  and  will  enable  the  engineer 
intelligently  and  unerringly  to  reach  the  final  conclusion  to  which  his 
opinion  in  detail  leads.     The  above  method  has  something  of  the 
mathematical  form ;  but  the  fact  should  not  be  forgotten  that  it  is 
based  upon  judgment,  and  that  therefore  it  can  not  be  expected  to 
give  results  of  a  high  degree  of  accuracy. 

In  practice  the  application  of  this  method  is  much  less  compli- 
cated than  appears  from  the  above  example,  for  usually  proximity 
of  some  natural  pavement  material  or  freight  rates  on  others,  limits 
the  choice  to  a  comparatively  few  kinds  of  pavements.  Further,  the 
decision  as  to  the  kind  of  pavement  to  be  laid  is  often  influenced  by 
the  fancy  or  ability  of  those  who  pay  for  it.  However,  the  engineer 
should  employ  a  logical  process  in  arriving  at  his  own  conclusions, 
and  thus  be  in  a  position  to  give  sound  advice  upon  the  funda- 
mental principles  involved. 

1269.  Finally,  in  any  important  case,  it  is  wise  to  determine  the 
best  pavement  by  both  the  economic  and  the  non-ecomonic  method, 
so  as  to  check  one  method  against  the  other. 


INDEX 


ASP 

Asphalt,  267 
American,  270 
Bermudez,  269 
California,  269 
cement,  268 

preparation,  274 
specifications,  275 

binder  for  macadam,  278 
bituminous  concrete,  279 
bituminous  surface,  277 
filler  for  block  pavements.  282 
seal  coat,  280 

sheet  asphalt  pavements,  281 
characteristics,  268 
cost,  283 
crude,  267 
Gilsonite,  270 
liquid,  275 

specifications  for,  275 
petroleum  residue,  270 
properties  of,  270 
binding  power,  271 
chemical  stability,  270 
freedom  from  decomposition,  271 
resiliency,  271 
waterproofness,  271 
refined,  267 
rock,  268 
shipping,  270 
sources,  268 

tests,  see  Bituminous     materials,  tests 
Trinidad,  268 
Asphalt  pavement,  411 
amount  in  U.  S.,  320 
block,  470 

composition,  471 
cost,  472 
merits,  472 
concrete,  461,  464 
Amiesite,  462 
bitulithic,  461 

area  in  U.  S.,  320 
cost  of  construction,  465 
Amiesite,  467 
bitulithic,  467 
Topeka  mixture,  465 
Warrenite,  467 
definition,  461 
laying,  464 
merits,  466 
mixing,  464 
specifications,  468 
stone-filled,  463 
Topeka  mixture,  463 
Warrenite,  462 
foundation,  412 
bituminous,  413 
hydraulic,  412 
kinds,  411 

block,  411,  470 
concrete,  411,  461,  464 
Amiesite,  462 
bitulithic,  461 
stone-filled,  463 
Topeka  mixture,  465 
Warrenite,  462 


of. 


659 


ASP 

Asphalt  pavement,  kinds,  rock,  469 

sheet,  411 
rock,  469 

construction,  469 
sheet,  411 

adjacent  to  track,  441 
binder  course,  415 
bitumen,  417 
kind,  415 
closed,  415 

specifications,  416 
open,  415 

specifications,  415 
paint  coat,  415 
laying,  419 
mixing,  418 
rolling,  422 
thickness,  422 
cause  of  failure,  443 

improper  manipulation,  444 
burned  asphalt,  444 
chilled  cement,  445 
damp  foundation,  445 
high  heat,  444 
improper  consistency,  444 
inadequate  compression,  446 
inadequate  mixing,  445 
insufficient  bitumen,  445 
rich  binder,  445 

separation  of  sand  and  cement,  445 
natural  causes,  446 
bonfires,  448 
cracks,  447 
decay,  446 
illuminating  gas,  447 
leaky  joints,  447 
ordinary  wear,  446 
porous  foundations,  446 
shifting  under  traffic,  448 
weak  foundation,  446 
unsuitable  materials,  443 
asphalt,  416,  417,  426 
sand,  416,  417,  423 
cost  of  construction,  451 
actual,  454 
estimated,  453 
cost  of  maintenance,  454 
contract,  457 

Buffalo,  458 
municipal  plant,  455 

Brooklyn,  457 
crown,  46 
foundation,  412 
bituminous,  413 
hydraulic,  412 
other  forms,  414 
grade,  maximum,  459 
history,  412 

merits,  460 

repairing,  method  of,  449 
cracks,  450 
disintegration,  449 
formation  of  waves,  449 
old  material,  451 
painting  gutters,  450 
settlement  of  subgrade,  449 


660 


INDEX 


ASP— BIT 

Asphalt  pavement,  sheet,  repairs,  method  of 

recording,  450 
price  of,  457,  458 
specifications,  460 
wearing  coat,  423 

absorptive  power,  433 
bitumen,  per  cent,  435 
cement,  426 
amount,  427 
testing,  427 

absorptive  power,  433 
density,  432 
impact,  433 
density,  432 
filler,  426 
impact  test,  433 
laying,  436 
mixing,  435 
proportions,  434 
rolling,  438 
sand,  423 
thickness,  441 

Bermudez  asphalt,  269 

Bitulithic  pavements,  area  in  U.  S.,  320 

see  also  Asphalt  pavements. 
Bitumen,  267 

Bituminous  concrete  roads,  310 
aggregate,  311 
binder,  311 
cost,  315 
laying,  312 
mixing,  311 
seal  coat,  315 

vs.  bituminous  macadam,  315 
Bituminous  macadam  roads,  306 
applying  binder,  309 
bituminous  binder,  308 
characteristics,  310 
cost,  310 
crown,  307 
definition,  306 
foundation,  306 
maintenance,  310 
maximum  grade,  307 
tar-sand  mastic,  310 
wearing  coat,  307 
width,  307 

Bituminous  materials,  267 
definition,  267 
tests  of,  271 

bitumen  soluble  in  disulphide,  273 
naphtha,  273 
tetrachloride,  272 
consistency,  272 

float  apparatus,  272 
penetration,  272 
viscosity,  272 
distillation,  273 
ductility,  274 
fixed  carbon,  273 
flash  point,  272 
float  test,  272 
foam  test,  271 
melting  point,  272 
paraffin  scale,  274 
penetration,  272 
specific  gravity,  271 
vaporization,  273 
viscosity,  272 

Bituminous  surface,  definition,  296 
kinds.  296 
carpet,  298 

applying  material,  299 
bituminous  material,  298 
cleaning  surface,  299 
cost,  303 

on  gravel,  304 
on  macadam,  304 
maintenance,  303 


BIT— BRI 

Bituminous  surface,   kinds,  carpet,   value   of 
302 

coating,  297 

bituminous  material,  297 
Blocks,  size  of  city,  337 
Brick,  475 

chemical  composition,  475 
clay,  475 
hillside,  480 
kinds,  478 

hillside,  482 

re-pressed,  478 

vertical  fiber,  481 

wire-cut  lug,  479 
manufacture,  476 

burning,  483 

cutting,  477 

moulding,  476 
re-pressed,  478 
service  test,  500 
size,  485 

specifications,  485 
testing,  486 

absorption,  489 

appearance,  486 

color,  487 

crushing  strength,  488 

rattler  test,  490 

changes  proposed,  499 
limit  of  loss,  495,  497,  499 
marking  brick,  493 
specifications,  491 

size,  487 

specific  gravity,  488 

transverse  strength,  489 
vertical  fiber,  481 
wire-cut  lug,  479 
Brick  pavements,  474 
adjacent  to  track,  539 
area  in  U.  S.,  320 
bedding  course,  505 

cement  and  sand,  511 

mortar,  512 

sand  cushion,  505 
comparison  of  types,  536 

cost,  539 

durability,  536 

noisiness,  536 

smoothness,  536 

thickness,  537 

time  in  construction,  539 
construction,  503 

bedding  course,  505 
cement-sand,  511 
comparison,  514 
mortar,  512 
sand,  505 
cost,  544 

discussion,  544 

examples,  546-51 
expansion  joint,  533 

at  anchors,  535 

longitudinal,  533 

transverse,  534 
foundation,  503 

abandoned  type,  503 

bituminous  concrete,  504 

hydraulic  concrete,  505 

macadam  504 
grade,  maximum,  540 
header,  535 
inspecting,  519 
joint  filler,  521 

applying,  524 

bituminous,  530 

cost,  529 

grout,  522 

merits,  529 

mixing,  523 

sand,  521 


INDEX 


661 


BRI— CON 

Brick  pavements,  joint  filler,  tar-sand,  532 
laying  brick,  514 
delivery,  514 
direction  of  courses,  515 
rolling,  519 
setting,  517 
maintenance,  552 
cost,  564 
repairs,  552 
bulges,  557 

contraction  joints,  555 
cracks,  560 

defective  grouting,  555 
longitudinal  cracks,  557 
re-laying,  558 
re-surfacing,  561 
asphalt,  561 
brick,  563 
tar,  563 

settlement  of  trench,  554 
shrinkage  of  cushion,  553 
sinking  of  foundation,  554 
soft  brick,  552 
transverse  joints,  534 
turning  brick,  563 
merits,  549 
monolithic,  512 
roads,  541 
specifications,  551 
streets,  541 
Brick  rattler,  492 

specifications,  491 
Bridges,  112 
Broken  stone,  see  Macadam  stone. 

Catch  basing,  362 

construction,  362 

cover,  366 

examples,  363,  364,  365 

inlet,  366 

location,  364 
Catch-water,  83 
Cement,  asphalt,  268 

hydraulic,  227 
Census,  travel,  25 

see  also  Travel  census. 

Chevy  Chase  experimental  brick  road,  501 
Clay-sand  roads,  see  Sand-clay  roads. 
Cobble-stone  pavements,  area  in  U.  S.,  320 

construction,  567 

hammer,  568 

Concrete,    bituminous,   see   Bituminous   con- 
crete. 

aggregate,  228 

cement,  227 

consistency,  248 

data  for  estimates,  235 

gravel,  229 

ingredients  for  cu.  yd.,  236 
Fuller's  rule,  237 

materials,  227 
aggregate,  228 
cement,  227 
gravel,  229 
stone,  229 

methods  of  proportioning,  230 

mixers,  247 

mixing,  246 

proportions,  245 
theory  of,  230 

sieve  analysis,  231 
Concrete  curb  and  gutter,  382 

see  also  Curb. 
Concrete  pavements,  263 

see  also  Concrete  roads. 
Concrete  roads,  portland-cement,  227 

characteristics,  263 

construction,  238 
consistency,  248 
cost,  259 


CON— DRA 

Concrete  roads,   Portland  cement,    construc- 
tion, curing,  252 

finishing,  251 
machine,  256 

mixing,  246 

one  vs.  two  course,  241 

placing,  249 

proportions,  245 

protecting,  252 

side  forms,  244 

striking,  249 

thickness,  244 

width,  244 

contraction  joints,  254 
cross  section,  242 
curbs,  258- 

data  for  estimates,  235 
drainage,  238 
grade,  maximum,  243 
maintenance,  264 

bituminous  surface,  265 

cost,  265 

work  required,  264 
materials,  227 

aggregates,  228 

cement,  227 

gravel  vs.  broken  stone,  229 
one  vs.  two  course,  241 
reinforcement,  256 
shoulders,  257 
specifications,  264 
subgrade,  239 
super-elevation,  243 
template,  250 
thickness,  244 
width,  244 

Connecticut  gravel  road,  170 
Crown,  pavements,  374 

amount  of,  275 

laying  out,  374 
roads,  65 
Culverts,  113 
Curb,  378 

combined,  382 

expansion  joints,  386 

finishing  surface,  386 

forms,  383 

foundation,  383 

laying  and  mixing,  384 
concrete,  380 

cost,  381 
integral,  258 
stone,  378 

cost,  380 
Curves,  horizontal,  in  road,  58 

vertical,  at  grade  intersection,  353 
Cut-back  product,  definition,  267 

Distance  equivalent  to  1  ft.  of  rise  and  fall,  54 
value  of  saving,  43 
vs.  rise  and  fall,  53 
Drainage,  road,  72 
catch-waters,  83 
side  ditches,  78 
surface,  81 
crown,  82 
side  ditches,  deep,  82 

shallow,  82 
street,  361  . 

catch  basins,  362 
constructions,  362 
examples,  363 
Champaign,  363 
Milwaukee,  365 
London,  365 
Providence,  364 
inlet,  366 
intersection,  370 
commercial,  369 
Champaign,  368 


662 


INDEX 


DRA— EAR 

Drainage,    street,    catch   basins,  intersection, 

Omaha,  368 
location,  364 
crown,  374 

dished-payements,  376 
foot-crossing,  372 
gutter,  367 
depth,  369 
grade,  370 
material,  367 
Dynagraph,  19 

Earth  rpads,  70 

artistic  treatment,  114 
construction,  70 

earthwork,  see  Earthwork, 
machinery,  see  Road-building  machinery, 
cross  section,  71,  82,  83 

super-elevation,  71 

crown,  65,  81 

definition,  70 

drainage,  72 

surface,  81 

side  ditches,  78,  82,  83,  84 
underdrainage,  72 
object,  72 
tile,  74 
cost,  75 
fall,  75 
laying,  77 
cost,  77 
location,  78 
size,  76 

earthwork,  see  Earthwork, 
embankments,  rolling,  89 
settlement,  88 
stability,  90 
grades,  71 
improving  old,  91 

machinery,  see  Road-building  machinery, 
maintenance,  115 
care  of  ditches,  124 
surface,  117 
roadside,  124 
trees,  125 
cost,  129,  130 
dragging,  129 
total,  130 

destructive  agents,  115 
equal  axles,  117 
horse  before  wheel,  117 
narrow  tires,  115 
dragging,  120 

cost  of,  121,  129 
filling  holes,  124 
improving  old  roads,  91 
machinery,  see  Road-building  machinery, 
preventing  dust,  133 
removing  stones,  124 
scraping,  cost  of,  123 
snow,  obstruction  by,  125 
systems,  126 

by  contract,  129 
continuous  maintenance,  127 
continuous  repairs,  127 
intermittent  repairs,  127 
V  road-leveler,  123 
surface  oiling,  133 
applying  the  oil,  135 
cost,  136 

effect  on  maintenance,  134 
oil,  see  Oil;    also  Petroleum, 
preparing  surface,  135 
width,  70 

on  curves,  71 
Earthwork,  83 

balancing  cuts  and  fills,  86 
computing,  86 
cost  of,  103 

drag-scoop  scraper,  104 


EAR— GRA 

Earthwork,  cost  of,  elevating  grader,  104 
finishing  slopes,  112 
profits,  112 
scraper,  four-wheel,  110 

two-wheel,  106 
scraping  grader,  103 
'  wagons,  110 
embankment,  rolling,  89 
settling,  88 
stability  of,  90 
overhaul,  89 
rolling,  89 

setting  slope  stakes,  86 
settlement,  88 
shrinkage,  87 
Elevating  grader,  101 

operating,  102 
Embankments,  85 
cross  section,  85 
finishing  slopes,  112 
rolling,  89 
settling,  88 
stability  of,  90 
Excavation,  84 
cross  section,  85 

Flux,  267 

specifications,  274 
Foot-way  crossing,  372 
Foundation,  pavements,  392 
bituminous  concrete,  406 
concrete  base,  hydraulic  cement,  399 

cost,  403 

curing,  403 

finishing,  402 

mixing,  402 

placing,  402 

proportions,  401 

thickness,  400 
drainage,  392 

see  also  Drainage, 
earthwork,  393 

see  also  Earthwork, 
filling  trenches,  395 

flooding,  396 

natural  settlement,  395 

re-filling  with  sand,  398 

replacing  material,  398 

tamping,  397 
macadam,  405 
railway  track,  407 

examples,  408 
subgrade,  392 

rolling,  394 
thickness,  400 

French  coefficient  of  wear,  188 
French  standard  macadam  road,  198,  199 

Grade,  effect  on  load,  50 
effect  on  location,  48,  69 
maximum,  54 
minimum,  57 
Grade  resistance,  21 
Grader,  elevating,  101 
scraping,  95 
Shuart,  208 

Granite  pavements,  amount  in  U.  S.,  320 
Granite  paving  blocks,  574 
Gravel,  road-building,  150 
binder,  151 

materials  of,  151,  152,  153 
characteristics,  158 

Buck  Hill,  160,  161,  163 
Decatur,  160,  161 
Lexington,  160,  161,  162 
Oaktown,  160,  161,  163 
Paducah,  160,  161,  164 
Peekskill,  160,  161,  162 
Rockford,  160,  161,  164 
Rock  Hill,  160,  161,  163 


INDEX 


663 


GRA-HOR 

Gravel,    road-building,    characteristics,    Ros- 

etta,  160,  161,  164 
Shaker  Prairie,  160,  161,  163 
Shark  River,  160,  161,  163 
Urbana,  159,  160,  161 
cherty,  155 
composition  of,  158 
defined,  150 
distribution  of,  154 
durability,  150 
exploring  for,  156 
mineralogical  analysis  of,  161 
requisites  for,  150 
binder,  151 
durability,  150 
sizes,  151 
screening,  173 
sieve  analysis,  160 
sizes,  151 

Gravel  pavements,  amount  in  U.  S.,  320 
Gravel  roads,  150 

bituminous  surface,  see  Bituminrus  surface. 
Connecticut  standard,  170 
construction,  165 
bottom  course,  172 
Connecticut  Standard,  170 
cost,  175 
cross  section,  169 
crown,  166,  355 
drainage,  165 

forcing  gravel  into  subgrade,  174 
forms  of  construction,  167 
comparisons  of,  171 
surface,  167 
trench,  169 
hauling  gravel,  174 
loading  gravel,  174 
measuring  gravel,  174 
rolling,  171 
specifications,  178 
durability,  178 
dust  palliative,  181 

moistening  with  salts,  182 
practice  in  Washington,  D.  C.,  183 
sprinkling  with  fresh  water,  181 
light  oil,  183,  192 
proprietary  compounds,  182 
sea  water,  181 
earth  track  beside,  171 
economic  value,  176 
grade,  maximum,  166 
maintenance,  178 
cost,  180 

destructive  agents,  178 
re-surfacing,  180 
sprinkling,  180 
Texas  standard,  170 
tractive  resistance,  15,  16,  18,  21 
travel  on,  effect  of,  177 
width,  166 
Guard  rails,  113 
Guide  posts,  114 
Gutter,  367,  381 
combined,  382 
concrete,  382 
cost,  388 

finishing  surface,  386 
forms,  383 
foundation,  383 
laying,  384 
depth,  369 
expansion  joint,  386 
grade,  370 
material,  367 
private  driveway,  389 
street  intersection,  370 

Hammer,  brick,  519 

stone-block,  579 
Horse,  power  of,  22 


HOR-PAV 

Horse,  power  of,  effect  of  grade  upon,  23 
maximum  load  on  grade,  24 

Iron  ore,  binder  for  gravel  roads,  152 
Jarrah  wood,  description  of,  603 
Karri  wood,  description  of,  603 

Labor  road  tax,  38 
Load,  effect  of  grade  on,  23 
Location  of  roads,  41 
curves,  58 

aesthetic  value,  61 

super-elevation,  60 
distance,  42 

value  of  saving,  41 
grade,  45 

effect  of,  47 

limiting  effect  of,  50 

maximum,  54 

minimum,  57 

rise  and  fall,  47 

vs.  distance,  53 
grade  line,  69 
placing  line,  65 
safety  at  summit,  56 
wheelway,  position  of,  64 
width,  61 

improved  portion,  62 

on  curves,  64 

right-of-way,  61 
Lute  for  sand  cushion,  508 

Macadam  pavement,  634,  635 

area  in  U.  S.,  320 
Macadam  road,  185 

see  also  Bituminous  Macadam,  306 

Water-bound  Macadam,  185 
Macadam  stone,  186 
binding  power,  186 
cementing  power,  186 
hardness,  186 
tests  of,  187 
abrasion,  188 
cementation,  188 
hardness,  187 
impact,  187 
toughness,  187 
toughness,  186 
Massachusetts  standard  macadam  road,    197, 


New  Jersey  standard  macadam  road,  197 
New  York  standard  macadam  road,  197 

Oil  for  roads,  cost  of,  288 
specifications  for,  286 

earth  roads,  287 

gravel  roads,  287 

macadam  roads,  287 

park  drives,  286 
Oiling  machines,  137 
Oil,  see  Petroleum. 
Over-haul,  89 

Pavement  administration,  321 
causes  of  inefficiency,  322 
conditions,  321 
importance  of  problem,  321 
remedy,  324 

Pavements,  apportionment  of  cost,  326,  328 
area  of,  in  U.  S.,  320 
asphalt,  see  Asphalt  pavements, 
assessments  for,  329 

area  rule,  330 

frontage  rule,  330 

legality  of  levy,  331 

terms  of  payment,  331 
benefits,  318 


664 


INDEX 


PAY— ROA 

Pavements,  brick,  see  Brick  pavements, 
cobble-stone,  see  Cobble-stone  pavements, 
comparisons,  642,  654 

cost  of  construction,  642,  656 
cost  of  maintenance,  643 
renewals,  645 
repairs,  643,  657 
sprinkling,  652 
total,  653 

transportation,  6fi2 
comfort  in  use,  650 
durability,  635 

ease  of  cleaning,  648,  651,  657 
freedom  from  mud  and  dust,  650 
healthfulness,  650 
noiselessness,  649 
slipperiness,  647 
temperature,  650 
tractive  resistance,  646,  657 
concrete,  see  Concrete  pavements, 
cross  section,  355 

side-hill  streets,  355 
crown,  355 
foundation,  392 

bituminous  concrete,  406 
hydraulic  concrete,  399 
macadam,  405 
gravel,  see  Gravel  roads, 
guaranteeing,  331 

maintenance  by  contract,  333 
investments  in,  319 
openings,  334 
selecting  the  best,  633 
stone-block,  see  Stone- block  pavements, 
tearing  up,  334 
widths  of,  345 

with  car  tracks,  345 
without  car  tracks,  3<  6 
wood-block,  see  Wood-t  lock  pavements. 
Pavement  foundation,  see  Foundation. 
Paving  railway  areas,  407.  441,  539,  587,  623 
Petroleum,  283 
'    asphalt  content,  285 
asphalt  residue,  270 
classification,  283 
cost  of,  288 

method  of  refining,  284 
shipping,  284 
specifications  for,  286 
Poll  tax,  35 
Preserving  timber,  see  Wood-block  pavement. 

Rails,  car  tracks,  409 
Railway  rails,  409 
Railway  ties,  409 
Rammer,  brick,  520 

stone-block,  582 

wood-block,  617 
Rattler  for  testing  brick,  492 

specifications,  491 
Retaining  walls,  113 
Road,  artistic  treatment,  114 

bituminous  concrete,  see    Bituminous    con- 
crete roads. 

bituminous  surfaces  for,  see  Bituminous  sur- 
faces. 

earth,  see  Earth  roads. 

gravel,  see  Gravel  roads. 

hydraulic  concrete,  see  Concrete  roads. 

macadam,  see  Bituminous  macadam  roads. 
see  Water-bound  macadam  roads. 

sand,  see  Sand  roads. 

sand-clay,  see  Sand-clay  roads. 

taxes,  see  Taxes. 
Road  administration,  30 

national,  34 

state,  32 

unit,  31 
Roads,  advantages  of  good,  3 

artistic  treatment  of,  114 


ROA-  STO 

Roads,  classification,  34 

estimated  cost  of  bad,  10 

expenditures  for  in  U.  S.,  40 

improving  old,  91 

toll,  35 
Road-building  machinery,  91 

drag,  see  Road  drag 

elevating  grader,  101 

roller,  212 
tandem,  213 
three-wheel,  212 

scrapers,  92,  93,  94 

scraping  grader,  95 
Road  drag,  117 

plank,  118 

rules  for  using,  120 

split-log,  118 

steel,  118 
Road  grader,  scraping,  95 

elevating,  101 

Road  improvement,  financial  value  of,  11 
Rollers,  213 

asphalt  type,  213 

macadam  type,  212 

tandem  type,  213 

three-wheel  type,  212 

Sand  roads,  139 
drainage,  139 
hardening  the  surface,  139 
shade,  139 

tractive  resistance,  16,  21 
Sand-clay  roads,  140 

clay  on  sand  subgrade,  145 
clay,  145 
construction,  146 
cost,  147 
design,  141 
maintenance,  148 

natural  mixtures  of  sand  and  clay,  141 
construction,  143 
tests  of,  141 

sand  on  clay  subgrade,  143 
construction,  145 
proportions,  144 
sand, 144 
thickness,  144 
travel  census  of,  149 
Scrapers,  92 
drag,  92 
Fresno,  93 
scoop,  92   ' 
slip,  92 
wheel,  94 

four-wheel,  95 
two-wheel,  95 
Scraping  grader,  95 

operating,  97 
Snow,  cost  of  clearing,  126 

obstruction  by,  125 
State  aid,  32 

Stone-block  hammer,  579 
Stone-block  pavement,  566 
adjacent  to  track,  586 
amount  in  U.  S.,  320 
classification,  566 
Belgian  block,  568 
cobble-stone,  567 
durax,  569 
oblong  block,  568 
Roman,  566 
rubble,  568 
construction,  572 
bedding  course,  572 
mortar,  574 
sand,  573 
blocks,  574 
dressing,  574 

re-cutting,  576 
measuring,  577 


INDEX 


665 


STO—  STR 

Stone-block   pavement,    construction,  blocks, 
ramming,  580 
setting,  579 
size,  577 

re-cutting,  576 
filling  joints,  582 
asphalt,  585 
gravel,  582 
grout,  586 
pea  gravel,  582 
tar  and  sand,  584 
foundation,  572 
cost,  591 
blocks,  591 
Buffalo,  595 
Chicago,  592 
Cleveland,  597 
contract  price,  598 
durax,  593 
grouting,  593 

Medina  stone,  595,  596,  597 
New  York,  593 
re-cutting  and  re-laying,  592 
Rochester,  597 
Schenectady,  573 
tar-sand  filler,  596 
various  cities,  598 
durax,  588 
expansion  joint,  587 
grade,  maximum,  587 
granite,  569 
hammer,  579 
limestone,  572 
maintenance,  597 
cost,  599 

raising  blocks,  599 
re-filling  joints,  599 
re-laying,  598 
repairs,  598 

settlement  of  trenches,  599 
sinking  of  foundation,  599 
spalling  joints,  599 
Medina  sandstone,  571 
merits,  588 
paver's  hammer,  579 
paver's  rammer,  520,  582 
quartzite,  572 
sandstone,  571 
Colorado,  571 
Kettle  River,  572 
Medina,  571 
Potsdam,  571 
Sioux  Falls,  572 
trap,  571 

Stone-block  rammer,  582 
Stone  crusher,  203 
gyratory,  204 
oscillatory,  203 
Stone-crushing  plant,  205 
Street,  cross  section  on  side-hill,    355 
design,  336 

area  of  streets,  344 
blocks,  size  of,  337 
location  of  streets,  339 
directness,  341 


topography,  339 
an  of  streets, 


plan  of  streets,  336 
blocks,  size  of,  337 
lots,  size  of,  337 

shade  trees,  338 

width  of  streets,  343 
drainage,  see  Drainage,  street. 
grades,  347 

elevations  at  street  intersection,  350 

maximum,  348 

minimum,  349 

vertical  curves  at  intersection,  353 
location,  339 

directness,  341 

topography,  339 


STR— VEH 

Street,  pavements,  width  of,  343 

with  car  tracks,  346 

without  car  tracks,  345 
plan  of  streets,  336 

checker-board,  341 

C9ncentric,  343 

diagonal,  341 
trees,  358 

vertical  curves,  354 
width,  343 
Swiss  standard  macadam  road,  198 

Tar,  289 

characteristics,  289 

cost  of,  295 

kinds,  289 

shipping,  290 

specifications,  289 

bituminous  concrete,  292 
bituminous  macadam,  291 
bituminous  surfaces,  291 
filler  for  block  pavements,  294 
trade  names,  294 

tests  of,  see  Bituminous  materials,  tests  of. 

trade  names,  294 
Tax,  road,  34 

automobile,  40 

labor,  38 

money,  38 

poll,  35 

property,  38 

toll,  35 

Telford  road,  185,  189,  191 
Template,  brick  pavements,  505,  513 
mortar  bedding-course,  513 
sand  cushion,  505 
concrete  roads,  249 
Texas  gravel  road,  170 
Thank-you-marms,  83 
Ties,  street-railway,  409 
Tile,  cost  of,  75 

drainage,  72 

laying,  77 

location,  78 

one  vs.  two  lines,  77 

size  of,  76 

weight,  75 

Tires,  width  of,  effect  on  traction,  14 
Tractive  resistance,  12 

American  experiments,  19 

axle  friction,  12 

data  on,  15,  16,  17,  18,  20,  21 

diameter  of  wheel,  effect  of,  13 

French  experiments,  17 

rolling  resistance,  13 

speed,  effect  of,  16 

springs,  effect  of,  17 

width  of  tire,  effect  of,  14 
Transportation,  cost  of  wagon,  6 

annual  saving,  10 
Travel  census,  25 

American  roads,  26 
streets,  28 

classification  of  travel,  28 
diverting  travel,  29 
weight  of  vehicles,  29 

French,  26 

history,  26 

Illinois,  27 

Iowa,  28 

Massachusetts,  26 

weight  of  vehicles,  30 

width  of  traveled  way,  29 

width  of  vehicles,  31 
Trees  on  street,  358 
Trinidad  asphalt,  268 

V  road-leveler,  123 
Vehicles,  weight  of,  30 
width  of,  30 


666 


INDEX 


WAS— WOO 

Washington,  sprinkling  gravel  with  oil,  183 

street  plan  of,  342 
Water-bound  macadam,  185 
binder,  216 

bituminous    surface,    see    Bituminous    sur- 
faces. 

construction,    189 
binding,    217 
cost,  220 
crown,  192 
foundation,  189 
rolling,  213 
setting  Teltord,  201 
shoulders,  191 
shrinkage,  209 
size  of  stone,  204 
spreading  stone,  207 
subgrade,  190,  200 
super-elevation,  194 
Telford's,  191 
thickness,  194 
width,  191 
wings,  196 
crown,  192 
crushing  stone,  202 
grade,  permissible,  199 
maintenance,  223 

cost,  226 
•   forms  of,  189 
patching,  225 
raveling,  224 
rolling,  226 
sprinkling,  226 
standard,  French,  198,  199 
Massachusetts,  197,  198 
New  Jersey,  191,  197 
New  York,  197 
Swiss,  198 
super-elevation,  194 
thickness,  194 
width,  191 
wings,  196 
Water-breaks,  83 
Water-ways,  113 
Wheelway,  position  of,  64 

width  of,  62 

Wings  for  macadam  roads,  196 
Wood-block  pavement,  601 
adjacent  to  track,  623 
area  in  U.  S.,  320 
blocks,  604 

care  after  treatment,  611 
causes  of  decay,  605 
laying,  615 
specifications,  604 
dimensions,  604 


WOO 

Wood-block  pavement,  blocks,  specifications, 

quality,  605 
testing,  611 
treatment,  609 
construction,  612 
bedding  course,  612 
bituminous,  614 
mortar,  613 
sand,  612 
cost,  623 
blocks,  623 
examples,  625 
various  cities,  627 
filling  joints,  618 
grout,  618 
sand,  618 
tar,  618 

foundation,  612 
laying  blocks,  615 
crown,  623 
decay,  cause  of,  605 
expansion  joints,  622 
grade,  maximum,  623 
history,  602 
kinds,  601 

rectangular  blocks,  602 
round  blocks,  601 
maintenance,  628 
bleeding,  630 
bulges,  630 
cost,  631 
low  spots,  629 
poor  blocks,  628 
re-laying,  630 
merits,  626 
open  joints,  621 
preservative,  605 
amount,  611 
specifications,  607 
creosote  oil,  607 
coal-tar  distillate,  607 
coal-tar  paving  oil,  60S 
water-gas  tar,  608 
rolling,  617 
specifications,  604 
treatment,  609 

open-tank  process,  609 
pressure  process,  609 
timber,  603 
hemlock,  604 
jarrah,  603 
karri,  603 
larch,  604 
pine,  604 
tamarack,  604 
yellow  pine,  603 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


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AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


APR  24  13679  2 


MAY  2  4  1967 


-5- 


LD  21-95m-7,'37 


382053  Z^ 


UNIVERSITY  OF  CAUFORNIA  LIBRARY 


